Skip to main content

Full text of "The chemistry of the diazo-compounds"

See other formats












' \ . 



\All rights reserved'] 





IN this book I have endeavoured to describe our 
present knowledge of the Diazo-eompounds and to 
give an account of the enormous progress made in 
this important branch of Organic Chemistry since 
Griess's epoch-making discovery just fifty years ago. 

I have adopted the plan of giving full details of the 
simpler preparations and reactions which are being 
continually carried out in the laboratory, but I have 
made only short reference to the more involved 
operations such as would be undertaken by the 
research chemist, not only to avoid a mass of detail, 
but also because in such cases the original paper is 
invariably consulted, and with this in view full 
references to the literature are given. 

Owing to the very large space necessarily given to 
a discussion of the theories of the constitution of 
the diazo-compounds, the practical and descriptive 
portions have been kept together and the theoretical 
part reserved until later. 

Partly on account of this the word ' diazonium ' is 
not used until the theoretical part is reached, where 
its meaning is explained. 

I have striven to give an exact account of the long 
controversy between Hantzsch and Bamberger on 
the constitution of the diazo-compounds. The most 


important contributions to the literature have hitherto 
been made by Hantzsch (Die Diasoverbindungen, Ahrens' 
Sammlung, 1902) and his former pupil, Eibner (Zur 
GeschiMe der aromatischen Diazoverbindungen, Olden- 
bourg, 1903), so that the opposite view has, perhaps, 
not been set forth quite so fully, although the 
admirable Eeport to the British Association by Morgan 
(Our Present Knowledge of Aromatic Dia#o-compounds, 
1902) leaves nothing to be desired. 

Another difficulty in the way of presenting a clear 
view of the subject is found in the many cases where 
work, even supported by numerous analyses, has been 
shown to be incorrect. This, taken in conjunction 
with the somewhat authoritative tone of Hantzsch's 
papers and the unfortunate illness of Bamberger 
which necessitated cessation of work for some years, 
may well be responsible for the incomplete accounts 
of Diazo-chemistry which are occasionally encountered. 
Finally, a new theory of the constitution of Diazo- 
compounds is presented in the Appendix to this 

J. C. C. 

December, 1907. 







I. Preparation of dry diazo-salts. 2. Other methods of 
preparation. 3. Diazotization of amino-phenols and -thio- 
phenols. Quinonediazides (diazophenols). 4. Thiodiazoles 
(Diazosulphides). 5. Preparation of diazo-salts in aqueous 
solution. 6. Other methods of preparing solutions of diazo- 
salts. 7. The action of nitrous acid on aromatic substances 
containing more than one amino-'group* 8. ' Solid ' diazo- 



1. Thermochemistry. 2. Explosibility of dry diazo- 
compounds. 3. Velocity 6f diazotization. 


1. Action of water. 2. Stability of diazo-solutions. 



1. Action of alcohols. 2. Influence of substituents. 
3. Influence of the alcohol used. 4. Influence of temperature 
and pressure. 5. Influence of other substances. 6. Other 
methods of reduction. 





1. Replacement of the diazo-group by the halogens Chlo- 
rine Bromine Iodine Fluorine. 



1. Replacement of the diazo-group by cyanogen. 2. Re- 
placement of the diazo-group by the cyano-group. 3. Re- 
placement of the diazo-group by the thio cyano-group. 
4. Replacement of the diazo-group by the group SH. 5. Re- 
placement of the diazo-group by sulphur. 6. Replacement of 
the diazo-group by the sulphonic acid group. 7. Replace- 
ment of the diazo-group by the nitro-group. 8. Replace- 
ment of the diazo-group by the nitroso-group. 9. Replacement 
of the diazo-group by the amino-group. 10. Replacement of 
the diazo-group by the acetoxy-group. 


1. Sulphur dioxide. 2. Replacement of the diazo-group 
by the sulphinic acid group. 3. Hydrogen sulphide. 4. Re- 
placement of the diazo-group by the azoimino'group. 5. Benzoyl 











1. Azoxy-compounds. 2. Azo-compounds. 3. Aminoazo- 
compounds. 4. Hydroxyazo-compounds. 5. Rate of 
formation of amino- and hydroxy-azo-compounds. 6. Constitu- 
tion of the hydroxyazo-compounds. 7. Mixed azo-compounds. 




1. Preparation. 2, Properties of diazoacetic esters. 
3. Reactions of the aliphatic diazo-compounds. 4. Metallic 
diazo-compounds of the aliphatic series. 5. Diazoamino-com- 
pounds of the aliphatic series. 


1. Constitution of the diazo-salts according to Griess. 
2. Constitution of diazo-compounds according to Kekule*. 
3. Constitution of the diazo-salts according to Blomstrand. 
4. Constitution of diazobenzene hydroxide to 1894. 



1. Constitution of the diazo-compounds according to Hantzsch. 


1. Constitution of the diazo'-compounds according to 
Bamberger. 2. Relation between diazonium compounds 
and normal or syn-diazo-compounds. 3. Double salts of 
diazonium halides and metallic salts. 4. Diazonium halides 
andsyn-diazo-halides. 5. Diazonium perhalides. 6. Rela- 
tion between syn- and awfo'-compounds. 7. The isomeric 
diazo-sulphonates and diazo-cyanides. 8. Constitution of the 
metallic diazo-oxides. 9. Diazo-ethers. 10. Diazo- 
anhydrides. 11. Diazo-hydroxides. 12. Condition of the 
non-ionized diazonium hydroxide. 13. Constitution of iso 
(anti) diazo-hydroxides. 




POUNDS FROM 1895 153 

1. Constitution of the coloured diazo-salts of Jacobson. 
2. Constitution of the diazo-salts according to Walther. 
3. Constitution of diazo-compounds according to Bruhl. 
4. Constitution of the diazo-compounds according to Bobbie 
and Tinkler. 5. Constitution of the diazo-oompounds 
according to Armstrong and Robertson. 


COMPOUNDS TO 1907 159 

1. Constitution of the diazo-salts (diazonium salts). 
2. The labile and stable isomeric diazo-compounds. 


COMPOUNDS . . . . . . . 163 

SUBJECT INDEX . . . . . . .169 




Amer. Chem. J. American Chemical Journal. 

Annalen Justus Liebig's Annalen der Chemie. 

Arch. Pharm Archiv der Pharmazie. 

Atti R. Accad. Lincei .... Atti della Reale Accademia dei Lincei. 
Ber. Bericlite der Deutschen chemischen 

Bull. Acad. Sci. Cracow . . . Bulletin international de 1'Academie 

des Sciences de Cracovie. 
Bull. Soc. chim Bulletin de la Societe chimique de 

Bull. Soc. ind. Mulhouse . . . Bulletin de la Societe industrielle de 


Chem. Weekblad Chemiseh Weekblad. 

Chem. Zeit Chemiker Zeitung. 

Compt. rend Comptesrendushebdpmadaires des Sean- 
ces de I'Academie des Sciences. 

Gazzetta Gazzetta chimica italiana. 

Jdhresber Jahresbericht uber die Fortschritte der 


J. pr. Chem Journal fur praktische Chemie. 

J. Russ. Phys. Chem. Soc. . . Journal of the Physical and Chemical 

Society of Russia. 
J. Soc. Chem. Ind Journal of the Society of Chemical 

J. Soc. Dyers Journal of the Society of Dyers and 


Journ. Chem. Soc Journal of the Chemical Society. 

Phil. Mag Philosophical Magazine (The London, 

Edinburgh and Dublin). 
Phil. Trans Philosophical Transactions of the Royal 

Society of London. 

Proc Proceedings of the Chemical Society. 

Proc. Roy. Soc Proceedings of the Royal Society. 

Trans Transactions of the Chemical Society. 

Zeitsch. angew. Chem Zeitschrift fur angewandte Chemie. 

Zeitsch. EleTctrochem Zeitschrift fur Elektrochemie. 

Zeitsch. Farb.-Ind Zeitschrift fur Farben-Industrie. 

Zeitsch. physikal. Chem. . . . Zeitschrift fur physikalische Chemie. 
Zeitsch. f. Chem Zeitschrift fur Chemie. 

D.R-P Deutsches Reichs-Patent. 

E.P English Patent. 

F.P. . . French Patent. 




Page 25, note f, for diazo-dyestuffs read disazo-dyestuffs 
Cain. Diazo-compounds 

the result that diazoaminobenzoic acid was isolated. Griess 
then extended his work so successfully that he discovered the 
existence of an entirely new class of substances, to which the 
name 'diazo' was given. An account of this discovery, 
given by Griess himself, will be of interest* || 

* Annalen, 106, 123. t Ibid., 1854, 91, 185. 

t Obituary notices of Griess, Ber., 1891, 24, 1007. 

$ Griess says : ' I have come to the conclusion that the two atoms (or 
the molecule) of nitrogen, N 2 , they contain, must be considered as 
equivalent to two atoms of hydrogen, and it is in accordance with this 
view that the names of the new compounds have been framed.' (Phil. 
Trans., 1864, 154, 668.) 

|| Roscoe and Schorlemmer, Treatise on Chemistry, vol. iii, part 3, 311. 
Griess, private communication to Watson Smith in 1887 ; compare also 
Watson Smith, J. Soc. Chem. Ind., 1907, 26, 134. 




THE diazo-compounds were discovered in 1858 by Johann 
Peter Griess,* who obtained them by treating aromatic amino- 
compounds with nitrous acid. Piria had already found, in 
1849, that asparagine or aminosuccinamic acid is converted 
into malic acid by the action of nitrous acid, the amino-group, 
NH 2 , being substituted by the hydroxyl group, OH, thus 

C 4 H 4 3 (NH 2 ) 2 + 2HN0 2 = C 4 H 4 3 (OH) 2 + 2N 2 + 2H 2 O. 
In the aromatic series, also, Hunt in the same year showed 
that aniline, by the same method, was converted into phenol. 
Then Gerland in 1853 f prepared hydroxybenzoic acid from 
aminobenzoic acid and also observed the formation of a red 
intermediate product, the quantity of which was found to 
increase by working with cold dilute solutions, Gerland was 
unable to decide as to the constitution of this substance owing 
to wide variations in the analytical figures. The further 
investigation of this was suggested to Griess by Kolbe,J with 
the result that diazoaminobenzoic acid was isolated. Griesa 
then extended his work so successfully that he discovered the 
existence of an entirely new class of substances, to which the 
name 'diazo' was given. An account of this discovery, 
given by Griess himself, will be of interest,|| 

Annalen, 106, 123. t Ibid., 1854, 91, 185. 

Obituary notices of Griess, Ber., 1891, 24, 1007. 

Griess says : ' I have come to the conclusion that the two atoms (or 
the molecule) of nitrogen, N 2 , they contain, must be considered as 
equivalent to two atoms of hydrogen, and it is in accordance with this 
view that the names of the new compounds have been framed.' (Phil. 
Trans., 1864, 154, 668.) 

|| Roscoe and Schorlemmer, Treatise on Chemistry, vol. iii, part 3, 311. 
Griess, private communication to Watson Smith in 1887 ; compare also 
Watson Smith, J. Soc. Chem. Ind., 1907, 26, 134. 



1 Dr. Gerland, when working in the laboratory of Prof. 
Kolbe, in Marburg, investigated the action of nitrous acid on 
amidobenzoic acid at the request of Kolbe. Thus oxybenzoic 
acid was prepared, indicating a chemical change then con- 
sidered of much importance. In like manner I investigated 
a means of converting picramic acid (amidodinitrophenylic 
acid) into the oxydinitrophenylic acid, C 6 H 2 (NO 2 ) 2 (OH) 2 , but 
I obtained instead of the latter a compound possessed of such 
striking and peculiar properties that I at once concluded it 
must belong to a completely new class of compounds. Analysis 
soon showed me that this peculiar compound had the com- 
position C 6 H 2 (N0 2 ) 2 N 2 0. Naturally I soon submitted many 
other amido-compounds in like manner to the action of nitrous 
acid, and obtained thus, in almost every case, the correspond- 
ing diazo-compound. But the circumstance to which I was 
indebted for my success in obtaining the diazo- compounds 
was that of the treatment of the amido-compounds with 
nitrous acid in the cold, whereas in the earlier experiments 
of Hunt and Gerland a higher temperature was always 
attained, and consequently no diazo-compounds could exist. 
Having obtained these diazo-compounds, I then tried their 
action on all possible substances, among which of course are 
the numerous class of amido-compounds. I found that the 
diazo-compounds combine directly with these, forming fre- 
quently brilliantly coloured substances which dye animal 
fibres directly. The first colouring matter thus prepared by 
me, which I obtained in the years 1861-2, was the benzeneazo- 
a-naphthylamine.'* It was first prepared on the large scale, 
to the best of my recollection, in the years 1865-6 by Caro, 
who was then chemist in the works of Messrs. Roberts, Dale 
& Co., of Manchester. I first recommended the oxyazobenzene 
obtained by me for use as a colouring matter in 1866.'f 

Griess continued his researches on diazo- and azo-com- 
poundsj during his three years' residence in London as 

* Phil. Trans., 1864, 154, 679. 

t Annalen, 137, 88. 

t Griess's first short preliminary announcement was published in 
Annalen, 1858, 108, 123 ; see also Proc. Roy. Soc., 1859, 9, 594 ; Phil. 
Mag. 1859 [iv], 17, 370; Compt. rend., 1859, 49, 77. The full paper 
appeared in Annalen, 1860, 113, 201. See also Annalen, 1860, 113, 337 ; 


Hofmann's assistant, and also afterwards while with Messrs. 
Allsopp, Burton-on-Trent. Here, although busily occupied in 
the vast brewery, Griess found time in which to prepare a 
large number of new diazo-compounds, and these were then 
handed over to his friend, Dr. R. Schmitt, at Dresden for 
analysis. Hempel's account of this is interesting : ' Regel- 
massig kamen von Burton an den Ufern des Trent die von 
Griess dargestellten neuen Korper in kleinen Packeten, urn in 
Dresden an der Elbe analysirt zu werden. Per Fracht kamen 
dann wohl gleichzeitig als willkommene Beilage Fasser von 
Allsopp's beriihmtem Pale Ale in ausgesuchtester QualitaV 
(The new compounds prepared by Griess were regularly sent 
in small packets from Burton-on-Trent to Dresden to be 
analysed. At the same time a welcome accompaniment took 
the form of barrels of Allsopp's finest Pale Ale). Griess's 
brilliant investigations extended to the preparation of a very 
large number of diazo-compounds; further, he discovered 
most of their reactions with other reagents and laid the 
foundation of the immense edifice of azo-dyestuffs which has 
since been, and is still being, erected. 

After having described the diazoamino-derivatives of amino- 
benzoic acid,* aminotoluic acid, and aminoanisic acid, and their 
reactions, Griess then obtained diazoaminobenzene f and the 
diazobenzene salts. J Of these the nitrate, the easiest to pre- 
pare, was used as the starting-point in the preparation of the 
crystallized sulphate, the platinichloride, the aurichloride, and, 
some years later, the ferricyanide, the nitroprussiate, and the 
tin chloride double salt. With the object of preparing the 
bromide, Griess treated diazoaminobenzene with bromine in 
ethereal solution and obtained the perbromide C 6 H 5 N 2 Br 3 . 

1861, 117, 1 : 120, 125: 1861, Suppl. I, 100; 1862, 121, 257 ; 1866, 137, 
39; Proc. Boy. Soc., 1860, 10, 309, 591; 1862, 11, 263 ; 1863, 12, 418 ; 
1864, 13, 375. A long paper, including most of the earlier work, is in 
Phil. Trans., 1864, 154, 667 ; and accounts were also published in Journ. 
Chem. Soc., 1865, 3, 268, 298; 186H, 4, 57 ; 1867, 5, 36. For the later 
work see Ber., 1869, 2, 369 ; 1874, 7, 1618 ; 1876, 9, 132, 627, 1653 ; 1878, 
11,624; 1879,12,2119; 1881,14,2032; 1882,15,2183; 1883,16,2028; 
1884, 17, 338. 

* Annalen, 1861, 117, 1. t Ibid., 1862, 121, 257. 

J Ibid., 1866, 1^7, 39. 

Ber., 1879, 12, 2119; 1885, IP, 965. 


On the addition of ammonia to this compound, the whole of 
the bromine was removed and a substance containing three 
atoms of nitrogen was isolated. This was called diazobenzene- 
imide and possessed the formula C 6 H 5 N 3 . Griess also studied 
the formation of metallic diazo-derivatives ; thus, by the action 
of a concentrated potassium hydroxide solution on a strong 
solution of diazobenzene nitrate, a substance containing 
potassium was obtained, and by mixing a solution of this 
with a silver solution, a substance containing silver was pre- 
cipitated. The formulae of these metallic derivatives were 
considered by Griess to be C 6 H 5 N 2 . OK and C 6 H 5 N 2 . OAg 
respectively (see, however, p. 96). By treating a solution of 
the potassium compound with acetic acid, a viscous yellow oil 
was obtained which Griess looked upon as free diazobenzene. 
With mineral acids it yielded the corresponding salts. 

We now pass on to consider briefly the various reactions 
which the diazo-salts, in Griess's hands, were found to undergo. 
By boiling with water, phenols were obtained ; in the case of 
the nitrate, nitrophenol was formed by the interaction of 
phenol and the liberated nitric acid. When alcohol was 
substituted for water benzene was formed, whilst the alcohol 
was reduced to aldehyde. By the action of hydriodic acid 
in the cold, iodobenzene was obtained, chlorobenzene by 
distilling the dry platinichloride with soda, and bromobenzene 
in the same way from the platinibromide, and also by boiling 
the perbromide with alcohol. 

By the action of phenols and amines on the diazo-compounds 
Griess discovered that highly-coloured condensation products 
were formed. These were the azo-dyestuffs, some of which 
were immediately prepared on the large scale ; the reaction 
itself giving the key to an industry which has since attained 
an enormous importance. 

The next important discovery to be noted is that of the 
first diazo-compound belonging to the aliphatic series in 1883. 
Curtius prepared the ethyl ester of diazoacetic acid, pro- 
ceeding from this to a series of brilliant researches on fatty 
diazo-compounds, culminating in his discovery of azoimide. 
In 1894 von Pechmann isolated the simplest member of 


the series, namely, diazomethane, since which time a large 
number of derivatives have been obtained. 

The remarkable influence of small amounts of copper salts 
on the reactions which the diazo-compounds undergo was 
discovered in 1884 by Sandmeyer, whose name is associated 
with this decomposition, and the substitution of finely divided 
copper for its salts was introduced by Gattermann in 1890. 

In the latter year Meldola discovered that the presence of 
the diazo-group has, in certain cases, a remarkable effect on 
the stability of a nitro-group present in the same benzene 
ring, whereby this group is very readily eliminated. 

This transformation of acidic groups under the influence of 
the diazo-group has been made the subject of comprehensive 
researches by Meldola and his colleagues, as well as by 
Bamberger, Orton, and Hantzsch, and the latter chemist 
showed in 1896 that in some cases the acidic group attached 
to the diazo-nitrogen could change places with a halogen atom 
in the benzene ring. 

In 1894 an important investigation carried out by Schraube 
and Schmidt, whereby the existence of two isomeric metallic 
salts of diazobenzene was indicated, led to a thorough 
examination of the metallic diazo-derivatives, and gave rise 
to a prolonged discussion of the constitution of the whole 
class of diazo-compounds. Among the many discoveries 
which were made in 1895 is specially to be recorded Bam- 
berger 's isolation of the diazoic acids by the oxidation of the 

The definite proof by Andresen, in that year, that light 
acted on diazo-compounds with the production of the corre- 
sponding phenols was followed by the remarkable observation 
of Orton in 1906, that quantitative yields of phenols were 
obtained from certain diazo-salts which gave practically no 
hydroxy-derivative when heated with water or acids. 



^ 1. Preparation of dry diazo-salts. In preparing the 
diazo-compounds Griess used, as a source of nitrous acid, the 
gases evolved by warming a mixture of nitric acid and 
arsenic trioxide. These gases were passed into either an 
alcoholic solution of the amine or an aqueous paste of an 
amino-salt, the experiment being carried out in the cold, 
when the resulting diazo-compound separated or was precipi- 
tated by the addition of alcohol and ether. 

In the case of diazobenzene nitrate, used by Griess as the 
starting-point for the preparation of other diazo-compounds, 
the nitrous gases were passed into a well-cooled paste of 
aniline nitrate and water until aniline ceased to be liberated 
on adding potassium hydroxide to a small test portion. The 
solution was then filtered and alcohol and ether added to 
precipitate the diazobenzene nitrate, which separated in white 

A considerable improvement on this method consists in the 
use of a solution of sodium nitrite as a source of nitrous acid.f 
By this process the calculated quantity of nitrous acid may 
be used, and this is so convenient that sodium nitrite is almost 
entirely employed at the present day in the preparation of 

The use of an aqueous solution of sodium nitrite is, how- 
ever, not very suitable for the preparation of those dry diazo- 
compounds which are very soluble in water, and in order to 
avoid the presence of the latter Knoevenagel J used amyl 
nitrite in alcoholic solution, a method which had been em- 

* The description of the metallic diazo-compounds is reserved until a 
later chapter (see p. 96). 

t Martius, J. pr. Chem., 1866, 98, 94. 
1 Ber., 1899, 23, 2995. 


ployed by V. Meyer and Ambuhl in the preparation of diazo- 
aminobenzene.* This was a great improvement on existing 
methods, and a large number of dry diazo-salts have been 
prepared in this way. Pure products are, however, only 
obtained in the absence of free mineral acid.f The reason 
of this is that in presence of excess of mineral acid additive 
compounds of the diazo-chloride with hydrochloric acid are 

The hydrochlorides are therefore prepared by passing 
dry hydrogen chloride into a solution of the amine in absolute 
alcohol or ether and heating the product at 40-50 until the 
last traces of acid have been removed. The dry salt is then 
dissolved in alcohol and the theoretical quantity of amyl 
nitrite added at the ordinary temperature. On precipitating 
with ether the diazo-chloride is obtained in the pure state. 
This is a somewhat tedious process, and it has been found { 
that the reaction proceeds even more satisfactorily in the 
presence of glacial acetic acid, thus avoiding the necessity of 
preparing the dry aminic hydrochloride. 

In order to prepare diazobenzene chloride the experiment 
is carried out as follows : Fifty grams of aniline hydro- 
chloride are dissolved or suspended in about three times the 
quantity of glacial acetic acid and the mixture stirred by 
a turbine. A little more than the theoretical quantity of 
amyl nitrite is now added, care being taken that the tempera- 
ture does not exceed 10. Any undissolved aniline salt 
disappears quickly, and the diazotization is complete as soon 
as a small portion withdrawn and treated with sodium acetate 
no longer gives a yellow coloration. On adding ether a thick 
crystalline precipitate of diazobenzene chloride is obtained, 
which is filtered, washed with ether, and dried in a desiccator. 
The yield is 53 grams. The sulphate is obtained in a similar 
way ; in this case aniline sulphate is diazotized in presence of 
the calculated quantity of sulphuric acid. The separation of 
the diazo-sulphate is effected by first adding a little alcohol to 
the mixture with acetic acid and then precipitating with 
ether. The halogenated diazo-chlorides are prepared in the 

* Annalen, 1889, 251, 56. t Hirsch, Per., 1897, 30, 1148. 

J Hantzsch and Jochem, Ber., 1901, 34, 3337. 


same way, but care must be taken to use the hydrochlorides 
obtained according to Hirsch's method, avoiding the presence 
of mineral acid. 

The preparation of diazo-salts which are sparingly soluble 
in water can, of course, be carried out in aqueous solution. In 
some cases the diazo-compound is precipitated on addition of 
sodium nitrite to the acid solution of the amine, whilst in 
others the insoluble diazo-compound is precipitated on adding 
the salt of a different acid. 

Many instances of the former case occur among the amino- 
sulphonic acids. Thus _p-diazobenzenesulphonic acid is easily 
obtained* by dissolving sulphanilic acid in dilute aqueous 
sodium hydroxide, acidifying with hydrochloric acid, 
and adding the calculated quantity of sodium nitrite, pre- 
viously dissolved in a small quantity of water. The tempera- 
ture should be about 5. As soon as the whole of the 
nitrite has been added the diazo-compound, in the form of 
the anhydride C 6 H 4 O 3 N 2 S, separates in fine white needles, 
which may be filtered, but should not be dried, as they are 
extremely explosive (see p. 28). The diazo-compound derived 
from a-naphthylamine-4-sulphonic acid (naphthionic acid) is 
obtained in a similar manner. The naphthionic acid is dis- 
solved in alkali and reprecipitated by the addition of mineral 
acid- On adding the nitrite solution, the white insoluble 
naphthionic acid is gradually converted into the yellow 
insoluble diazo-compound. 

Instances of the second method are numerous; thus on 
adding a solution of sodium picrate to a solution of diazo- 
benzene nitrate a precipitate of the insoluble diazobenzene 
picrate is obtained, f A very stable diazo-picrate has also 
been prepared from ^-aminobenzanilide in the same way, J 
and the picrate of diazophenylindole, CgoH^OfNg, as also the 
picrate of diazomethylindole, C 15 H 10 O 7 N 6 , can be crystallized 
from alcohol. The chromate was prepared by Griess and 
Caro, || who diazotized aniline nitrate by means of a solution 

* Schmitt, Annalen, 1859, 112, 118 ; 1861, 120, 144. 

t Baeyer and Jaeger, Ber., 1875, 8, 894. 

1 Morgan and Wootton, Proc., 1906, 22, 23. 

Castellana and d'Angelo, Atti R. Accad. Lincei, 1905 [v], 14, ii. 145. 

|| Jahresler., 1867, 915. 


of calcium nitrite, and then added an equivalent of potassium 
dichromate and hydrochloric acid, when a precipitate was 
obtained. They suggested the use of this chromate as an 

Diazo-chromates are now usually prepared f by precipita- 
ting a diazo-solution with sodium dichromate. 

Insoluble diazo-thiosulphates, hydrof erricyanides, and tung- 
states have been also prepared in a similar manner. { 

An interesting diazo-carbonate is obtained by pouring 
the diazo-chloride derived from benzoyl-p-phenylenediamine 
(p-aminobenzanilide) into cold aqueous sodium carbonate, 
when a yellow precipitate results which has the formula 
C 6 H 6 . CO.NH.C 6 H 4 . N 2 . HC0 3 . 

By adding sodium acetate to the diazo-chloride, and then 
treating with excess of sodium nitrite, a crystalline yellow 
diazo-nitrite is obtained. 

Certain diazo-salts of hydrazoic acid have also been 
prepared by treating an ethereal solution of the diazo- 
hydroxide with ethyl azoiminocarboxylate, N 3 . C0 2 Et. These 
salts have the composition Ar.N 2 .N 3 , and are extremely 
unstable. || 

Diazo-fluorides containing one molecule of hydrofluoric acid 
are obtained by diazotizing the amine with amyl nitrite in 
presence of hydrofluoric acid,^[ and diazo-perchlorates are pro- 
duced by diazotizing amines in presence of perchloric acid. 
These perchlorates are extremely explosive.** 

2. Other methods of preparation. A modification of 
Griess's method of diazotizing was used by his co-worker 
Schmitt,tt wno saturated absolute alcohol with nitrous fumes 
and poured this over aminophenol hydrochloride. On adding 
ether the diazophenol was precipitated. 

* Bull. Soc. chim., 1867 [ii], 7, 270 ; F. P. 73286. 

t Meldola and Eynon, Trans., 1905, 87, 1 ; Castellana and d'Angelo, 
loc. cit. 

J Hepburn, J. Soc. Dyers, 1901, 17, 279. 
Morgan and Micklethwait, Trans., 1905, 87, 921. 
|| Hantzsch, Her., 1903, 36, 2056. 
IF Hantzsch and Vock, Ber., 1903, 36, 2059. 
** Ber., 1906, 39, 2713, 3H6. 
tt Ber., 1868, 1, 67. 


The direct interaction of fuming nitric acid and amines for 
the preparation of diazo-nitrates is hardly to be classed as 
a separate method, as fuming or brown nitric acid always 
contains nitrous fumes which diazotize the nitrate of the 
amine. It is interesting, however, in this connexion to note 
that Stenhouse * obtained the diazodinitrophenol of Griess by 
pouring boiling nitric acid on picramic acid, and several 
diazonitrotoluenesulphonic acids have been obtained in the 
dry state by diluting a solution of the corresponding toluidine- 
sulphonic acid in fuming nitric acid; nitration and diazo- 
tization both having been effected, f A more recent example 
of this is the preparation of 2-nitro-4-diazo-m-xylene-6-sul- 
phonic acid from m-xylidinesulphonic acid 

(CH 3 ) 2 :NH 2 :S0 3 H = 1:3:4:6. J 

A surprising reaction is the formation of the diazo-deriva- 
tive of aminophenolsulphonic acid by treating it with nitric 
acid and carbamide, for one would expect the nitrous acid 
present to be destroyed by the carbamide and thus prevent 
diazotization, but recent experiments || have shown that this 
destruction is very incomplete in the case of concentrated 
nitric acid. Various other nitrous derivatives have been used 
occasionally in the preparation of diazo-compounds ; it is even 
stated that nitric oxide can be substituted for nitrous fumes 
in the preparation of diazobenzene nitrate.^ 

Other substances which have been used are nitrosyl 
bromide** and chloride,ft and nitrosulphonic acid. It 

The use of barium nitrite instead of sodium nitrite has 
been suggested for preparing dry diazo-salts. By using the 
calculated quantity of sulphuric acid the whole of the mineral 

* Journ. Chem. Soc., 1868, 6, 150. 

t Limpricht, Ber., 1874, 7, 452. 

J Zincke, Annalen, 1905, 339, 202. 

Bennewitz, J. pr. Chem., 1874 [ii], 8, 50. 

|| Silberrad and Smart, J. Soc. Chem. Ind., 1906, 25, 156. 

IF Ladenburg, Ber., 1875, 8, 1212. 

** Koninck, Ber., 1869, 2, 122. 

tt Pabst and Girard, D. R-P. 6034 of 1878, and Ber., 1879, 12, 365 ; 
compare also Kastle and Keiser, Amer. Chem. J., 1895, 17, 91, who ob- 
tained a double salt, diazobenzene aniline chloride, C 6 H 5 N 2 C1, C 6 H 5 NH 8 C1, 
by treating aniline hydrochloride with nitrosyl chloride. 

J{ Pabst and Girard, loc. cit. Witt, Ber., 1903, 36, 4388. 


matter is precipitated, and, on filtering, the diazo-salt may be 
precipitated (with alcohol and ether) free from admixture with 
inorganic salts. 

It has also been found possible to obtain diazo-compounds 
without using an amine as the starting-point; thus dry 
diazobenzene nitrate has been prepared* by the action of 
nitrous fumes on mercury diphenyl, 

Hg(C 6 H 5 ) 2 + 2N 2 3 = HgC H 5 . N0 3 + C 6 H 5 . N 2 . NO 3 
and when mercury ^?-ditolyl is substituted for the diphenyl 
compound, ^-diazotoluene nitrate is formed. f By treating 
a solution of nitrosobenzene in chloroform with nitric oxide, 
diazobenzene nitrate was obtained by Bamberger, 

C 6 H 5 .NO + 2NO = C 6 H 5 .N 2 .N0 3 

and the same diazo-salt has been isolated by passing nitrous 
fumes into an ethereal solution of nitrosophenylhydrazine.t 
Finally, by the action of alcoholic hydrochloric acid on nitroso- 
anilidoacetic acid, the chloride of p-diazophenylhydroxylamine, 
OH.NH.C 6 H 4 . N 2 C1, is produced. 

3. Diazotization of amino-pheuols and -thioplienols. 

Quinonediazides (Diazophenols). The diazo-chlorides of 
o- and >-aminophenol are obtained by diazotizing the corre- 
sponding bases in alcoholic solution with amyl nitrite and 
hydrochloric acid at 0, and precipitating with ether. || The 
diazo-salts thus obtained are white. m-Diazophenol chloride 
is extremely unstable, and loses nitrogen even at 0. 

When the two former salts are dissolved in water and 
treated with potassium hydroxide or moist silver oxide, 
hydrochloric acid is split off, and the free diazophenols, or 
quinonediazides,^ are formed 

HO.C 6 H 4 . N 2 C1 = HC1 + O : C 6 

These quinonediazides are yellow and soluble in water. 

* Ber., 1897, 30, 509. t Kunz, Ber., 1898, 31, 1528. 

I Riigheimer, Ber., 1900, 33, 1718. 
0. Fischer, Ber., 1899, 32, 247. 

|| Schmitt, Ber., 1868, 1, 67. Hantzsch and Davidson, Ber., 1896, 29, 
1522. See also Cameron, Amer. Chem. J., 1898, 20, 229. 

II For constitution see p. 126. 


The substituted aminophenols are converted directly into 
the quinonediazides on diazotization. The compound derived 
from aminodinitrophenol (picramic acid) was the first diazo- 
compound obtained by Griess. 

Quinonediazides are also formed by allowing neutral solu- 
tions (or solutions containing no free mineral acid) of certain 
substituted diazo-salts to stand for some time ; thus 2:4: 6-tri- 
chlorodiazobenzene hydrogen sulphate or nitrate loses one 
atom of chlorine and becomes converted into 3:5-dichloro- 

0:C 6 H 2 C1 2 /| 


and many other halogen substituted anilines behave in the 

same way.* 

4. Thiodiazoles (Diazosulphides). When o-aminophenyl- 

NH 2 


is treated with nitrous acid an anhydride is obtained, as in 
the case of the aminophenols, but, unlike the quinonediazides, 
the o-diazosulphides are colourless, resembling in this respect 
the azimides obtained by the action of nitrous acid on the 
o-diamines. They generally crystallize well, have a charac- 
teristic sweetish odour, and are very feebly basic. Their 
constitution is probably represented as 

5. Preparation of diazo-salts in aqueous solution. From 
the foregoing it will have been seen that the preparation 
of solutions of diazo-salts is a comparatively simple matter, 
nevertheless there are many amines, mostly substituted, which 

* Orton, Proc. Roy. Soc., 1903, 71, 153 ; Trans., 1903, 83, 83, 796. 
t Jacobson, Annalen, 1893, 277, 209, 218, 232, 237. 


either resist the action of nitrous acid or, owing to the forma- 
tion of secondary products, are incapable of yielding diazo- 
salts. These will be dealt with later. 

In all cases the method of preparation on the large and the 
small scale is the same, so that a technical recipe may be 
exactly imitated in the laboratory and vice versa. 

The amine to be diazotized is usually dissolved in about 10 
parts of water with addition of one equivalent of hydrochloric 
acid, if necessary by the aid of heat. The solution is then 
cooled to 0-5 by the direct addition of ice and one and a half 
to two equivalents of hydrochloric acid added. (When the 
hydrochloride of the base is easily soluble in hydrochloric 
acid the whole of the acid may be used in dissolving the 
amine.) A solution of the calculated quantity of sodium 
nitrite is now added ; in most cases slowly until only a weak 
reaction is obtained with starch-iodide paper (this is best 
prepared from cadmium iodide and starch) after the solution 
has stood for 3-4 minutes. But in certain cases, especially 
where there is a great tendency towards the formation of 
diazoamino-derivatives, as in the case of a-naphthylamine and 
p-nitroaniline, the nitrite solution is added all at once, the 
precaution being taken of adding sufficient ice to prevent the 
temperature rising unduly. Occasionally the secondary reac- 
tion may be avoided by using a nitrite solution which has been 
previously acidified with hydrochloric acid or by using a 
larger excess of acid. The tendency towards the formation of 
diazoamino-compounds increases if organic acids are used, 
thus, for example, if two and a half equivalents of acetic acid 
are substituted for the same equivalent quantity of hydro- 
chloric acid, in the case of aniline only about 20 per cent, is 
converted into the diazo-salt; the diazotization is complete 
only by the use of eleven equivalents of acetic acid. The use 
of less hydrochloric acid has a similar effect ; aniline hydro- 
chloride is only partly diazotized by sodium nitrite, but the 
quantity converted increases with the concentration of the 
solution, thus in solutions containing respectively 10,1, and 
0-1 per cent, of aniline, about 30, 20, and 10 per cent, of the 
aniline is diazotized.* 

* Altschul, J. pr. Chem., 1896 [ii], 54, 508. 


One of the most frequently prepared diazo-compounds is 
that derived from >-nitroaniline ; indeed it is stated* that 
more than ] ,000 tons of >-nitroaniline are yearly converted 
into the diazo-compound for the purpose of producing ' para- 
nitraniline red ' by combination with /3-naphthol on the cotton 

A large number of methods of preparing this important 
diazo-compound have been published, f one of which (Cassella 
& Co.) is here quoted. 

jo-Nitroaniline (21 grams) is dissolved in water with addition 
of 42 c.c. of hydrochloric acid of 22 Bd, and the solution cooled 
to 5-10. The water and ice used weigh 307 grams. A solu- 
tion of 11-5 grams of technical sodium nitrite (95 per cent.) in 
103-5 grams of water is now added all at once and the mixture 
well stirred until a clear solution is obtained. If the resulting 
diazo-solutioii is to be used for combination with /3-naphthol 
it is first treated with a solution of 25 grams of sodium acetate 
dissolved in 50 grams of water. J 

Generally speaking, amines such as aniline, the toluidines, the 
xylidines, p-armnoacetanilide, are diazotized at 0-2. Others, 
as for example, a- and /2-naphthylamines, the nitroanilines, 
and diamines, such as benzidine, tolidine, dianisidine, are 
converted into the diazo-compounds more suitably at about 
10. Hydrochloric acid is most commonly employed, but 
sulphuric and acetic acids are also used. 

There are many cases where the diazotization of an amino- 
compound is not effected quite so easily as is described above, 
and special methods have to be employed. Thus many 
aminoazo-compounds are insoluble in water or acids and are 
attacked by nitrous acid only with difficulty. At the same 
time the diazo-compound is often insoluble in water. Such 
compounds are, for example, p-sulphobenzeneazo-a-naphthyl- 
amine and ^-acetylaminobenzeneazo-a-naphthylamine, and 
these are diazotized by using an excess of sodium nitrite and 

* Schwalbe, Zeitsch. Farb. Ind., 1905, 4, 433. 

f Schwalbe, loc. cit. 

1 Compare also Schwalbe, Zeitsch. Farb. Ind., 1905, 4, 433; Erban 
and Mebus, Chem. Zeit., 1907, 31, 663, 678, 687, 1011. 

For a detailed description of the preparation of a number of these 
see Cain and Thorpe, The Synthetic Dyestuffs, 1905, 226 et seq. 


stirring for several hours, keeping the mixture ice-cold in order 
to avoid escape of nitrous acid. In order to prevent this 
escape it has been proposed to diazotize under increased pres- 
sure.* The amine is introduced into a closed vessel together 
with the corresponding quantity of mineral acid, and the 
pressure is then raised by admitting compressed air or other 
indifferent gas, after which the nitrite solution is added. 

Difficulties have been met with in attempting the diazotiza- 
tion of substituted amines containing a number of acidic 
groups ; thus V. Meyer and Stiiber f found it impossible to 

decompose trinitroaniline 


by treatment with ethyl nitrite in alcoholic solution, and 
pentabromoaniline also resists diazotization J unless a large 
excess of sulphuric acid is employed. 

This method || is found to be advantageous in diazotizing 
derivatives of aniline containing several negative groups. 
The base is dissolved in sulphuric acid (monohydrate), the 
solution cooled to 10 to 15, and a very concentrated solu- 
tion of sodium nitrite added in excess during 1-1J hours, the 
liquid being well stirred. On diluting the solution any un- 
altered amine is often precipitated and can be removed by 
filtration. Fuming, 40 per cent., hydrochloric acid may some- 
times be used instead of sulphuric acid. 

This method has been successfully applied in diazotizing 

* Seidler, D. R-P. 143450. t Annalen, 1873, 165, 187. 

t Noelting, Bull. Soc. ind. Mulhouse, 1887, 57, 30. 

Hantzseh, Ber., 1900, 33, 520. 

f| Clans and Wallbaum, J. pr. Chem., 1897 [ii], 56, 48. 

11 Claus and Beysen, Annalen, 1891, 266, 224. 


Although, as Griess found, the ortho-aminophenols can be easily 
diazotized (see p. 11), when the corresponding compounds of the 
naphthalene series are similarly treated, difficulties often arise 
owing to the oxidizing action of the nitrous acid. This is 
especially applicable to the 1 : 2- and 2 : 1-amino-naphthols. 
In order, therefore, to obtain diazo-salts derived from these 
substances a number of methods have been employed with the 
object of avoiding this action. Thus the addition of copper or 
zinc salts to the solution of the amine or the use of the nitrites 
of zinc, nickel, mercury, &c., has been found efficacious.* For 
example, 12 kilos of l-amino-2-naphthol-4-sulphonic acid are 
mixed with 50 litres of water and ice and a solution of 1 kilo 
of copper sulphate added. A solution of 3-5 kilos of sodium 
nitrite is now slowly run in, and after diazotization is complete 
the solution is filtered and the diazo-compound precipitated 
with hydrochloric acid. This diazo-compound may be dried 
and powdered. f A second example is the following : J 48 
kilos of the above acid are mixed and well stirred with 
a solution of 33 kilos of zinc sulphate in 33 litres of water 
containing a little zinc hydroxide. The latter is formed by 
the addition of about 3 kilos of ammonia to the solution. 
A concentrated aqueous solution of 14 kilos of sodium nitrite 
is then added. The reaction is completed by warming for two 
hours at about 40, and the mass is then acidified with acetic 
acid. By filtration and crystallization, brilliant bronze needles 
of the diazo-compound are obtained. 

Another method consists in carrying out the diazotization 
of these amino-compounds in presence of excess of acetic or 
oxalic acid. The diazo-compounds derived from such amino- 
hydroxynaphthalenesulphonic acids are so stable that they 
can be sulphonated || and nitrated. 1f 

It is singular that the 2 : 3-aminonaphthols can be smoothly 
diazotized in the usual manner.** 

6. Other methods of preparing solutions of diazo-salts. 
In addition to the methods given in 1 and 2 which were 
used to obtain dry diazo-salts, many other ways of producing 

* E. P. 10235 of 1904. See also D.R-P. 171024, 172446. 

t E. P. 15025 of 1904. t F. P. 353786 of 1905. 

D. R-P. 155083, 175593. || D. R-P. 176618, 176620. 

D. R-P. 164665, 176619. ** E. P. 28107 of 1897. 


these compounds in solution have been used. For example, 
diazobenzene chloride can be obtained by the action of zinc 
dust and hydrochloric acid on a solution of aniline nitrate,* thus 
C 6 H 5 . NH 2 , HN0 3 + Zn + 3HC1 = C 6 H 5 . N 2 C1 + ZnCl 2 + 3H 2 O. 
It is obvious that the action is a reducing one, the nitric acid 
being converted into nitrous acid by the nascent hydrogen. 

A reaction similar to this is the production of diazo-salts 
from the nitrites of aromatic amines by treatment with 
a mineral acid.f 

Bamberger found J that when nitrosoacetanilide, 

C 6 H 5 .NAc.NO, 

was triturated with excess of 50 per cent, potassium hydroxide, 
the resulting solution showed the presence of a diazo-compound. 
(The nature of this, existing in an alkaline solution, will be 
explained later.) 

Certain nitroso-compounds, which contain the nitroso-group 
in the benzene nucleus may be directly converted into diazo- 
compounds by the action of three molecular proportions of 
nitrous acid, thus 

R.NO + 3HN0 2 = R.N 2 . N0 3 + HN0 3 + H 2 0. 

The method has been successfully applied to the preparation 
of the diazo-derivative of diphenylamine from the ^-nitroso- 

Quinoneoxime also, when treated with nitrogen trioxide in 
ethereal solution, yields the corresponding diazo-salt.H 

The formation of diazo-compounds by the interaction of 
nitrogen peroxide and quinonedioximes ** is of much interest 
from a theoretical point of view (see p. 163). When, for 
example, thymoquinonedioxime is treated with nitrogen 
peroxide a nitrosodiazo-derivative is obtained. 


N0 3 .N 

* Mohlau, D. R-P. 25146, Ber., 1883, 16, 3080. 
t Wallach, Annalen, 1907, 353, 322. 
t Ber., 1894, 27, 915 ; compare E. P. 13577 of 1894. 
$ 0. Fischer and Hepp, Annalen, 1888, 243, 282. 
|| Hantzsch, Ber., 1902, 35, 894. If Jaeger, Ber., 1875, 8, 894. 

** Oliveri-Tortorici, Gazzetta, 1900, 30, i. 526. 



Diazo-compounds are also obtained by the oxidation of 
phenylhydrazines with mercuric oxide * or acetate, f with 
nitrous acid in presence of a strong mineral acid,J and with 
bromine^ and also by the action of acidic chlorides on 
thionylphenylhydrazone. || 

An electrolytic process for the preparation of diazo-salts has 
been patented by Boehringer & Sons.^" As the method is 
carried out at temperatures of from 40 to 90, under which 
conditions the diazo-salt would be very quickly decomposed, 
the latter is immediately combined with a hydroxyl compound, 
such as )3-naphthol-3 : 6-disulphonic acid ('R salt'). A solution 
containing a mixture of aniline, sodium nitrite, and R salt is 
charged into a suitable cell at the platinum electrode whilst 
dilute sodium hydroxide surrounds the nickel cathode. On 
electrolyzing, the diazo-salt is formed and at once condenses 
with the R salt, with production of the azo-dyestuff. 

A somewhat indirect method of obtaining diazo-salts was 
observed by Lauth,** who found that certain azo-dyestuffs 
were split up into quinones and diazo-compounds by treatment 
with an oxidizing agent, such as lead peroxide and sulphuric 
acid. It has been found also ft that by the action of red 
fuming nitric acid on azo-dyestuffs, a reaction first studied by 
Meldola 5 JJ oxidation and nitration takes place, and diazo- 
compounds, together with nitro- derivatives of the second 
constituent of the dyestuff, are formed. 

7- The action of nitrons acid on aromatic substances 
containing more than one ammo-group. In investigating 
the action of nitrous acid on diamines or triamines of the 
aromatic series, one would expect each amino-group to become 
converted into the corresponding diazo-group. Although in 
most cases, perhaps, this is the primary action, yet very often 
some secondary reaction ensues with such rapidity that no 

* E. Fischer, Annalen, 1879, 199, 320. 

t Bamberger, Ber., 1899, 32, 1809. 

t Altschul, Chem., 1896 [ii], 54, 496. 

Michaelis, Ber., 1893, 26, 2190. 

If Annalen, 1892, 270, 116. 

IT D. R-P., 152926 of 1902 ; E.P. 2608 of 1904. 

** Bull. Soc. chim., 1891 [iii], 6, 94. 

tt O. Schmidt, Ber., 1905, 38, 3201. 

}} Proc. y 1894, 10, 118 ; Trans., 1889, 55, 608; 1894, 65, 841. 


diazo-salt can be isolated by the usual means, and special 
methods have to be employed. 

Very striking differences in behaviour are exhibited by the 
three phenylenediamines. When a dilute solution of sodium 
nitrite is added to a dilute solution of the sulphate of 
o-phenylenediamine, aziminobenzene is formed * according to 
the equation 

C 6 H 4 *S + HN0 2 = C.H/ \NH + 2H 2 0. 

Griess obtained this substance by acting on o-phenylene- 
diamine hydrochloride with >-diazobenzenesulphonic acid.f 
It has not yet been found possible to prepare the tetrazo- 
compound. In the case of o-tolylenediamine a similar reaction 
takes place, and it has been shown J that the amino-group 
which is in the meta-position relative to the methyl group is 
converted into the diazo-group before internal condensation, 
resulting in the formation of the azimino-compound, takes 

m-Phenylenediamine behaves very differently. When a 
solution of the hydrochloride is treated with sodium nitrite, 
the well-known dyestuff ' Bismarck Brown ' is obtained. This 
is the hydrochloride of bisbenzeneazophenylenediamine. If 
the nitrite is added suddenly, a certain amount of nitroso-m- 
phenylenediamine is formed. 

When the reaction, however, is carried out in a different 
manner, both amino-groups may be diazotized, forming a 
tetrazo- or bisdiazo-compound. Thus Griess || showed that 
the reaction could be successfully brought about by taking 
care that both the nitrite solution and hydrochloric acid are 
always in excess of the diamine. A two per cent, solution of 
m-phenylenediamine hydrochloride is prepared, and, on the 
other hand, a dilute solution of sodium nitrite of specific 
gravity 1.1. To the latter is added an equal volume of 
hydrochloric acid of specific gravity 1.15, and then the 
diamine solution added slowly, keeping the mixture well 

* Ladenburg, Ber., 1876, 9, 221. t Ber., 1882, 15, 2195. 

1 Noelting and Abt., Ber., 1887, 20, 2999. 

Tauber and Walder, Ber., 1900, 33, 2116. || Ber., 1886, 19, 317. 

c a 


stirred until the dark yellow solution of the tetrazo-compound 
is obtained. A later modification of this method is the 
following:* 80 c.c. of fuming hydrochloric acid are diluted 
with about 400 grams of ice and cooled with a freezing mix- 
ture. To this is added a solution of 15 grams of sodium 
nitrite in cold water, so that a strong solution of nitrous acid 
is obtained. To this solution is added quickly a cold solution 
of 9 grams of m-phenylenediamine hydrochloride to which 
10 c.c. of strong hydrochloric acid had been added. The 
mixture is well stirred during the operation, and a clear 
yellow solution of the tetrazo-compound is obtained. Other 
methods consist in adding the nitrite solution to a mixture of 
the diamine with a large excess of hydrochloric acid,f or in 
pouring a mixture of the diamine and nitrite into ice-cold 
dilute hydrochloric acid.J The dry tetrazo-chloride has also 
been prepared. 

The tetrazo-compound derived from m-tolylenediamine is 
prepared similarly. || 

Substituted m-phenylenediamines, containing the sub- 
stituent attached to that carbon atom which is in the ortho- 
position to both amino-groups are, as a rule, easily tetrazo- 
tized. Thus the m-tolylenediaminesulphonic acid of formula 

CH 3 

NH/\NH 2 

S0 3 H 

is tetrazotized without difficulty,^ as are also such diamino- 
hydroxy- compounds as those of the formulae 


NH/\NH 2 

C0 2 H 

* Tauber and Walder, Ber., 1897, 30, 2901. 

t E. P. 1593 of 1888. t Epstein, D. R-P. 103660 of 1899. 

Hantzsch and Borghaus, Ber., 1897, 30, 93. 

|| D. R-P. 103685 of 1899. 

f E. P. 17546 of 1892. ** E. P. 18624 of 1900, D. R-P., 168299. 


In order to diazotize only one amino-group in sulphonated 
m-diamines, the solution of the base is mixed with the calcu- 
lated quantity of alkali nitrite and then mineral acid added ; 
by this means the diamine is always in contact with the 
requisite quantity of free nitrous acid, and the diazotization 
proceeds smoothly.* 

Griess also studied f the action of nitrous acid on p-pheny- 
lenediamine, and stated that the principal product of the 
reaction when carried out in the usual way consisted of 
aminodiazobenzene chloride, one only of the amino-groups 
having been diazotized. It was found, however, that by this 
method a mixture of the diazo- and the tetrazo-compounds 
was obtained. { 

Later, Griess was successful in preparing the tetrazo-com- 
pound by using the same method as he had employed in the 
preparation of the m-tetrazobenzene chloride, and the dry 
tetrazo-sulphate has been obtained in small amount. || 

The use of a diazo- and tetrazo-compound derived from 
p-phenylenediamine has become of very great practical im- 
portance in the manufacture of azo-dyes, but as it is essential 
that a single compound and not a mixture of diazo- and 
tetrazo- should be prepared, and, further, that no large excess 
of nitrous acid should be present, these compounds are now 
prepared indirectly. 

For this purpose either p-nitroaniline or p-aminoacetanilide 
is used as the starting-point. If a compound involving the 
use of the diazo-chloride is required, the above substances 
are diazotized in the usual way,1J and, after coupling with the 
desired component, the nitro-group is reduced by sodium 
sulphide solution, or the acetyl group is removed by heating 
with sodium hydroxide. In each case, if X denotes the 
component, we obtain the compound 

* D. R-P. 152879. t Ber., 1884, 17, 697. 

I Nietzki, Ber., 1884, 17, 1350. Ber., 1886, 19, 317. 

|| Hantzsch and Borghaus, loc. cit. 

-luvu uc U1J.UO Lj.cbiiaiuiJ.ucu., niiiiu^eiJ. ucnig CVUAVCU. cvcu. 

the action of nitrous acid. (Hantzsch, Ber., 1902, 35, 896.) 


If the tetrazo-compound had been desired, this product is 
now diazotized in the usual way, and the diazo-compound 
coupled with a molecule of the same component X, or a diffe- 
rent one Y, giving us a dyestuff derived from the tetrazo- 
compound of p-phenylenediamine, of formula 


In the event of the component X containing an amino- 
group, and at the same time (as is usual) belonging to the 
naphthalene series, care is taken to use the calculated quantity 
of sodium nitrite (one molecule), when the NH 2 group united 
to the benzene ring is completely diazotized, leaving the other 
NH 2 group intact. This can generally be also diazotized by 
using a second molecule of nitrite. 

We have seen already (p. 15) that certain substituted 
amines present difficulties to the diazotizing process, some, in 
fact, being incapable of diazotization by the usual method. 
Similar examples occur amongst the substituted diamines: 
thus o-nitro-p-phenylenediamine cannot be directly converted 
into the tetrazo-compound, but only the diazo-compound is 
formed. Even an excess of nitrite fails to convert more than 
one amino-group into the diazo-group,* the constitution of 
the product being in all probability 


NH 2 <T >N C1 

The nitro-p-phenylenediamine is best diazotized by dis- 
solving the hydrochloride in water, adding an excess of acetic 
acid, and then excess of sodium nitrite at 5-10. It is very 
remarkable that if the diazo-compound is coupled with a 
component such as E. salt and an azo-dye formed, the remain- 
ing amino-group may now be easily converted into the 
diazo-group. f Other instances of this are known in the 
naphthalene series (see p. 24). Differences in the behaviour 
of two amino-groups in the substituted benzene molecule had 
indeed been detected by Griess, who found that p-diamino- 

* Billow, Ber., 1896, 29, 2285. t E. P. 6630 of 1892. 


benzole acid yielded p-aminodiazobenzoic acid and not the 

The diamines of the diphenyl series have attained very 
great importance owing to their use in the production of 
dyestufis which dye cotton without the aid of a mordant. 
The simplest of these is benzidine, 

which presents no difficulty in undergoing diazotization (con- 
trary to the statement of Kauflerf), the most suitable tem- 
perature being 8-10, and both amino-groups being easily 

It is also possible to obtain the monodiazo-compound by 
mixing solutions of benzidine hydrochloride and tetrazo- 
diphenyl chloride and allowing the mixture to remain for two 
or three days at 10-204 After filtering off the dark-coloured 
insoluble by-products, the solution contains principally 
aminodiazodiphenyl chloride 

X >N 2 C1 

Other diamines, such as tolidine, dianisidine, ethoxy benzi- 
dine, dichlorobenzidine, nitro- and dinitro-benzidine, diamino- 
stilbenedisulphonic acid, are tetrazotized in exactly the same 
manner as benzidine. 

Turning now to the naphthalene series the phenomena 
observed in the diamines of the benzene series are again 

Those diamines containing the amino-groups in the ortho 
or peri positions yield with nitrous acid azimino-compounds 
(2:3-naphthylenediamine) . Their sulphonic acids behave 
similarly. |[ 

The meta-diamines behave like the m-diamines of the 
benzene series, giving brown colouring matters. 

1 : 4-Naphthylenediamine is diazotized with still greater 
difficulty than ^9-phenylenediamine. Nitrous acid acts in this 
case also as an oxidizing agent, and 1 : 4-naphthaquinone is 

* Ber., 1884, 17, 603. t Annalen, 1907, 351, 151. 

t Tauber, Ber., 1894, 27, 2627. Ber., 1894, 27, 765. 

H E. P. 8645 of 1895. 


formed. In order to obtain the diazo- or tetrazo-derivative 
the same method is adopted as in the case of p-phenylene- 
diamine, namely, to convert one amino-group into the acetyl- 
amino-group and then to diazotize the remaining amino- 
group. * If this is then coupled with a suitable component, 
forming an azo-dyestuff, the acetyl group may be split off 
and the free amino-group now diazotized. 

The same procedure is used in preparing the diazo- or 
tetrazo-derivatives of the l:4-naphthylenediaminesulphonic 
acids, except in the case of the acid containing the S0 3 H group 
in the position 2 


3 H 

NH 2 

This acid exhibits a great tendency towards the formation 
of oxidation products when treated with sodium nitrite in 
the presence of mineral acids, but the diazotization proceeds 
smoothly when acetic or oxalic acids are used.f It is re- 
markable that only one amino-group is attacked, and it has 
been found impossible to prepare a tetrazo-derivative. This 
behaviour is to be attributed, perhaps, to the protective action 
of the sulphonic acid group, and consequently the diazo- 
compound probably possesses the constitution 

X being the organic acid radical. 

If now this diazo-compound is coupled with a phenol or 
naphthol, the resulting azo-dyestuff is easily diazotized. This 
behaviour is analogous to that exhibited by o-nitro-p-pheny- 
lenediamine (p. 22). 

It has been found also that if the monoacetyl derivative of 

* E. P. 18783 of 1891. 
t E. P. 2946 of 1896. 


this sulphonic acid is prepared, the formula of which is 



the free amino-group readily undergoes diazotization.* 

The remaining naphthylenediamines and their sulphonic 
acids are easily converted into the tetrazo-compounds.f 

The transference of triamines into the corresponding diazo- 
compounds cannot be illustrated by many examples, as cases of 
this are rare. The best known are probably those of rosaniline 
and para-rosaniline. These bases, containing, of course, three 
free amino-groups, were diazotized by Caro and Wanklyn J and 
E. and 0. Fischer, who thus prepared compounds containing 
three diazo-groups. 

The formation of diazoamino-compounds in this reaction 
has also been observed. || 

8. ' Solid diazo-componnds '. Mention has already been 
made of the great technical importance of diazotized p-nitro- 
aniline, owing to its use in the production of the ' para-red ' 
by combination with /?-naphthol on the cotton fibre. 

In order to enable the dyer to avoid the preparation of this 
and other diazo-compounds in the dyehouse, several processes 
have been adopted for the purpose of supplying the users 
with the diazo-compound ready made. 

Such preparations mostly consist of a paste of the diazo- 
compound in a very concentrated form, or of a sparingly 
soluble diazo-salt. A remarkable compound, produced by the 
action of alkalis on the diazo-chloride, which is very stable 

* E. P. 17064 of 1896. 

t E. P. 26020 of 1896; see also Lange, Chem. Zeit., 1888, 12, 856. 
Kaufler and Karres (Ber., 1907, 4O, 3263) could only diazotize one 
aminp-group of 2 : 7-naphthylenediamine, using amyl nitrite in alcoholic 
solution, but patents have been taken out formerly for diazo-dyestuffs 
from the tetrazo-compound. 

I Zeitsch.f. Chem., 1866, 511. Annalen, 1878, 194, 269. 

|| Pelet and Redard, Bull. Soc. chim., 1904 [iii], 31, 644. 


and which yields the diazo-chloride on acidifying, has also 
been put on the market. The nature of this substance is 
fully discussed on p. 96. 

It is prepared by treating the >-nitrodiazobenzene chloride 
or other diazo-salts containing nitro- or halogen-groups with 
caustic alkali at 60-70.* The diazo-salts prepared from 
aniline and its homologues are treated at 120 .f 

The substances formed may be dried or used as a paste ; by 
the action of a mineral acid the free diazo-chloride is regenerated. 

The diazo-compound of >-nitroaniline, after having been 
treated in this manner with alkali, is known as ' Nitrosamine 
red in paste '. Another way in which to obtain the diazo-salt 
in a more stable condition is to mix it with a solution of 
sodium a-naphthalenesulphonate, J sodium nitrobenzenesul- 
phonate, or sodium naphthalenedisulphonates, || and the 
tetrazo-salts of benzidine, &c., can be condensed with sodium 
2-naphthol-3 : 6 : 8-trisulphonate, or sodium 2-naphthol-l-sul- 
phonate, when additive compounds, and not azo-dyestuffs, are 
obtained. If Further, the zinc chloride double salts of diazo- 
tized aminoazo-compounds are also prepared.** All these 
stable compounds may be dried. 

A simpler method is to diazotize the nitroaniline in a very 
concentrated solution by passing nitrous acid gas through 
a solution of ^-nitroaniline in sulphuric acid, or even to 
evaporate the diazo-solution (prepared from sulphuric acid) 
in a vacuum at a temperature not exceeding 45 . Anhydrous 
sodium sulphate is now added, which, with the excess of sul- 
phuric acid, is converted into the bisulphate, and the paste, 
which soon solidifies, may be powdered, ft The substance 
obtained from diazotized ^-nitroaniline in this way is called 
'Azophor red P.N. ', 'Nitrazol C', Azogen red, and Benzo- 
nitrol; and that from diazotized dianisidine 'Azophor blue 
D '. (For instances of the elimination of groups during diazo- 
tization see p. 63.) 

* E. P. 20605 of 1893. f E. P. 3397 of 1894, 13460 of 1895. 

I E. P. 18429 of 1894. D. R-P. 88949 of 1894. 

|| D. R-P. 94280 of 1894. IT E. P. 8989 of 1895, 11757 of 1895. 

** E. P. 1645 of 1896 ; D. R-P. 89437 of 1896. 
tt E. P. 21227 of 1894 ; D. R-P. 85387 of 1894 ; E. P. 15353 of 1897. 


1. Thermochemistry. The formation of diazo-compounds 
proceeds with absorption of heat; the reaction is thus an 
endothermic one. 

The development of heat which is observed in the usual 
process of preparing these compounds is due to the formation 
of water and sodium chloride. 

The values which have been recorded for the heat of forma- 
tion of diazo-compounds are as follows : 

Diazobenzene nitrate . . . 474 calories.* 
Diazobenzene chloride . . . 44-0 
o-Diazotoluene chloride . . . 41*8 
p-Diazotoluene chloride . . . 42-3 J 

2. Explosibility of dry diazo-componnds. From the fact 
that the formation of diazo-compounds is accompanied by 
absorption of heat, it was to be expected that these substances 
would be unstable, and it is found that nearly all diazo-salts 
are very liable to explode when in the dry state; the most 
unstable in this respect being those containing nitro-groups. 
Thus diazobenzene nitrate is more explosive than the sulphate, 
and a case is on record where p-nitrodiazobenzene nitrate 
exploded violently when lightly touched with a platinum 

Great care must be taken, therefore, in handling these sub- 
stances, as they are extremely unreliable, and may never be 
regarded as safe. Diazobenzene chloride, usually looked on 
as comparatively stable, exploded on one occasion, apparently 

* Berthelot and Vielle, Compt. rend,, 1881, 92, 1076. 
t Vignon, Bull. Soc. chim., 1888 [ii], 49, 906. 
t Knoevenagel, Ber., 1890, 23, 2994. 
Bamberger, Ber., 1895, 28, 538. 


spontaneously, with very great violence ; * and a violent ex- 
plosion of dry diazobenzenesulphonic acid, which had been 
prepared some years previously, occurred in 1901. f An 
exactly similar accident befell the author of this book 
in 1896. 

In spite of the danger of working with such substances, 
determinations of the temperature at which diazo-compounds 
explode have been made. Thus dry m- and >-nitrodiazo- 
benzene chlorides explode at 118 and 85 respectively,! and 
diazobenzene nitrate explodes above 90. 

3. Velocity of diazotization. The rate at which amines 
are diazotized has been determined by Hantzsch and Schu- 
mann. || Diazotization, of course, proceeds with extreme 
rapidity under ordinary conditions, and the experiments were 
therefore conducted with ^V/1000 solutions. Using a colori- 
metric method for estimating the nitrous acid, it was found 
that, in presence of an excess of acid, the rate of diazotization 
of aniline, p-toluidine, m-xylidine, p-bromoaniline, andp-nitro- 
aniline is the same in each case. Further, if the temperature 
is raised, the rate is increased. 

The reaction which takes place is of the second order, and 
the values obtained for the velocity-constant 

C = x 

were 0-036 for aniline, 0-038 for ^-toluidine, 0-041 for m-xyli- 
dine, and 0-045 for p-bromoaniline in N/IOQO solution with one 
molecule of free acid at 0. 

Schumann then measured the velocity by observing the 
fall of electrical conductivity which takes place during 
diazotization. If He was able to confirm the previous experi- 
ments, and concluded that all aromatic amines are diazotized 
at approximately the same speed. 

* Hantzsch, Ber., 1897, 30, 2342, footnote. 

t Wichelhaus, Ber., 1901, 34, 11. 

\ Oddo, Gazzetta, 1895, 25, i. 327. 

Berthelot and Vielle, Compt. rend., 1881, 92, 1074. 

|J Ber., 1899,32, 1691. 

1 Ber., 1900, 33, 527. 


1. Action of water. When a diazo-salt is heated with 
water a phenol is formed * according to the equation 
X.N 2 .HS0 4 + H 2 = X.OH + H 2 S0 4 + N 2 , 
X denoting the aromatic nucleus. 

The reaction is best carried out in the presence of sulphuric 
acid. If the diazo-nitrate is used the nitric acid liberated 
attacks the phenol, forming nitrophenols. 

The ordinary method of carrying out the operation is to 
diazotize in presence of sulphuric acid and then to add, if neces- 
sary, a further quantity of sulphuric acid. The solution is 
then either directly boiled until no further evolution of nitrogen 
takes place, or steam may be passed into the solution, or the 
solution may be added slowly to boiling dilute sulphuric acid. 

In one or other of these ways most diazo-compounds yield 
the corresponding phenol, which is isolated by the usual 
means. For example, 4 : 4'-dihydroxydiphenyl is obtained 
in the following manner. 25 grams of benzidine are dis- 
solved by the aid of heat in 500 c.c. of water and 30 c.c. of 
concentrated hydrochloric acid. The solution is cooled to 5 
by adding ice, and then 18 grams of sodium nitrite dissolved 
in a small quantity of water are poured in slowly, the tempera- 
ture not being allowed to rise above 10 ; this is effected by 
adding more ice if necessary. 

100 grams of concentrated sulphuric acid are now added, 
and steam passed into the mixture until crystals of dihydroxy- 
diphenyl begin to separate out and the solution gives no 
further colour with an alkaline solution of R salt or /S-naphthol. 
On cooling, the precipitate is filtered, dissolved in dilute caustic 
soda, the solution filtered from any insoluble matter, and 
* Griess, Annalen, 1866, 137, 67. 


reprecipitated by hydrochloric acid. The dihydroxydiphenyl 
is recrystallized from dilute alcohol when it is obtained pure.* 

The process is carried out on the large scale in the manufacture 
of several naphthol- and dihydroxynaphthalene-sulphonic 
acids, and a classical example of this decomposition, as applied 
to diazo-compounds derived from triamines, is the production 
of aurin from para-rosaniline. f 

Owing to the very great reactivity of the diazo-salts and their 
well-known capacity of coupling or combining with phenols, 
it is obvious that there is a great tendency for secondary 
reactions to take place, interfering, to a certain extent, with 
the quantitative production of the hydroxy-compounds ; 
further, owing to the extreme differences in the relative 
stability of diazo-salts, other more obscure side reactions are 
liable to intervene, particularly in those cases where the 
decomposition can be completed only by long heating. 

Secondary reactions occur indeed even in the simplest case ; 
thus in the decomposition of diazobenzene sulphate a small 
quantity of hydroxydiphenyl is formed in consequence of the 
action of some of the undecomposed diazo-compound on 
phenol, J 
C 6 H 5 .N 2 .HS0 4 + C 6 H 6 .OH = C 6 H 6 .C 6 H 4 .OH + H 2 S0 4 + N 2 . 

In very many cases also, particularly in the naphthalene 
series, the solution becomes deeply coloured owing to the 
coupling of the diazo-compound with the naphthol formed, with 
production of the azo-dyestuff. Thus in the decomposition 
of diazo-a-naphthalene-4-sulphonic acid this reaction in variably 
occurs, even in the presence of sulphuric acid ; consequently 
a large excess of acid is usually taken in order to limit this 
formation of colouring matter as far as possible. 

There are many cases recorded in the literature where it 
has been found impossible to obtain even a trace of an 
hydroxy-compound by carrying out the decomposition in the 
manner described above. Most of these occur among extremely 
stable diazo-compounds, such as those derived from the halogen 
or nitro-substituted amines. || 

* Compare also Hirsch, Ber., 1889, 22, 335. 

t Annalen, 1878, 194, 301. J Hirsch, Ber., 1890, 23, 3705. 

Trans., 1903, 83, 221. || Amer. Chem. J., 1889, 11, 319. 


Recognizing that the cause of this might be due to an 
insufficiently high temperature, Heinichen* adopted the 
method of heating the strong diazo-solution with concentrated 
sulphuric acid, whereby the boiling-point becomes raised to 
150. In this way he obtained 2 : 6-dibromophenol from the 
corresponding diazo-salt after the usual method had failed. 

As, however, the stability of diazo-compounds has been 
shown to increase with addition of sulphuric acid,f this 
method is not always successful. 

A novel way of attacking the problem is that described by 
Kalle & Co. J The non-production of phenols in certain cases 
being evidently due, as already indicated, to condensation 
between the diazo-compound and the phenol formed, any 
process depending on the removal of the latter when set free 
would be expected to stand more chance of success. The 
method adopted by this firm is therefore to carry out the 
decomposition by dropping the diazo-solution into a mixture 
of dilute sulphuric acid and sodium sulphate heated to 135- 
145, and allowing any volatile products to distil over. In 
this way the temperature is kept high without using con- 
centrated sulphuric acid. 

By this means a good yield of guaiacol is obtained from the 
diazo-salt of o-anisidine,|| and the diazo-salts of s-tribromo- 
and s-trichloro-aniline, which under no other conditions could 
be made to yield phenols, gave a small yield of s-tribromo- 
phenol and s-trichlorophenol respect! vely.lT Another method 
of procedure is to decompose the diazo-compound at the 
moment of its formation by adding a solution of sodium 
nitrite to a boiling solution of the base in hydrochloric acid. 
In this way a good yield of >-nitro-o-cresol can be obtained 
from p-nitro-o-toluidine, but if the decomposition is carried 
out in the usual way internal condensation occurs, with 

* Annalen, 1889, 253, 281. t Ber., 1905, 38, 2511. 

t E. P. 7233 of 1897. 

An alternative method consists in adding the diazo-solution to 
a boiling 50 per cent, aqueous solution of copper sulphate. (D. R-P. 
167211, Soc. Chim. des Usines du Bhdne.) 

|| The diazo-sulphate of jp-anisidine yields quinol on heating with 
water to 140. (Salkowski, Ber., 1874, 7, 1008.) 

T Cain, Trans., 1906, 87, 19. 


formation of nitroindazole.* Certain derivatives of o-anisi- 
dine, the diazo-compounds of which have failed to yield the 
corresponding substituted guaiacol, would, in all probability, 
behave in the normal manner if one of the above methods 
were applied. f 

Various other apparent exceptions to the general rule have 
been described by Wroblewski,J who obtained the substituted 
hydrocarbons only, and not the phenols, from the diazo-salts 
derived from dibromoaniline, dibromo->-toluidine, and bromo- 
and chloro-^-toluidine. These diazo-salts have been recently 
examined, with the result that, in each case, the corresponding 
phenol was obtained. Wroblewski's results were probably 
due, as indeed he himself suggests, || to the presence of alcohol 
used in the preparation. The production of dibromophenetole 
by heating the diazo-compound of dibromo-o-phenetidine If is 
probably explained in the same way. 

A case which does not seem to accord with this explanation 
is that of ethyl diazogallate, which can be crystallized from 
water, and when heated with water in a sealed tube for four 
hours to 220 yields ethyl gallate, the nitrogen having been 
completely eliminated.** 

By treating aminoindazole with nitrous acid and warming 
the resulting diazo-compound with water, Bamberger ff pre- 
pared a new class of diazo-compounds, to which the name 
1 triazolens ' is given. He formulated the compound according 
to the equation 

NH 2 

Aminoindazole. Indazoletriazolen. 

but Hantzsch JJ regarded it as a diazide 

* Witt, Noelting, and Grandmougin, Ber., 1890, 23, 3635. 

t Meldola, Woolcott, and Wray (Trans., 1896, 69, 1327) obtained 
resins only by boiling the diazo-salts of ^>-bromo- and p-nitro-o-anisidine 
with water or dilute sulphuric acid. 

t Ber., 1874, 7, 1061. 

Cain, Trans., 1906, 89, 19. || Ber., 1884, 17, 2704. 

IT Mohlau and Oehmichen, Chem. t 1881 [ii], 24, 476. 

** Power and Sheddon, Trans., 1902, 81, 77. 

tt Ber., 1899, 32, 1773. JJ Ber., 1902, 35, 89. 


C 6 H 4 


In the diphenyl series certain exceptions to the general rule 
have been observed. On heating the tetrazo-salts prepared 
from dianisidine and 3 : 3'-dichlorobenzidine 




with dilute sulphuric acid, no phenol whatever was obtained,* 
the products being apparently of a quinonoid character. By 
using Heinichen's method a small amount of the dihydroxy- 
compound was obtained only in the latter case. An attempt 
to replace both the diazo-groups in ethoxytetrazodiphenyl 

N 2 .HS0 4 
C 2 H 5 

.HS0 4 

led to an interesting result.f It was found that the diazo- 
groups varied greatly in stability, and an intermediate product 
was isolated having the formula 

* Cain, Trans., 1903, 83, 688. 
t Cain, Trans., 1905, 87, 5. 



N 2 .HS0 4 

C 2 H 5 


2. Stability of diazo-solutions. Very great differences 
occur between the various diazo-salts with regard to their 
power of resisting decomposition by water. Many decompose 
rapidly at the ordinary temperature, whilst others remain 
apparently unchanged after prolonged boiling. 

Several cases of great stability are described by Griess; 
most of them occur among the halogen or nitro-substituted 
diazo-salts. Experiments of a somewhat qualitative character 
were performed by Oddo,* who diazotized a number of amines 
at various temperatures, and determined the quantity of the 
diazo-compound formed. He found that at 100-105 much 
diazo-compound is produced from m- and ^-nitroaniline, 
p-chloroaniline, l:3:4-dinitroaniline, and 1:2:5- and 1:3:6- 
nitrotoluidine, whilst little is obtained with m-chloro- and 
bromo-aniline, o-nitroaniline, ^-aminobenzoic acid, and 1:5:2- 
nitrotoluidine. At 80-85 much diazo-compound is obtained 
with the substances named above as giving little at 100-105, 
whilst small yields are furnished by aniline, ^-toluidine, and 
a- and /3-naphthylamine ; at 60-65 the four bases last named 
give good yields of diazo-compounds, whilst o-toluidine and 
>-xylidine give very poor ones. At 40-45 ^-xylidine in 
turn gives a good yield of diazo-compound. 

A considerably more exact method of determining the 
stability consists in titrating from time to time a portion 
of a diazo-solution with a fixed amount of sodium /3-naph- 
tholsulphonate (Schafier's salt) solution.f The increasing 
amount of diazo-solution required to combine with the whole 

* Gazzetta, 1895, 25, i. 327 ; 1896, 20, ii. 541. 
t Hirsch, Ber., 1891, 24, 324. 


of the naphthol solution is a measure of the advance of the 

The main results obtained in this way are given in the 
table on p. 37. 

This titration method has been used by some later investi- 
gators, * but it is not suitable for exact measurements, owing 
to the possibility of secondary reactions taking place between 
the diazo-compound and the alkali or sodium acetate, which 
must be added to the naphtholsulphonic acid in order to effect 
complete combination. (For an account of this secondary 
reaction see p. 96.) 

Hausser and Muller f introduced an entirely different 
method from the foregoing. They heated solutions of various 
diazo-compounds at fixed temperatures and measured the 
volume of nitrogen evolved. 

The decomposition belongs to the class of unimolecular 
processes, and is represented by the well-known expression 

n li A 

C= t lo ^X=l 

By comparing the values of the constant obtained the relative 
stability of the diazo-compounds may be determined. 

A constant value was obtained only in the case of the 
diazo-salts from sulphanilic acid and ^-toluidinesulphonic 
acid. In the case of the other amines examined the values 
for C were not constant, and from these results somewhat 
erroneous conclusions were drawn. Hantzsch, J using the 
same method, measured the rate of decomposition of the 
diazo-chlorides prepared from aniline, p-bromoaniline, p-tolu- 
idine, p-anisidine, and ^-cumidine, and showed that at 25 all 
these substances gave a constant value for 

1, A 


t & A-x 

It must be noted that the experiments of Hausser and 
Muller and of Hantzsch were carried out with solutions pre- 

* Buntrock, Leipziger Monatsschrift fur Textil-Industrie, 1898, 608 ; 
Schwalbe, Zeitsch. Farb.-Ind., 1905, 4, 433. 

t Butt. Soc. chim., 1892 [iii], 7, 721 ; 1893, 9, 353. Compt. rend., 1892, 
114, 549, 669, 760, 1438. 

J Ber., 1900, 33, 2517. 

D 2, 


pared by dissolving the dry diazo-salt in water. Recent 
investigations have shown* that solutions of diazo-salts 
prepared directly from the following amines aniline, the 
toluidines, sulphanilic acid, the nitroanilines, ^-aminoacet- 
anilide, 3 : 3'-dichlorobenzidine, a- and /2-naphthylamine, and 
a number of sulphonic acids derived from the two last, decom- 
pose in accordance with the above formula. This holds good 
at temperatures ranging from 20 to 100. The diazo-salt 
prepared from m-toluidine is the most unstable of those 
examined, the value of C at 20 being 0-00208. o-Toluidine 
comes next with a value of C of 0-00187; aniline gives a 
value for C of 0-00072. 

The diazo-salts of the nitroanilines are extremely stable, 
the ortho-compound being the most and the para- the least 
stable. The value of C for o-nitrodiazobenzene chloride is 
0-00555 at 100. Exceptions to the rule are shown by certain 
tetrazo-salts and those diazo-salts which are insoluble in 

The rate of decomposition of diazo-salts increases rapidly 
with the temperature, f the values of C obtained being in 
accordance with Arrhenius's formula for the temperature 
coefficient, namely 

The rate of decomposition (in the case of diazobenzene 
salts) is independent of the quantity of mineral acid present 
(except sulphuric acid, which tends to withdraw water from 
the sphere of action), and is independent of the nature of the 
acid. Equivalent solutions of diazobenzene chloride, bromide, 
sulphate, nitrate, and oxalate decompose at the same rate. J 

The presence of colloidal platinum or silver increases the 
rate of the decomposition, owing to catalytic action. Finally, 
it may be useful to append a table showing the relative 

-* Cain and Nicoll, Trans., 1902, 81, 1412; 1903, 83, 206. 

t Cain and Nicoll, Trans., 1903, 83, 470; Euler, Annalen, 1902, 325, 

I Cain, Ber., 1905, 38, 2511 ; Euler, loc. cit. 

Euler, Ofversigt af Kongl. Vetenskaps. Akad. Forhandl. Stockholm, 
1902, No. 2, 227. Compare also Schwalbe, Ber., 1905, 38, 2196, 3071 ; 
Cain, Ber., 1905, 38, 2511. 


stability of various diazo-salts as determined by various 
observers. The amine giving the most stable diazo-salt is 
at the top. 


| | 



C5 03 

S -2 

s 3 nl 




1. Action of alcohols. The action of alcohol on diazo- 
compounds was, of course, studied by Griess, who obtained 
benzene from diazobenzene salts, and dinitrophenol from 

The production of the hydrocarbon or complete elimination 
of the diazo-group by the action of boiling alcohol was, for 
many years, regarded as a general reaction, in spite of the 
observation of Wroblewski, * who found that the diazo-salt 
of chlorotoluidine gave, not the chloro-hydrocarbon, but 
the corresponding chlorophenetole. Four years later also 
Hayduckf showed that when o-toluidinesulphonic acid was 
diazotized, and the resulting diazo-salt boiled with alcohol, 
phenetolesulphonic acid was obtained. 

A striking application of the reaction was made by E. and 
O. Fischer in 1878, J who showed that when the diazo- 
compound of paraleucaniline was boiled with alcohol the three 
diazo-groups were eliminated, with formation of triphenyl- 
methane ; diazo-leucaniline, in the same way, gave tolyl- 

When the bisdiazo-derivative of benzidine is warmed with 

ethyl alcohol to 40-45 only one diazo-group is eliminated, 

the second requiring a higher temperature for its removal 

C1.N 2 .C 6 H 4 .C 6 H 4 .N 2 C1 -* C 6 H 5 .C 6 H 4 .N 2 C1 

-* C 6 H 5 . C 6 H 5 . 

Examples of the formation of ethers in this reaction were, 
however, rapidly multiplying, amongst which may be men- 
tioned the cases of m-aminobenzenedisulphonic acid, || cumi- 

* Ber., 1870, 3, 98. t Annalen, 1874, 172, 215. 

% Annalen, 1878, 194, 242. Ber., 1898, 31, 479. 

|| Zander, Annalen, 1879, 198, 1. 


dinesulphonic acid,* aminotetramethylbenzene, cumidine,f 
and o-toluidinedisulphonic acid, J all of which yielded, when 
diazotized and then treated with alcohol, the corresponding 
ethyl ethers. 

The reaction may thus proceed in two ways, according to 
the following equations : 

i. R.N 2 X + C 2 H 5 . OH = RH + C 2 H 4 + N 2 + HX 
ii. R.N 2 X + C 2 H 5 . OH = R.O.C 2 H 5 + N 2 + HX ; 
where R denotes a hydrocarbon radical and X an acid 
radical. A systematic investigation into the whole question 
was next undertaken by Remsen and his pupils, and it was 
very quickly demonstrated that the normal reaction is the 
formation of ethers in accordance with the second of the 
foregoing equations. || The course of the reaction is, how- 
ever, somewhat complicated, and depends on many factors, 
such as the position and nature of the substituents, the 
pressure at which the operation is carried on, &c. 

2. Influence of substituents. The presence of the acid 
radicals, C0 2 H, Cl, Br, N0 2 , &c., tends to induce the complete 
elimination of the diazo-group, and this influence is greatest 
when these radicals are in the ortho-position with respect to 
the diazo-group; their influence is less in the meta-position 
and least in the para-position. 

Thus of the chlorodiazobenzene nitrates, the ortho- and 
meta-compounds yield only chlorobenzene with ethyl alcohol, 
but the para-compound gives rise to the formation of a little 
>-chlorophenetole.^[ In the case of the diazobenzoic acids the 
ortho-compound gives benzoic acid only, whilst the meta- and 
para- yield the alkyloxy-derivatives. ** 

Another interesting example is that of the nitrodiazo- 
benzene salts. When heated with methyl alcohol the ortho- 

* Haller, Ber., 1884, 17, 1887. 
t Hofmann, Ber., 1884, 17, 1917. 
J Basse, Annalen, 1885, 230, 286. 
Per., 1885, 18, 65. 

|| Remsen and Palmer, Amer. Chem. J., 1886, 8, 243. 
1 Cameron, Amer. Chem. J., 1898, 20, 229. 

** Remsen and Orndorff, Amer. Chem. J., 1887, 9, 387. Compare also 
Griess, Ber., 1888, 21, 978. 


compound gives 87 per cent, of the theory of nitrobenzene ; 
from the meta-compound 51 per cent, is obtained, together 
with a little m-nitroanisole, whilst the para-derivative gives 
about 40 per cent, of nitrobenzene and 8 to 17 per cent, of 

In the naphthaline series the 1 : 2-, 2 : 1-, and 1 : 4-nitrodiazo- 
naphthalene sulphates all yield nitronaphthalene with ethyl 
alcohol,* whereas the ethoxy-derivative is obtained from both 
a- and )3-diazonaphthalene sulphates, f 

3. Influence of the alcohol used. The tendency towards 
the formation of hydrocarbons is increased as the molecular 
weight of the alcohol increases. Diazobenzene chloride and 
sulphate with methyl alcohol yield anisole as the sole product, 
no benzene being formed. J With ethyl alcohol the chief pro- 
duct is phenetole, but a little benzene is also obtained. o-Diazo- 
toluene sulphate with methyl alcohol yields tolyl methyl ether 
and only a trace of toluene. 

The diazo-salts of m-chloro- and m-bromo-aniline also yield 
only the corresponding halogen derivatives of benzene when 
heated with ethyl alcohol, but when methyl alcohol is used 
the chief product in each case is the halogenated anisole, only 
small quantities of chloro- and bromo-benzene being produced. 
The diazo-sulphates of ^-chloro- and p-bromo-aniline illustrate 
this point very clearly ; with ethyl alcohol no ethers are ob- 
tained, whilst with methyl alcohol the ethers are the sole 
products. || The higher alcohols behave in a similar manner 
to methyl and ethyl alcohols. With diazobenzene chloride 
7i- and ^so-propyl alcohol yield phenyl propyl ethers but no 
trace of propaldehyde or acetone; amyl alcohol gives both 
phenyl amyl ether and valeraldehyde or its condensation pro- 
ducts ; and benzyl alcohol gives benzaldehyde with only a 
little phenyl benzyl ether. Glycerol behaves like propyl 
alcohol, giving the monophenyl ether, whilst mannitol and 
benzoin are not attacked.^ 

* Orndorff and Cauffman, Amer. Chem. J., 1892, 14, 45. 
t Orndorff and Kortright, Amer. Chem. J., 1891. 13, 153. 
t Hantzsch and Jochem, Ber., 1901, 34, 3337. 

Bromwell, Amer. Chem. J., 1897, 19, 561. || Cameron, loc. cit. 

1 Hantzsch and Vock, Ber., 1903, 36, 2061. Compare also Orndorff 
and Hopkins, Amer. Chem. J., 1893, 15, 518. 


Phenol acts similarly, thus, when a solution of diazobenzene 
sulphate is warmed with phenol, diphenyl ether is obtained.* 
In alkaline solution, however, an azo-compound is produced 
(see p. 86). 

4. Influence of temperature and pressure. The influence 
of these factors in the decomposition is somewhat difficult to 
separate, as when the pressure is varied, the boiling-point of 
the solvent changes. 

In the case of the diazo-compound prepared from p-tolu- 
idine-o-sulphonic acid,f the decomposition proceeds slowly at 
the ordinary pressure. When this pressure is raised by 500 
m.m. an almost quantitative yield of the ethoxy-compound is 
obtained, but below this pressure the yield is diminished, as 
shown by the following numbers : 

Pressure in mm. 800 700 600 500 400 300 210 120 

69 ' 8 63 ' 2 57 ' 7 52 ' 8 48<7 434 40 ' 6 37 ' 2 

With methyl alcohol the methoxy-compound is obtained, and 
alteration of pressure has no influence on the course of the 

From a large number of cases, however, which have been 
examined, involving the use of both methyl and ethyl alcohol, 
it is found that the yield of alkyloxy-derivative increases with 
the pressure. 

5. Influence of other substances. If the decomposition 
is carried out with the addition of sodium ethoxide, sodium 
hydroxide, potassium carbonate, or zinc dust, a remarkable 
effect is produced. The alkyloxy-formation is almost entirely 
inhibited, and the reaction proceeds mainly with elimination 
of the diazo-group. Thus in the case of >-diazotoluene nitrate 
and sulphate the ordinary treatment with methyl alcohol 
results in the production of a good yield of the methoxy- 
derivative. When, however, sodium methoxide or any of the 

* Hofmeister, Annalen, 1871, 159, 191. 

t Remsen and Palmer, Amer. Chem. J., 1886, 8, 243; Eemsen and 
Dashiell, ibid., 1893, 15, 105. 

1 Parks, Amer. Chem. J., 1893, 15, 320. 

Shober, Amer. Chem. J., 1893, 15, 379 ; Metcalf, ibid., 301 ; Beeson, 
ibid., 1894, 16, 235 ; Shober and Kiefer, ibid., 1895, 17, 454 ; Chamber- 
lain, ibid., 1897, 19, 531. 


above substances are present no alkyloxy-compound is obtained, 
but the product consists mainly of toluene.* 

The rule holds good also for tetrazo-compounds of the 
diphenyl series; thus the tetrazo-chloride of o-ditolyl gives, 
with methyl alcohol, dimethoxy-o-ditolyl, and with ethyl 
alcohol a mixture of diethoxy-m-ditolyl and ?7i-ditolyl ; but in 
the presence of sodium methoxide, hydroxide, or zinc dust, no 
alkylated compound is formed. f 

6. Other methods of reduction. The reduction of diazo- 
salts to the corresponding hydrocarbon may, of course, be 
effected by reducing agents instead of alcohol : thus Baeyer 
and Pfitzinger J introduced the method of reducing the diazo- 
salt to the hydrazine with stannous chloride, and removing the 
group NH.NH 2 , by oxidation with boiling cupric sulphate 
solution, and by treating diazobenzene formate with stannous 
formate solution, benzene, together with a little diphenyl, &c., 
is produced. 

C 6 H 6 . N 2 C1 + SnCl 2 + H 2 = C 6 H 6 + N 2 + SnOCl 2 + HC1. 

The reduction is also effected by adding sodium stannite to 
a solution of a diazo-compound in sodium hydroxide, || 
C 6 H 6 . N 2 C1 + NaOH + Na 2 SnO 2 

= C 6 H 6 + N 2 + Na 2 Sn0 3 + NaCl, 

by the use of hypophosphorous acid,l an alkaline solution of 
sodium hyposulphite,** and also when diazides of sulphonic 
acids are boiled with copper powder and formic acid.ft 

* Beeson, loc. cit. ; Chamberlain, loc. cit. ; Griffin, Amer. Chem. J., 
1897, 19, 163 ; Moale, ibid., 1898, 20, 298. 

t Winston, ibid., 1904, 31, 119. J Ber., 1885, 18, 90, 786. 

Gasiorowski and Waijss, Ber., 1885, 18, 337; Culmann and Gasio- 
rowski, J. pr. Chem., 1889 [ii], 40, 97. 

|| Friedlander, Ber., 1889, 22, 587. Compare also Eibner, Ber., 1903, 
36, 813. 

IT Mai, Ber., 1902, 35, 162. ** Grandmougin, Ber., 1907, 40, 858. 

tt Ber., 1890, 23, 1632. 




1. Replacement of the diazo-gronp by the halogens. 

1. Chlorine. Although chloro- derivatives are obtained 
when a diazo-salt is heated with concentrated hydrochloric 
acid,* the yield is usually very poor,f and Griess observed that 
the replacement was more successful when the platinichloride 
of the diazo-compound was heated with sodium hydroxide, 

(CAN^PtCle = 2C 6 H 5 C1 + Pt + 2C1 2 + N 2 . 

A much more convenient method, however, was introduced 
by Sandmeyer in 1884.J 

In investigating the action of cuprous acetylide on diazo- 
benzene chloride, he noticed that chlorobenzene was produced, 
and showed that this was due to the cuprous chloride formed 
during the reaction. 

The replacement is carried out by adding the diazo-solution 
to a boiling 10 per cent, solution of cuprous chloride in 
hydrochloric acid. Nitrogen is evolved and the mass distilled 
with steam when chlorobenzene passes over. 

The cuprous chloride may be prepared by heating to boiling 
a mixture of copper sulphate (250 parts), sodium chloride 
(120 parts), and water (500 parts). Concentrated hydrochloric 
acid (1,000 parts) and copper turnings (130 parts) are no wadded, 
and the temperature maintained until the mixture loses its 
colour. The solution is decanted from any undissolved copper 
and the weight made up to 2,036 parts by the addition of 

* Griess, Ber., 1885, 18, 960. 

t When, however, a-diazoanthraquinone is treated with hydrochloric 
acid, the chloro-derivative is readily obtained, and the presence of 
cuprous salts is not essential (D. R-P. 131538). 

I Ber., 17, 1633, 2650; 1885, 18, 1492, 1496; 1890, 23, 1880; see also 
Ber., 1886, 19, 810; 1890, 23, 1628; Annalen, 1893, 272, 141. 


concentrated hydrochloric acid. A 10 per cent, solution of 
cuprous chloride is obtained, which is preserved in an atmo- 
sphere of carbon dioxide.* 

Gattermann then demonstrated f that the addition of very 
finely-divided copper to a solution of the diazo-chloride in 
hydrochloric acid effected the replacement at the ordinary tem- 
perature. Gattermann used copper precipitated from copper 
sulphate solution with zinc, but Ullmann has shown that the 
' copper bronze ' of commerce may be used equally effec- 
tively, t 

A modification of this method consists in using copper 
sulphate solution to which is added hydrochloric acid and 
sodium hypophosphite. 

The ' Sandmeyer ' reaction, as it is usually called, is con- 
sidered to be accompanied by the intermediate formation of 
a compound of the diazo-chloride with the cuprous chloride, 
and it is important in carrying out this operation that the 
possibility of the formation of phenols and azo-compounds 
should be avoided as far as possible. The production of 
a phenol is due to the decomposition of the diazo-compound 
before it has been converted into the cuprous chloride com- 
pound, or if it is added too slowly to the latter. 

According to Erdmann,|| the normal decomposition of the 
diazo-cuprous chloride compound takes place rapidly and 
smoothly only above a certain temperature, which is different 
for each compound ; these temperatures are about 0, 27, and 
30-40 in the case of the cuprous chloride derivatives of 
diazobenzene, o-diazotoluene, and ^-diazotoluene respectively. 
Below these points, the evolution of nitrogen takes place too 
slowly and is incomplete, part of the diazo-cuprous chloride 
compound being reduced to an azo-compound by the liberated 
cuprous chloride. It has been found that the quantity of cuprous 
chloride required may be reduced to 1/21 and 1/28 molecule 
per molecule of amine without appreciably reducing the yield 
of chlorobenzene and m-chloronitrobenzene respectively. If This 

* Feitler, Zeitsch. physikal. Chem., 1889, 4, 68. 

t Ber., 1890, 23, 1218; 1892, 25, 1091. 

J Per., 1896, 29, 1878. Angeli, Ber., 1891, 24, 952. 

|| Annalen, 1893, 272, 141. 

IT Votofcek, Chem. Zeit. Rep., 1896, 20, 70. 


is considered to be due to the diazobenzene chloride becoming 
first reduced to phenylhydrazine by the cuprous chloride, which 
then becomes cupric chloride ; the phenylhydrazine is then 
oxidized in presence of hydrochloric acid to chlorobenzene by 
the cupric chloride, and the cuprous chloride would then be 
re-formed to play the same part again. In confirmation of 
this explanation, it is found that phenylhydrazine is oxidized 
under the conditions named to chlorobenzene by both cupric 
and ferric chlorides, but no phenylhydrazine can be detected 
in the Sandmeyer reaction, owing possibly to its momentary 
existence. It is also worthy of note that a copper salt is not 
necessary in the preparation of iodobenzene by this method, 
and this may be due to the fact that hydriodic acid is itself 
a reducing agent.* 

Still another variation of Sandmeyer's method consists in 
electrolysing a solution of a diazo-compound to which cupric 
chloride has been added. A thick copper wire is used as 
the anode, and a cylinder of sheet copper as the cathode ; 
with a current density of 2-1 amperes per sq. dcm., and an 
E. M. F. of 10 volts, nitrogen was evolved, and a yield of 64 
per cent, of the theory of chlorobenzene was obtained. f 

In certain cases the reaction takes a different course from 
that already described ; thus Gattermann { found that two 
benzene nuclei could condense to form diphenyl derivatives, 
and the reaction has been extended by Ullmann, who has 
prepared a large number of diphenyl compounds by acting on 
nitrodiazo-compounds with cuprous chloride. 

2. Bromine. The diazo-group is replaced by bromine in the 
same manner as by chlorine. || In Griess's method a perbromide 
is obtained by adding hydrobromic acid and bromine water to 
the diazo-compound,! and this on being boiled with alcohol 
yields the bromo-derivative thus 

* Walter, Chem., 1896 [ii], 53, 427. 

t Votocek and Zenisek, Zeitsch. Elektrochem., 1899, 5, 485. 

t Ber., 1890, 23, 1226. 

Ber., 1901, 34, 3802 ; D. R-P. 126961. See also p. 61. 

II Phil. Trans., 1864, 154, 673 ; Annalen, 1866, 137, 49. 

IF If a diazophenol is used, a bromodiazophenol is formed ; cp. J. pr. 
Chem., 1881 [ii], 24, 449; Annalen, 1886, 234, 1; and it is remarkable 
that diazosulphanilic acid is indifferent to bromine (Armstrong, Proc., 
1899, 15, 176). 


C 6 H 5 . N 2 . N0 3 + HBr + Br 2 = C 6 H 5 . N 2 Br.Br 2 + HN0 3 
C 6 H 5 . N 2 Br.Br 2 + C 2 H 5 . OH 

= C 6 H 5 Br + N 2 + 2HBr + CH 3 . CHO. 

In addition to bromobenzene, p-bromophenetole is formed 
(cp. p. 38) ; when ether or glacial acetic acid is used instead 
of alcohol, bromobenzene alone is produced.* The platini- 
bromide of the diazo-bromide may also be treated in the same 
way as the platinichloride. 

In Sandmeyer's reaction cuprous bromide is substituted for 
cuprous chloride. 

In order to prepare /3-bromonaphthalene, Oddo f modified 
Gattermann's process as follows : 14-3 grams of /?-naphthyl- 
amine are diazotized and added to a mixture of 36 grams 
of potassium bromide with 100 grams of water and 30 
grams of moist copper powder previously heated to 50-70. 
The whole is heated in a reflux apparatus for 15 minutes, and 
then distilled in steam. A yield of 46-48 per cent, of the 
theory is obtained. 

3. Iodine. lodo- derivatives are easily prepared from the 
diazo-compounds by treating the latter with hydriodic acid. 
A solution of a little more than the theoretical quantity of 
sodium or potassium iodide is added to the solution of the 
diazo-chloride or sulphate. After standing and warming 
until the evolution of nitrogen has ceased, the liquid is 
usually made alkaline and the iodo-compound, if it is volatile, 
distilled with steam. In other cases it may be filtered off. 

4. Fluorine. The diazo-group may be replaced by fluorine 
by treating the diazo-salt with a solution of hydrogen fluoride 
in water.]: 

The substitution has also been effected from diazoamino- 
compounds. Thus, if diazoaminobenzene is added to fuming 
hydrofluoric acid, fluorobenzene is produced, and on mixing 
diazobenzene piperidide (from diazobenzene nitrate and piper- 

* Saunders, Amer. Chem. J., 1891, 13, 486. 

t Gazzetta, 1890, 2O, 631. 

t Ber., 1879, 12, 581 ; 1889, 22, 1846. 

Schmitt and Gehreu, J. pr. Chem., 1870 pi], 1, 395. 


idine *) with concentrated hydrofluoric acid, fluorobenzene is 
formed, thus 

C 6 H 5 . N 2 . NC 5 H 10 + 2HF = C 6 H 5 F + N 2 + NHC 5 H 10 , HF. 

The Sandmeyer reaction is considered by Hantzsch and 
Blagden f to be a somewhat complicated one, the final result 
being due to the simultaneous effect of three concurrent 
actions, namely, (1) the formation of a labile (diazonium, 
see p. 133) cuprous double salt, which then decomposes in 
such a way that the radical originally attached to the copper 
migrates to the aromatic nucleus ; (2) a catalytic action, which 
is the main action when copper powder is used, whereby 
nitrogen is eliminated from the diazo-salt, and the acid 
radical becomes united with the aromatic nucleus ; (3) the 
formation of azo-compounds, the cuprous being oxidized to 
a cupric salt. 

The first two reactions proceed when >-bromodiazobenzene 
bromide is subjected to the action of cuprous chloride dis- 
solved in methyl sulphide. The product consists chiefly of 
chloro-p-bromobenzene mixed with a little ^-dibromobenzene. 
i. 2C 6 H 4 Br.N 2 Br + Cu 2 Cl 2 = Cu 2 Br 2 + 2N 2 + 2C 6 H 4 ClBr. 

ii. C 6 H 4 Br.N 2 Br=N 2 + C 6 H 4 Br 2 . 

When cuprous bromide is allowed to react with >-bromo- 
diazobenzene chloride, p-dibromobenzene and a little chloro- 
_p-bromobenzene are produced. In both examples the first 
reaction is the chief one, and, under certain conditions, is the 
only one. Thus cuprous iodide furnishes iodo-derivatives 
only, with various diazo- chlorides and bromides, and cuprous 
chloride and diazobenzene iodide yield chlorobenzene, no iodo- 
benzene being produced. 

The third reaction, namely, the formation of azo-compounds, 
occurs when cuprous chloride, dissolved in hydrochloric acid, 
is added to the cold solution of the diazo-salt. Under these 
conditions, aniline, o-chloroaniline, and the o- and ^9-toluidines 
yield considerable quantities of azo-compound, but the nitro- 
amines give diphenyl derivatives (see p. 61). 

* Baeyer and Jaeger, Ber n 1875, 8, 893. 
t Ber., 1900, 33, 2544. 




1. Replacement of the diazo-gronp by cyanogen. This 
is one of the most important of the diazo-decompositions, as it 
serves to introduce an additional carbon atom into the mole- 
cule ; moreover, the nitriles formed in this way mostly yield 
the corresponding carboxylic acids.* 

The preparation of ^-toluonitrile is carried out as follows : 
50 grams of copper sulphate are dissolved in 200 c.c. of water 
by heating on the water-bath, and a solution of 55 grams of 
potassium cyanide in 100 c.c. of water added gradually with 
continuous heating. Care must be taken to perform the 
operation under a hood as cyanogen is evolved. To this hot 
solution is now added during about ten minutes a diazo-solution 
prepared from 20 grams of ^9-toluidine, 50 grams of concen- 
trated hydrochloric acid and 16 grams of sodium nitrite. The 
whole is now heated on the water-bath for a quarter of an 
hour and the toluonitrile distilled over with the steam. Here 
again care must be taken to get rid of the vapours as hydrogen 
cyanide is evolved. The nitrile distils as a yellow oil, which is 
purified by distillation, f 

By treating a solution of diazobenzene chloride with potas- 
sium cyanide in the cold, a double compound of the diazo- 
cyanide and hydrogen cyanide, C 6 H 5 .N 2 .CN, HCN, is formed.:]: 

The replacement is also effected by adding copper powder to 
a mixture of the diazo-salt and potassium cyanide, exactly as 
in the case of the preparation of the chloride. 

* Sometimes, however, these are formed with difficulty, owing probably 
to steric hindrance. Hofmann, Ber., 1884, 17, 1914 ; Kiister and Stall- 
berg, Annalen, 1894, 278, 207 ; Cain, Ber., 1895, 28, 967. 

t Gattermann, Practical Methods of Organic Chemistry. 

| Gabriel, Ber., 1879, 12, 1637. 

^ Gattermann, Hausknecht, Cantzler, and Ehrhardt, Ber., 890, 23, 


2. Replacement of the diazo-gronp by the cyano-gronp. 

This is effected by adding potassium cyanate to a diazo- 
sulphate and treating the mixture with copper powder, when 
the corresponding carbimide is obtained, thus 

C 6 H 5 . N 2 . HS0 4 + KCNO = C 6 H 5 . N : CO + N 2 + KHS0 4 . 

The potassium cyanate is prepared in the following manner : 
100 grams of finely-powdered and sieved potassium ferro- 
cyanide are mixed with 75 grams of powdered potassium 
dichromate, each ingredient being first thoroughly dried. 
This mixture is added, in portions of 3-5 grams at a time, to 
an iron dish heated over a three-flame burner. The mass 
becomes black and is well stirred but should not be heated 
to the melting-point. On cooling it is extracted with five times 
its volume of 80 per cent, alcohol, and the cold solution stirred, 
when a white crystal powder of potassium cyanate separates, 
which is filtered and washed with small quantities of ether. 

For the decomposition, 10 grams of aniline are dissolved in 
100 grams of water and 20 grams of concentrated sulphuric 
acid, the solution cooled with ice, and diazotized with 7*5 
grams of sodium nitrite. To the diazo-solution is added a 
concentrated aqueous solution of 9 grams of potassium cyanate 
and then 5 grams of copper powder, when evolution of nitrogen 
begins. A second 5 grams of copper powder is added and an 
oily layer of phenylcarbimide separates on the top of the 
liquid. This is skimmed off with a glass spoon, extracted 
with chloroform, and the chloroform solution filtered by the 
aid of the pump. More copper powder is added to the original 
solution until no more nitrogen is evolved and any further 
quantity of phenylcarbimide collected in the same way. The 
chloroform solutions are now separated from water, dried and 
freed from chloroform by evaporation. The residual oil, on 
distillation, yields pure phenylcarbimide.^ 

3. Replacement of the diazo-gronp by the thiocyano- 
gronp. This reaction is carried out by the aid of copper 
thiocyanate. For example : 31 grams of aniline are dissolved 
in 100 grams of concentrated sulphuric acid and 200 grams of 
water, and diazotized with 23 grams of sodium nitrite. To this 

* Ber. t 1890, 23, 1220 ; ibid., 1892, 25, 1086. 


solution is added a concentrated solution of 35 grams of potas- 
sium thiocyanate, and a paste of copper thiocyanate, obtained 
by dissolving 80 grams of copper sulphate and 150 grams of 
ferrous sulphate in water, precipitating with 35 grams of 
potassium thiocyanate and filtering. Nitrogen is evolved 
when this paste is added to the diazo-solution, and the reaction 
is complete after the whole has stood for three hours; the 
phenylthiocarbimide is then distilled with steam and rectified,* 

4. Replacement of the diazo-group by the group SH. 

When a diazo-sulphonate is warmed with an alcoholic solution 
of potassium sulphide nitrogen is evolved, and a thiophenol- 
sulphonic acid is formed, f Thus if the diazo-derivative of 
sulphanilic acid is used potassium >-thiophenolsulphonic acid 

N, SK 

C 6 H| + K 2 S = C 6 H 4 < +N 2 

X S0 3 X S0 3 K 

These thiophenols or mercaptans are also obtained by 
hydrolysing the xanthates produced by treating a diazo-salt 
with potassium xanthate.J 

By hydrolysing the xanthate from diazotized sulphanilic acid, 
in addition to the mercaptan, there is formed the ethosulphide 

S0 3 K.C 6 H 4 . S.CS.OEt -> S0 3 K.C 6 H 4 .SH 
and S0 3 K.C 6 H 4 .SEt. 

5- Replacement of the diazo-group by sulphur. When 
hydrogen sulphide or ammonium sulphide acts on a solution 
of diazobenzene chloride or sulphate, the diazo-group is re- 
placed by sulphur, and phenyl sulphide (C 6 H 6 ) 2 S is produced ;|| 
and if a solution of o-diazobenzoic acid sulphate is added to a 
cold saturated solution of sulphur dioxide in which copper 
powder is suspended, nitrogen is evolved, much copper passes 
into solution, part of the sulphur dioxide being oxidized to 
sulphuric acid, and the chief product is dithiosalicylic acid, 

* Per., 1890, 23, 738; compare Ibid., 770. 

t Klason, Ber., 1887, 20, 349. 

1 Leuchart, J. pr. Chem., 1890 [ii], 41, 179. 

| Walter, Proc., 1895, 11, 141. 

|| Graebe and Mann, Ber., 1882, 15, 1683. 


(C 6 H 4 . C0 2 H) 2 S, which is obtained in a yield of about 50 per 
cent, of the theoretical.* 

Another way in which sulphides are formed is by treating 
diazobenzene chloride with a colourless solution of copper 
sulphate (1 mol.) in sodium thiosulphate (6 mols.), that is, 
cuprous sodium thiosulphate. Phenyl sulphide is formed 
along with benzeneazodiphenyl. 

Sulphanilic acid and o- and ^-toluidine yield also the corre- 
sponding sulphides, but no diphenyl derivatives are produced. 
When a-naphthylamine is similarly treated there is no forma- 
tion of sulphide, but a-azonaphthalene is obtained, f 

6. Replacement of the diazo-group by the sulphonic acid 
group. Both the thiophenols and the disulphides yield the 
corresponding sulphonic acids on treatment with alkaline 
permanganate solution. { 

7- Replacement of the diazo-group by the uitro-group. 

This is brought about by treating a diazo-salt with nitrous 
acid and cuprous oxide. The amine is dissolved in two mole- 
cules of dilute nitric acid (hydrochloric acid is to be avoided), 
and, after being diazotized, a second molecule of sodium nitrite 
is added. The solution is then poured on finely-divided 
cuprous oxide and the reaction usually proceeds in the cold. 

For example, the cuprous oxide is prepared by dissolving 
together 50 grams of copper sulphate and 15 grams of grape 
sugar in 100 grams of water. The solution is boiled and 20 
grams of caustic soda, dissolved in 60 grams of water, added 
all at once. The mixture is neutralized with acetic acid. 

On the other hand, 9 grams of aniline are dissolved in 50 
grams of water and 20 grams of concentrated nitric acid 
(sp. gr. 1.4) ; 15 grams of sodium nitrite, dissolved in 50 grams 
of water, are added and then the diazo-solution is poured on 
the cuprous oxide gradually. When the reaction is finished 
the nitrobenzene is extracted by distillation with steam. 

The replacement also proceeds to a small extent in the 

* Henderson, Amer. Chem. /., 1899, 21, 206. 
t Bernstein, Eer., 1901, 34, 3968. 

F. Bayer & Co., D. R-P. 70286 of 1892; E. P. 11865 of 1892. 

JBer., 1887, 20, 1495. 

E 2 


absence of cuprous oxide ; thus when 2:4: 6-tribromodiazo- 
benzene sulphate is treated with 20 molecular proportions 
of potassium nitrite, the corresponding 2:4: 6-tribromo-l- 
nitrobenzene is formed, together with the quinonediazide 

Nitrobenzene is also formed when diazobenzene perbromide 
is shaken with aqueous sodium hydroxide in the cold.f 

The diazo-group of diazobenzene nitrate may also be re- 
placed by the nitro-group by making use of the crystalline 
double salt of formula Hg(N0 2 ) 2> 2C 6 H 5 .N 2 .N0 3 , which is 
obtained by mixing solutions of diazobenzene nitrate and 
potassium mercuric nitrate. When this salt is boiled with 
water, it yields phenol and nitrophenol, but when treated 
with copper powder, a quantitative yield of nitrobenzene is 

Another method consists in mixing diazo-sulphates with 
a freshly prepared suspension of cupro-cupric sulphite and 
treating the mixture with excess of an alkali nitrite. By 
this means 2:4: 6-tribromodiazobenzene sulphate gives a 65 
per cent, yield of 2 : 4 : 6-tribromo-l-nitrobenzene, and /?-diazo- 
naphthalene sulphate furnishes a 25 per cent, yield of /3-nitro- 

8. Replacement of the diazo-gronp by the nitroso-gronp. 

This is effected by treating a diazobenzene chloride solution 
with an alkaline solution of potassium ferrocyanide.|[ 

9. Replacement of the diazo-gronp by the ammo-group. 
This replacement is effected by adding hydroxylamine to 

* Orton, Trans., 1903, 83, 806. 

t Bamberger, Ber., 1894, 27, 1273. 

t Hantzsch and Blagden, Ber., 1900, 33, 2544. 

Hantzsch and Blagden, loc. cit. 

|| Bamberger and Storch, Ber., 1893, 26, 471. 


a solution of a diazo-salt ; * thus, aniline may be obtained 
from diazobenzene chloride, and^>-toluidine from its diazo-salt. 
An interesting example occurs in the anthracene series. When 
the anhydride of l-diazoanthraquinone-2-sulphonic acid 


is suspended in water and treated with ammonia or ammonium 
carbonate, nitrogen is evolved, and the original aminosulphonic 
acid is obtained. Further, when this diazo-compound is 
treated with hydroxylamine, a diazohydroxyamide 

OH . NH . N 2 . C 10 H 6 2 . S0 3 Na 

is formed, which is transformed by concentrated sulphuric acid 
into l-amino-4-hydroxyanthraquinone-2-sulphonic acid. 

A similar reaction takes place when hydrazine is substituted 
for hydroxylamine ; in this way, both the amino- and hydroxyl- 
groups are introduced into the molecule when the diazo-group 
is eliminated.! 

10. Replacement of the diazo-group by the acetoxy- 
group. Meldola and EastJ found that when certain azo- 
derivatives of jS-naphthylamine, containing an ammo-group, 
are diazotized in warm glacial acetic acid, the diazo-group is 
replaced by the acetoxy-group, and Orndorff has shown that 
this reaction may be applied generally for the preparation of 
aromatic acetates. 

The acetoxy-group may be readily converted into the 
hydroxy-group by hydrolysis, so that this method is useful 
in effecting the replacement of the diazo- by the hydroxy- 
group in such cases where the normal decomposition with 
water does not take place. 

* Mai, Per., 1892, 25, 372. t Wacker, Ber., 1902, 35, 2593, 3920. 

J Trans., 1888, 53, 460. Amer. Chem. J., 1888, 10, 3b8. 



1. Sulphur dioxide. When diazobenzene chloride is 
treated with sulphur dioxide in aqueous solution in the cold, 
reduction takes place with formation of a hydrazine, and at 
the same time a second reaction proceeds by which the nitro- 
gen is eliminated and the sulphonic acid group takes its place. 
These two products condense together in the nascent state 
and a sulphazide is formed* 

2C 6 H 5 . N 2 C1 + 3S0 2 + 4H 2 

= C 6 H 5 . NH.NH.S0 2 . C ft H 6 + N 2 + 2H 2 S0 4 + 2HC1. 

These compounds are also formed by dissolving the amine 
in 95 per cent, alcohol, saturating the solution with sulphur 
dioxide, and adding a concentrated aqueous solution of potas- 
sium nitrite, f 

A differently constituted product results when the neutral 
diazo-compound, prepared from ^-nitroaniline (p-nitrodiazo- 
benzene hydroxide, NO 2 . C 6 H 4 . N 2 . OH), is dissolved in abso- 
lute alcohol and subjected to the action of dry sulphur dioxide 
at 0-5. _p-Nitrobenzenediazo->-nitrobenzenesulphone 

N0 2 . C e H 4 . N 2 . S0 2 . C 6 H 4 . N0 2 
is formed.]: 

These sulphones are also obtained by treating a diazo-salt 
with benzenesulphinic acid. 

Condensation products in which two benzene nuclei exist 
are also obtained when a diazo-salt is subjected to the action 
of sulphur dioxide in presence of a not too large excess of 

* Koenigs, Ber., 1877, 10, 1531. t Ulatowski, Ber., 1887, 20, 1238. 

t Ekbom, Ber., 1902, 35, 656. 

| Hantzsch and Singer, Ber., 1897, 30, 312. 


sulphuric acid.* Diazobenzene chloride, under these conditions, 
yields a compound of formula 

C H 6 . N : N.C 6 H 4 . NH.NH.S0 3 H, 

and m-diazotoluene chloride a compound of analogous con- 
stitution. The reaction takes a different course when sulphites 
are employed. With neutral alkali sulphites the correspond- 
ing diazo-salts are obtained 

C 6 H 6 . N 2 C1 + K 2 S0 3 = C 6 H 6 . N 2 . S0 3 K + KCl.f 
Acid sulphites furnish hydrazinesulphonic acids of formula 
C 6 H 5 . NH.NH.SOgX.J Also when a solution of sodium hypo- 
sulphite is allowed to react with diazobenzene sulphate or 
chloride, the chief product is sodium phenylhydrazine-)3- 
sulphonate. There are also formed small quantities of 
diazobenzeneimide and benzenesulphonphenylhydrazine. In 
alkaline solution, the diazo-group is replaced by hydrogen 
(see p. 42). 

2. Replacement of the diazo-group by the sulphinic 
acid group. The formation of sulphinic acids by the direct 
action of sulphurous acid on diazo-salts was first observed by 
Miiller and Wiesinger,|| but the replacement is best carried 
out by Gattermann's method, using copper powder. If A solu- 
tion of the diazo-sulphate, containing an excess of sulphuric 
acid, is saturated with sulphur dioxide, the solution being 
kept cold. Each 100 c.c. of the solution should absorb about 
15 grams of the gas. Copper powder is now added gradually 
to the solution (which should be clear), ice being added to 
keep the solution cold during the operation. The addition of 
copper is continued with vigorous stirring until no more 
nitrogen is evolved. As some sulphur dioxide is carried off 

* 15 grams of aniline and 50 grams of concentrated sulphuric acid. 
Troger, Hille,and Vesterling, J. pr. Chem., 1905 [iij, 72, 511; Troger 
and Schaub, Arch. Pharm., 1906, 244, 302 ; Troger and Franke, ibid., 
307; Troger, Wamecke, and Schaub, ibid., 312; Troger, Berlin, and 
Franke, ibid., 326. 

t Griess, Ber., 1876, 9, 1653. 

j Schmitt and Glutz, Ber., 1869, 2, 51 ; Strecker and Romer, Ber., 
1871, 4, 784; E. Fischer, Ber., 1875, 8, 589. 

Grandmougin, Ber., 1907, 40, 422. || Ber., 1879, 12, 1348. 

IT Ber., 1899, 32, 1136. 


with the nitrogen, a further quantity is passed through the 
mixture during the reaction. The sulphinic acid is extracted 
from the product by means of ether. In the case of the 
diazotized naphthylamines it is better to add the diazo- 
solution to a mixture of copper powder and a saturated 
solution of sulphurous acid. 

3. Hydrogen sulphide. When hydrogen sulphide is 
passed through an aqueous, nearly neutral, solution of >-nitro- 
diazobenzene chloride at 0, the diazo-sulphide 

is produced. 

In hydrochloric acid solution, the mercaptan hydrosulphide, 
N0 2 . C 6 H 4 . N 2 SH.H 2 S, is first formed, and on prolonging 
the passage of the gas, the disulphide, (NO 2 . C 6 H 4 .N 2 ) 2 S 2 , 

On warming diazotized sulphanilic acid with alcoholic 
potassium sulphide, the diazo-nitrogen is expelled and the 
dipotassium salt of _p-thiophenolsulphonic acid is produced 

and mercaptan combines with diazo-salts to form an inter- 
mediate compound which loses nitrogen on warming 

C eH< S N 2 >0 _ C 6 H 4 <ggCA _ C 6 H /|H 5 

Similarly phenyl mercaptan forms corresponding thiophenol 
ethers. J 

4. Replacement of the diazo-gronp by the azoimino-gronp. 

1. Action of ammonia. Griess examined the action of 
concentrated aqueous ammonia on diazobenzene nitrate, and 
obtained an extremely unstable compound which decomposed 
into phenol, aniline, and nitrogen. 

This substance was shown by von Pechmann \\ to consist of 
bisdiazobenzeneamide, the reaction proceeding as follows 

2C 6 H 5 . N 2 C1 + 3NH 3 = C 6 H 5 . N 2 . NH.N 2 . C 6 H 5 + 2NH 4 C1. 

* Bamberger and Kraus, Ber., 1896, 29, 272. 

t Ber., 1887, 20, 350. J Hantzsch and Freese, Ber., 1895, 28, 3237. 

Annalen, 1866, 137, 81. 

\\ Ber., 1894, 27, 898; ibid., 1895, 28, 171. 


A similar substance is obtained fromp-diazotoluene chloride, 
but ^>-nitrodiazobenzene chloride yields only ^-dinitrodiazo- 
aminobenzene under the same conditions. 

On extending this reaction to diazobenzene perbromide, 
Griess obtained the first of a very important series of new 
compounds, namely, the diazoimides, containing three atoms of 
nitrogen united together. The empirical formula is C 6 H 5 N 3 , 
and Kekule* proposed for it the constitutional formula 

65 .N< 



C 6 H 5 . NBr.NBr 2 + NH 3 = C 6 H 5 . N 3 .+ 3HBr. 

Diazobenzeneimide or phenylazoimide is a yellow oil 
possessing a stupefying odour. It boils at 59 under a 
pressure of 12 mm. and explodes when heated at the ordinary 
pressure. When it is heated with hydrochloric acid nitrogen 
is evolved and chlorobenzene is obtained*; with sulphuric 
acid two-thirds of the nitrogen is eliminated and aminophenol 
is producedf. Diazobenzeneimide is also obtained when 
hydroxylamine acts on diazobenzene sulphate,! 

C 6 H 6 . N a . HS0 4 + NH 2 . OH = C 6 H 6 . N 3 + H 2 + H 2 SO 4 
and by the elimination of water from nitrosophenylhydrazine 

X2 , 

C 6 H 6 .N< =C 6 H 6 .N<||+H 2 


2. Action of hydrazine. The action of diazo-salts on 
phenylhydrazine was first studied by Griess, who obtained 
diazobenzeneimide by treating phenylhydrazine with m-diazo- 
benzoic acid 

,C0 2 
2C 6 H 4 <| +2C 6 H 6 .NH.NH, 

X N 2 
= C 6 H 6 . N 3 + C 6 H 6 . NH 2 + C0 2 H.C 6 H 4 . N, + NH 2 .C 6 H 4 . C0 2 H 

* Ber., 1886, 19, 313. t Ber., 1894, 27, 192, 

I Ber., 1892, 25, 372 ; 1893, 26, 1271 ; compare also Forster and Fierz, 
Trans., 1907, 91, 855, 1350. 
Ber., 1876, 9, 1659. 


and also by the interaction of diazobenzene and m-hydrazino- 
benzoic acid 

= C 6 H 6 . N 3 + C 6 H 6 . NH 2 + C0 2 H.C 6 H 4 . N 3 

+ NH 2 . C 6 H 4 . C0 2 H + 2H 2 0. 

The same compound was obtained by E. Fischer by acting on 
phenylhydrazine with diazobenzene sulphate* 
C 6 H 6 . N 2 . S0 4 H + C 6 H 6 . NH.NH 2 

= C 6 H 5 . N 3 + C 6 H 5 .NH 2 . H 2 S0 4 * 

It has been shown, however, that in addition to the above 
compound a substance is formed of formula 
C 6 H 6 .N 2 .N(C 6 H 5 ).NH 2 , 

to which the name diazobenzenephenylhydrazide is given.f 
When this compound is oxidized with dilute permanganate 
solution, bisdiazobenzenediphenyltetrazone is obtained. This 
compound, of formula 

C 6 H 5 . N 2 . N(C 6 H 6 ).N : N.N(C 6 H 6 ).N : N.C 6 H 6 , 
contains a chain of no less than eight nitrogen atoms. 

When hydrazine itself is substituted for the phenyl deriva- 
tive two reactions proceed : on the one hand, we have the 
formation of diazobenzeneimide and ammonia, and on the 
other, aniline and azoimide are produced, thus 

C 6 H 5 . N 2 . NH.NH 2 = C 6 H 6 . N 3 + NH 8 
and C 6 H 6 . N 2 . NH.NH 2 = C 6 H 5 . NH 2 + N 3 H ; J 

the latter reaction, however, proceeds to only a slight extent. 

3. Action of azoimide. The azoimides are also obtained 
by adding a solution of azoimide or its sodium salt to a diazo- 
solution containing excess of sulphuric acid. The resulting 
azoimide is extracted with ether. (For the preparation of 
azoimide by reactions which do not involve the use of diazo- 

* Ber., 1877, 10, 1334 ; Annalen, 1878, 190, 94 ; compare also Griess, 
Ber., 1887, 20, 1528 ; ibid., 1888, 21, 3415. 
t Wohl and Schiff, Ber., 1900, 33, 2741. 
t Ber., 1893, 26, 88, 1263. 
Noelting and Michael, Ber., 1893, 26, 86. 


compounds, textbooks on Organic Chemistry should be con- 

5. Benzoyl chloride. When diazo-salts are treated 
with an aqueous suspension of benzoyl chloride and copper 
powder, dibenzoylhydrazines, RN(CO.C 6 H 5 )N(CO.C 6 H 6 )R, are 

* Biehringer and Busch, Ber., 1902, 35, 1964. 



BY acting on diazobenzene nitrate with potassium ferro- 
cyanide, Griess * obtained azobenzene, a substance having the 
formula C 18 H 14 N 2 , and a brownish oil. The second of these 
was shown later to be benzeneazodiphenyl 

C 6 H 6 .N 2 .C 6 H 4 .C 6 H 5 .t 

Griess also observed the formation of p-diphenol by the 
decomposition of the double salt, (C 6 H 5 . N 2 C1) 2 , SnCl 4 . { 

As will be seen later (p. 73) diazo-salts combine with amines 
to form diazoamino-compounds, and these pass, by molecular 
change, into aminoazo-compounds. In the simplest case, that 
of diazobenzene chloride and aniline, in addition to aminoazo- 
benzene, o- and ^)-aminodiphenyl are formed. 

A similar case has not been observed, however, when a diazo- 
salt acts on a phenol, but when nitrosophenol is thus treated, 
diphenyl derivatives are largely produced. || 

Also in the preparation of phenol from diazobenzene sul- 
phate o- and >-hydroxydiphenyl are formed. ^[ 

It is evident, therefore, that at the moment when the diazo- 
nitrogen separates from the benzene nucleus, two of the latter 
unite at this point. 

Diphenyl may be prepared in good yield by Gattermann's 
method of adding copper powder to a solution of diazobenzene 
sulphate in alcohol.** 31 grams of aniline are dissolved in 

* Annalen, 1866, 137, 39 ; Ber., 1876, 9, 132. 

t Locher, Her., 1888, 21, 911 ; compare also p. 62. 

t Ber., 1885, 18, 960. 

Hirsch, Ber., 1892, 25, 1973 ; see also Heusler, Annalen, 1890, 260, 

|| Borsche, Ber., 1899, 32, 2935 ; Annalen, 1900, 312, 211. 

U Hirsch, Ber., 1890, 23, 3705; J. pr. Chem., 1885 [ii], 32, 117; corn- 
pare also Norris, Macintyre, and Corse, Amer. Chem. J., 1903, 29, 120. 

** Ber., 1890, 23, 1226. 


40 grams of concentrated sulphuric acid and 150 grams of 
water, and the solution diazotized in the usual manner with 
23 grams of sodium nitrite. 100 grams of alcohol (90 per 
cent.) are now added, and then 50 grams of copper powder. 
Nitrogen is evolved and the temperature rises to about 30-40. 
After about one hour, when the reaction is finished, the whole 
is distilled with steam; alcohol passes over, and when the 
distillate gives, on addition of water, a solid substance, the 
receiver is changed, and crystals of diphenyl are collected. 
Instead of copper powder, 100 grams of zinc dust or iron 
powder may be used. 

Diphenyl is also obtained by the action of stannous chloride 
on diazobenzene chloride or formate.* 

A large number of similar condensation products are 
obtained by subjecting mixtures of diazo-salts and hydro- 
carbons or similar ring-compounds to the action of aluminium 
chloride; thus diphenyl is obtained from diazobenzene chlo- 
ride and benzene, and the corresponding phenyl derivative 
results from the condensation of this diazo-salt with thiophen, 
pyridine, and quinoline.f 

In applying the cuprous chloride and copper powder methods 
for the production of chloro-derivatives to the case of many 
nitrodiazo- and chloronitrodiazo-salts, a remarkable tendency 
towards the formation of diphenyl derivatives has been 

Thus, when o-nitrodiazobenzene chloride is acted on by 
copper powder, a yield of 60 per cent, of 2 : 2 / -dinitrodiphenyl 
is obtained ;t by using cuprous chloride a yield of 68 per cent, 
was observed. 

The diphenyl reaction is also brought about by treating 
diazo-salts with cuprous oxide dissolved in ammonia,|| and 
corresponding derivatives are formed by treating diazo-salts 
with zinc ethyLIT 

* Culmann and Gasiorowski, Chem., 1889 [ii], 40, 97. 

t Mohlau and Berger, Ber., 1893, 26, 1994 ; see also Kiihling, Ber., 
1895, 28, 41 ; 1896, 29, 165, and Bamberger, Ber., 1895, 28, 403. 

i Niementowski, Ber., 1901, 34, 3325. 

Ullmann and Forgan, Ber., 1901, 34, 3802 ; D. R-P. 126961. 

|| Vorlander and F. Meyer, Annalen, 1902, 320, 122. 

IT Bamberger and Tichwinsky, Ber., 1902, 35, 4179. Tichwinsky, 
J. Russ. Phys. Chem. Soc., 1903, 35, 155, 675 ; 1904, 36, 1052. 


A similar condensation takes place in the naphthalene series ; 
thus when jS-diazonaphthalene sulphate is dissolved in alcohol 
and treated with zinc dust, to which has been added a very 
little powdered copper sulphate, /3/3-dinaphthyl is formed,* and 
when a cold neutral solution of diazobenzene chloride is mixed 
with a solution containing 1 molecular proportion of copper 
sulphate and 6 molecular proportions of sodium thiosulphate 
(a solution of the salt Cu 2 S 2 3 , 3 Na 2 S 2 3 , 6 H 2 O), benzene- 
azodiphenyl, C 6 H 5 . N 2 . C 6 H 4 . C 6 H 6 , is produced, together with 
phenyl sulphide, (C 6 H 6 ) 2 S.t 

* Chattaway, Trans., 1895, 67, 653. 
t Bernstein, Ber., 1901, 34, 3968. 



A CUKIOUS reaction was noticed by Meldola,* who found 
that when 3 : 4-dinitro-o-anisidine 

is diazotized in acetic acid solution, the resulting diazo-com- 
pound only contains one nitro-group; the other having been 
eliminated during diazotization, a substance of formula 

CH 3 .0, 
or, more probably, , 

being obtained.f The nitro-group has thus been replaced by 
hydroxyl in the process, being itself liberated in the form of 
nitrous acid. 

In a similar manner dinitro-p-anisidine 

NH 2 

O.CH 3 

on being diazotized in presence of acetic acid J loses a nitro- 
group, the diazo-compound formed giving with -naphthol 
a substance of formula 

Tmn*., 1900, 77, 1172. t Proc., 1901, 17, 135. 

Meldola and Eyre, Trans., 1902, 81, 988. 

N .C 10 H 6 .OH 

the diazo-compound itself not having been isolated. 

In nitric or sulphuric acid solution the nitro-group remains 
unaffected, but in presence of hydrochloric acid the nitro- 
group adjacent to the diazo-group is replaced by chlorine. 

Meldola and his pupils have found that when a nitro-group 
is in the ortho- or para-position with'respect to an amino-group, 
no displacement of the nitro-group takes place on diazotization 
unless there is a second nitro-group adjacent to the first 
(mobile) group. 

It has further been observed that when a methoxy-group 
is in the para-position with respect to the amino-group, and at 
the same time has a nitro-group in an adjacent position, de- 
methylation takes place on diazotization.* 

Thus the compounds 

OMe OMe 


NO, . . 


yield the corresponding quinonediazides of dinitrobenzene 

NO/YO, and 

N 2 N 2 

Also when m-phenylenediaminedisulphonic acid is tetrazo- 
tized, a sulphonic acid group is replaced by hydroxyl with 
formation of tetrazophenolsulphonic acid.f 

In some other cases which have been observed, it has been 
possible to obtain a nitrodiazo-compound which, even on dilu- 

* Meldola and Stephens, Trans., 1905, 87, 1205. 
t E. P. 18283 of 1903. 



tion with water, soon loses a nitro-group. Thus if the dinitro- 
/9-naphthylamine of formula 


is diazotized in concentrated sulphuric acid solution and 
poured into ice-cold water, a precipitate is formed after a short 
time consisting of a diazo-oxide, to which is assigned the 

The nitro-group in the a-position is thus replaced by 
hydroxyl. Similarly from the dinitronaphthylamine 


the corresponding mononitrodiazo-oxide 

is obtained. The formation of the nitrodiazo-oxide 


* Gaess and Ammelburg, Ber., 1894, 27, 2211. 


from the corresponding dinitro-a-naphthylamine 


takes place in exactly the same way.* 

In all the above cases it will have been noted that the 
nitro-group which is eliminated reappears as free nitrous acid. 
This has led Meldola and Eyre f to make the experiment of 
starting the diazotization of the above-mentioned dinitro-o- 
anisidine with a small quantity of nitrous acid (one quarter of 
the theoretical amount was used) ; and they observed that the 
diazotization was continued by the nitrous acid thus eliminated. 

This transformation is not confined to those diazo-compounds 
containing only nitro-groups. Many other cases are known ; 
thus Meldola and Streatf eild { found that when the sulphate 
of dibromo-j3-naphthylamine 


was diazotized in presence of acetic acid and the resulting 
mixture raised to the boiling-point, the normal reaction, 
namely, replacement of the diazo-group by hydroxyl (see 
p. 29) did not take place, but bromine was displaced and a 
diazo-oxide was formed 


In a similar manner chlorobromo-/3-naphthylamine yielded 
a bromodiazo-oxide 

* Friedlander, Ber., 1895, 28, 1951. 

t Trans., 1901, 79, 1076. } Trans., 1895, 67, 908. 



Similar substitutions of a halogen-group by hydroxyl have 
been observed to take place by merely treating the diazo-salt 
with alkalis. Thus 2:4: 6-tribromodiazobenzene chloride 
yields the dibromodiazo-oxide 

N C1 


The same reaction takes place in the case of 2-chloro-3- 
nitroaniline-5-sulphonic acid, 



and the tetrazo-derivative of 2-chloro-m-phenylenediamine 
5-sulphonic acid, under the same conditions, loses chlorine, 
a hydroxyl-group taking its place 

Cl OH 

_^ C1N/ / \N 2 C1 GIN, 





S0 3 H 

A sulphonic acid group also undergoes this change ; thus the 

* Bamberger and Kraus, Vierteljdhrssch. Ges. Zurich, 1899, 24, 257 ; 
Ber., 1906, 39, 4248 ; Bamberger, Annalen, 1899, 305, 289. Compare 
also Silberstein, J. pr. Chem. 1883 [ii], 27, 98. 

t Badische Anilm- und Soda-Fabrik, D.R-P. 141750. 

| E. P. 16811 of 1901. 

F 2, 




when diazotized and rendered alkaline.* 

The replacement proceeds even when the diazo-salt of a 
weak acid such as the acetate, carbonate, bicarbonate, oxalate, 
&c., is allowed to stand ; this takes place in the case of 2 : 5 : 6- 
trichloroaniline-m-sulphonic acid, o-nitroaniline-p-sulphonic 
acid, and 2 : 4-dinitroaniline.f 

The transformation of 2:4: 6-tribromo- and trichloro-diazo- 
benzene takes the same course as shown above,! and a similar 
phenomenon occurs in the case of a considerable number of 
halogen-derivatives of the benzene and naphthalene series. 

Some striking molecular transformations have been observed 
by Hantzsch.|| If ^-chlorodiazobenzene thiocyanate (prepared 
by adding potassium thiocyanate to the diazo-chloride) ia 
dissolved in alcohol containing a trace of hydrochloric acid, 
the thiocyano-group changes place with the chlorine atom, 
and on adding ether to the solution, p-thiocyanodiazobenzene 
chloride is precipitated, thus 

N 2 .SCN N 2 C1 


Similarly, many brominated diazo-chlorides pass into chlori- 
nated diazo-bromides ; If for example, 2:4: 6-tribromodiazo- 
benzene chloride is converted into chlorodibromodiazobenzene 

* E. P. 23993 of 1902. t E. P. 20551 of 1901. 

1 Orton, Proc. Boy. Soc., 1902, 71, 153. 

Orton, Proc., 1902, 18, 252; Trans., 1903, 83, 796; 1907, 91, 1554; 
Badische Antlin- und Soda-Fabrik, E. Ps. 1561, 6615 of 1902 ; 16995, 
27372 of 1903 ; 4997 of 1904; Noeltingand Battegay, Ber., 1906, 39, 79. 

|| Ber., 1896, 29, 947. 

IT Hantzsch, Schleissing, and Jager, Ber., 1897, 30, 2334 ; see also Ber., 
1898, 31, 1253. 

** Hantzsch and Smythe, Ber., 1900, 33, 505. 


This transformation has been studied quantitatively, and 
has been found to proceed according to the following laws : 

(1) The bromine atoms are replaced only when present in 
the para- or ortho-position with respect to the diazo-group, 
those in the ortho-position being most readily removed. A 
bromine atom in the meta-position is not affected. 

(2) The ease of transformation increases with the number 
of bromine atoms present. 

(3) The transformation constant, calculated from the equa- 
tion for a unimolecular reaction, 

1, A 
k = J lo SATx' 

increases with the temperature and is also influenced by the 
solvent, having its minimum value in water, and becoming 
greater as the series of alcohols is ascended. 

(4) The diazo-salts containing two bromine atoms are 
stable when dry, but are rapidly transformed in ethyl alcohol ; 
2:4: 6-tribromodiazobenzene chloride becomes transformed 
even in the dry state. 

A corresponding isomeric change does not take place in the 
case of tri-iododiazobenzene chloride or tribromodiazobenzene 

Lastly, a remarkable change is undergone by 1-nitrodiazo- 
-naphthalene chloride which is transformed in presence of 
glacial acetic acid into l-chlorodiazo-/3-naphthalene nitrite. f 

* Hantzech, Ber., 1903, 36, 2069. 
t Morgan, Trans., 1902, 81, 1376. 


MOST investigators who have worked with diazo-compounds 
have noticed that they are very easily changed by the action 
of light. Thus Berthelot and Vielle in 1881 * recorded the 
observation that when diazobenzene nitrate was exposed to 
light it became rose-coloured. 

This decomposition has been made the basis of photographic 
processes ; thus Feer in 1889 f exposed a film coated with a 
mixture of a diazo-sulphite and a phenol or amine to light. 

A decomposition of the former occurred which was followed 
by the formation of an azo-compound, and hence the produc- 
tion of a coloured negative. 

Green, Cross, and Bevan J coated films with diazotized pri- 
muline, the decomposition of which was proportional to the 
intensity of the light ; this formed the ' negative ', and a ' posi- 
tive ' was developed by treatment with an amine or a phenol. 
Those parts of the negative which had been exposed to bright 
light gave no colour with the component, owing to the destruc- 
tion of the diazo-compound with evolution of nitrogen and 
formation of a phenol. They concluded, however, that 
union of the diazo-compound with the medium (cellulose) was 
necessary, for the free diazo-primuline when exposed to light 
in a thin film was either not decomposed at all or only after 
long exposure. 

Andresen examined the behaviour of the diazo-salts of the 
two naphthylamines, and showed that the reaction was similar 
to that effected by heat, namely, that phenols were formed 


R.N 2 C1 + H 2 = R.OH + N 2 + HC1. 

Ruff and Stein || arrived at the following conclusions with 

* Compt. rend., 1881, 92, 1074. t D. R-P. 53455. 

t D.R-P. 56606, Ber., 1890, 23, 3131 ; J. Soc. Chem.Ind., 1890, 9, 1001. 

Photographische Correspondenz, 1895. 

|| Ber., 1901, 34, 1668. 


regard to the action of light on substituted diazobenzene 

Those which contain a negative group (OH, NO 2 , C0 2 H) in 
the para-position are ' more sensitive than those containing 
a similarly situated positive group (Cl, CH 3 ) ; the influence of 
the nitro-group is greatest. Ortho- and para-substituted 
groups have about the same effect, either in increasing or 
decreasing sensitiveness ; this effect is always less than that of 
a meta-group. In the case of diazo-salts derived from different 
nuclei, the sensitiveness to light increases with the number of 
atoms in the nucleus ; thus the diazo-salt from 3-aminocarba- 
zole is nearly five times as sensitive as that from p-toluidine. 

As regards the decomposition of diazo- and tetrazo-com- 
pounds an equal number of diazo-groups are destroyed by light 
in the same time ; thus the same number of minutes is neces- 
sary to decompose completely the diazo-salt from an N/IQ 
solution of p-aminodiphenyl as from an N/2Q solution of 

Orton, Coates, and Burdett* were the first to investigate 
extensively this reaction. Solutions of the diazo-salts of 
various aromatic amines were found to decompose under the 
action of light in the manner indicated by Andresen, but the 
mechanism whereby the phenolic decomposition is effected 
must be very different from that induced by the action of 
heat, for the remarkable fact was discovered that many of the 
diazo-compounds which are decomposed by water or acids 
only with very great difficulty, and then only to a very slight 
extent, for example, the diazo-salt of 2:4: 6-tribromoaniline, 
undergo rapid transformation under the action of light with 
quantitative formation of the corresponding phenol. 

A similar instance of this difference in stability towards 
heat and light had been noticed by Meldola, Woolcott, and 
Wray,f who found that the compound 

Trans., 1907, 91, 35. t Trans., 1896, 69, 1327. 


was stable towards boiling water, but that it decomposed 
gradually and became brown on exposure to light. 

The s2/w,-diazo-cyanide of 2:4: 6-tribromoaniline in benzene 
solution changes under the action of light into the corre- 
sponding cm^-compound.* 

* Ciusa, Atti. R. Accad. Lincei, 1906 [v], 15, ii, 136 ; for an explanation 
of the terms syn and anti see p. 123. 


THE diazoamino-compounds are formed by the condensation 
of a diazo-salt with primary or secondary .amines in presence 
of sodium acetate, thus 

(1) C 6 H 5 . N 2 .C1 + NH 2 . C 6 H 5 = C,,H 5 . N 2 . NH.C 6 H 6 + HC1. 

(2) C 9 H 6 .N 2 C1+NH(C 2 H 5 ).C 6 H 6 = 

C H 5 .N 2 .N<C 2 H 6 ).C 6 H 6 + HC1. 

The preparation of diazoaminobenzene is carried out as 
follows : 10 grams of aniline are dissolved in 100 c.c. of 
water and concentrated hydrochloric acid corresponding to 
12 grams HC1. The solution is diazotized by adding a solu- 
tion of 8 grams of sodium nitrite with the usual precautions. 
On the other hand, 10 grams of aniline are dissolved in 
50 grams of water and exactly the theoretical quantity of 
hydrochloric acid. After cooling this solution with ice it is 
added to the diazo-solution, and then, immediately, a cold 
concentrated solution of 50 grams of sodium acetate. After 
standing for half an hour the diazoaminobenzene is filtered 
off, washed with water, dried on a porous plate, and crystal- 
lized from light petroleum. 

When aromatic diazo-compounds are allowed to act on 
aliphatic amines, similar diazoamino-compounds are obtained. 
Thus methylamine and ethylamine yield with diazobenzene, 
diazobenzenemethylamide (phenylmethyltriazen), 

C 6 H 6 .N 2 .NH.CH 3 , 
and diazobenzene-ethylamide (phenylethyltriazen), 

C 6 H 6 .N 2 .NH.C 2 H 6 , 


These compounds possess the formulae assigned to them, 
and are not tautomeric. f 

* Dimroth, Per., 1903, 36, 909 ; 1905, 38, 670, 2328 ; compare also 
Goldschmidt and Holm., Ber., 1888, 21, 1016. 
t Dimroth, Eble, and Gruhl, Ber., 1907, 40, 2390. 


The formation takes place also when an alkali nitrite is 
added to a solution of an amine containing no free mineral 
2C 6 H 6 . NH 2 . HC1 + NaN0 2 

= C 6 H 6 . N 2 . NH.C 6 H 5 + NaCl + HC1 + 2H 2 O. 

If two molecules of a diazo-salt condense with one of a 
primary amine, a bisdiazoamino-compound is formed 

2C 6 H 6 . N 2 C1 + C 6 H 5 . NH 2 = (C a H 6 . N 2 ) 2 N.C,H 6 + 2HC1. 

A modification of this method is to allow a molecule of 
a diazo-salt to act on a molecule of a diazoamino-compound 
C 6 H 5 . N 2 C1 + C 6 H 5 . N 2 . NH.C 6 H 6 = (C 6 H 5 .N 2 ) 2 N.C 6 H 5 + HCL* 

The primary monoamines of the benzene series all yield 
diazoamines, those containing the groups Cl, NO 2 , CN, &c., 
most readily, but the monoalkylated monoamines of this 
series show a tendency to form azo-compounds ; for example, 
methylaniline, when treated with diazobenzenesulphonic acid, 
yields a mixture of the diazoamino-compound, 
' S0 3 H.C 6 H 4 . N 2 . N(CH 3 ) . C 6 H 6 , 
and the isomeric aminoazo-compound, 

S0 3 H.C 6 H 4 . N 2 . C 6 H 4 . NH.CH 3 . f 

A number of bases, as for example diphenylamine, the 
naphthylamines and their monoalkyl-derivatives, m-phenyl- 
enediamine and certain of its homologues and substitution 
products, form aminoazo-compounds direct. Dimethylaniline 
and some other tertiary amines also yield aminoazo-compounds; 
here, of course, no diazoamine can be formed. Griess discovered 
the remarkable fact that the same compound is obtained from, 
for example, diazobenzene chloride and p-toluidine on the 
one hand, and p-diazotoluene chloride and aniline on the 

According to the above equations one would expect two 
different diazoamino-compounds to be formed thus 

(1) C 6 H 6 . N 2 C1 + NH 2 . C 6 H 4 . CH 3 

= C 6 H 5 . N 2 . NH.C 6 H 4 . CH 3 + HCL 

* Ber., 1894, 27, 703. 

t Bernthsen and Goske, Ber., 1887, 20, 925 ; Bamberger and Wulz, 
Ber., 1891, 24, 2082. 


(2) CH 3 . C 6 H 4 . N 2 C1 + NH 2 . C 6 H 6 

= CH 3 . C 6 H 4 . N 2 . NH.C 6 H 5 + HC1. 

If one supposes, however, that an intermediate product is 
formed of the formula 

C 6 H 6 . NH.NC1.NH.C 6 H 4 . CH 3 , 
(a) (b) 

then, by the elimination of hydrogen chloride, either of the 
above formulae is obtained according as to which hydrogen 
atom (a) or (b) is removed.* 

In order to decide which of the above formulae is correct, 
use is made of the compound with phenylcarbimide. This 
combines with the diazoamino-compound to form a substance 
of formula 

f\ \ r 1 TT 
(1) L 6 ti { 


When this is decomposed with dilute sulphuric acid, phenyl- 
2>-tolylcarbamide, phenol, and nitrogen are formed, so that its 
constitution must be represented by (1), for (2) would give 
diphenylcarbamide. This conclusion is also confirmed by the 
fact that whichever way the compound is prepared it yields 
only one acetyl derivative, namely, diazobenzene p-aceto- 
toluidide, which, when decomposed by acids, yields aceto- 
toluidide.f The constitution of the diazoamino-compound is 
therefore C 6 H 6 .N 2 .NH.C 6 H 4 .CH 3 , J and it is found that in 
these reactions the imino-group is always attached to the 
electronegative, and the diazo-group to the electropositive 

When the alkyl derivatives of the mixed diazoamino-com- 
pounds (that is, compounds in which the two radicals combined 
with the group N 3 H are different) are examined, it is found 
that three isomeric substances exist. These are formed : 
I. By the action of X.N 2 C1 on Y.NH.R. 

II. By the action of Y.N 2 C1 on X.NHR. 

.* V. Meyer, Ber., 1881, 14, 2447 ; 1888, 21, 1016, 3004. 
t von Pechmann, Ber., 1895, 28, 869. 
I Goldschmidt, Ber., 1888, 21, 2578. 


III. By the alkylation of X.N 3 HY with III and caustic 
potash. X and Y represent the two radicals united with the 
group N 3 H, and R represents a univalent alkyl-group. 

The isomerides obtained by direct alkylation are also 
formed when the compounds obtained according to (I) and 
(II) are heated together in equimolecular proportions.* 

Migration of the diazo-gronp. An interesting variation in 
this reaction is that in which the migration of the diazo-group 
occurs. Thus when diazotized sulphanilic acid and ^-toluidine 
hydrochloride are mixed together at the diazo- and amino- 
groups change places, and there results a mixture of p-diazo- 
toluene chloride and sulphanilic acid.f In neutral solution, 
however, the normal diazoamino-compound, 

CH 3 .C 6 H 4 .N 2 .NH.C 6 H 4 .S0 3 H, 

is formed. 

A corresponding interchange takes place between m- or 
_p-nitrodiazobenzene chloride and p-toluidine. When, how- 
ever, p-diazotoluene chloride is mixed with m- or _p-nitro- 
aniline or sulphanilic acid no migration of the diazo-group 
takes place. J If diazobenzene chloride and p-bromoaniline 
are allowed to interact, aniline and ^-bromodiazobenzene 
chloride are formed. 

This migration of the diazo-nucleus is probably associated 
with the changes which occur when this group passes from 
the ' diazonium ' (see chap, xviii) to the diazo condition. 

Bamberger || found that when an alkali ^so-diazo-oxide is 
dissolved in cold mineral acid, nitrous acid is formed 

R.N : N.OH + H 2 O = R.NH 2 + HNO 2 
R.NH 2 + HN0 2 + HC1 = R.N 2 C1 + 2H 2 0. 

It is interesting to note that diazoamino-compounds may 
be obtained without the use of diazo-compounds ; thus they 

* Meldola and Streatfeild, Trans., 1886, 49, 624 ; 1887, 51, 102, 434 ; 
1888, 53, 664; 1889, 55, 412 ; 1890, 57, 785. 
t Griess, Ber., 1882, 15, 2190. 
1 Schraube and Fritsch, Ber., 1896, 29, 287. 
Hantzsch and F. M. Perkin, Ber., 1897, 30, 1412. 
|| Ber., 1895, 28, 826. 


are formed by the interaction of nitrosoamines and primary 
aromatic amines ; for example 

C 6 H 6 (CH 3 )N.NO + H 2 N.C 6 H 5 = C 6 H 6 (CH 3 )N.N 2 . C 6 H 5 . 
Nitrosoacetanilide also reacts in a similar way 

= C 6 H 6 . NH.N 2 . C 6 H 6 + CH, . CO 2 H, 

and two molecules react with one molecule of aniline in alka- 
line solution to form a bisdiazoamino^compound, 

An isomeride of diazoaminobenzene is said to result when 
aniline is diazotized in presence of acetic acid instead of 
a mineral acid.* Its constitution is supposed to be 

but the existence of such a compound must be accepted with 
reserve, f 

Reactions of the diazoamino-componnds. The diazoamino- 
compounds usually have a yellow colour, and do not dissolve 
in acids. They may generally be crystallized without decom- 
position, and are much more stable than the diazo-compounds. 

When boiled with hydrochloric acid, nitrogen is evolved 

C 6 H 6 . N 2 . NH.C 6 H 6 + H 2 = C 6 H 5 . OH + C 6 H 6 . NH 2 + N 2 . 
On heating with cuprous chloride and hydrochloric acid, 
chlorobenzene and aniline are formed 

C 6 H 5 . N 2 . NH.C 6 H 6 + HC1 = C 6 H 5 C1 + C 6 H 5 . NH 2 + N 2 . 

Hydrazines are obtained by reduction with zinc dust and 
acetic acid 

C 6 H 6 . N 2 . NH.C 6 H 5 + 2H 2 = C 6 H 6 . NH.NH 2 + C 6 H 5 . NH 2 , 

* Orloff, J. Russ. Phys. Client. Soc. t 1906, 38, 587. 
t Compare also Vaubel, Zeitsch. angew. Chem., 1900, 13, 762 ; 1902, 15, 


and with nitrous acid two molecules of a diazo-compound are 

C 6 H 6 . N 2 . NH.C 6 H 6 + HN0 2 + 2HC1 = 2C 6 H 5 . N 2 C1 + 2H 2 O. 

By boiling a diazoamino-compound with sulphurous acid in 
alcoholic solution, the diazo-group is replaced by the sulphonic 
acid group 

C 4 H 6 . N 2 . NH.C 6 H 5 + 2S0 2 + 2H 2 

= C e H 6 . S0 3 H + N 2 + C 6 H 6 . NH 2 . H 2 S0 3 . 

A very important reaction is that which takes place when 
a diazoamino-compound is warmed with a mixture of an 
amine and its hydrochloride ; a molecular change occurs with 
formation of aminoazo-compounds 

C 6 H 5 .N 2 .NH.C 6 H 6 - C 6 H 5 .N 2 .C 6 H 4 .NH 2 . 

The velocity of the transformation of diazoamino- into 
aminoazo-compounds under the influence of aniline hydro- 
chloride has been shown by Goldschmidt and his pupils to be 
in accordance with the law of unimolecular reactions, in 

1 a * 

k=ilog . 

t a x 

In the case of diazoaminobenzene dissolved in aniline con- 
taining aniline hydrochloride, the rate of reaction is pro- 
portional to the concentration of the aniline hydrochloride, 
and increases with the temperature ; as is usual in the case of 
a unimolecular reaction, the velocity is independent of the 
concentration of the diazoaminobenzene. 

The transformation is also effected by other aniline salts, 
such as the dichloroacetate or trichloroacetate, but the 
rate in this case is slower than when the hydrochloride is 

Benzenediazoamino-p-toluene becomes converted into diazo- 
aminobenzene and ^-toluidine, the former of which then 
undergoes transformation in accordance with the above rules. 

* Per., 1896, 29, 1369, 1899 ; Zeitsch. physikaL Chem., 1899, 29, 89. 
t Compare also Jungius, Chem. WeekUad, 1905, 2, 246. 


The conversion takes place more slowly when the diazo- 
group is in the ortho-position with respect to the amino- 
group. Thus the value of the constant in the case of diazo- 
aminobenzene at 45 is 0-081, whilst the corresponding value 
for diazoamino-p-toluene is only 0-0095, the solution in each 
case being semi-normal. 


THE azo-compounds, like the diazo-, contain the group .N 2 ., 
but with the important difference that, whereas in the latter 
only one organic radical is united to the ,N 2 . group, the other 
free linking being combined with an acid radical, thus R.N 2 . 01 ; 
in the former two organic radicals are united to the N 2 group, 
thus R.N 2 . E. 

The groups attached to the nitrogen atoms may be either 
(1) aromatic, (2) aliphatic, or (3) one aromatic and one ali- 
phatic group, giving the mixed azo-compounds. 

The first representative of this class of compounds was 
obtained by Mitscherlich * by the distillation of nitrobenzene 
with alcoholic potash. Mitscherlich called the substance ' azo- 
benzide', the modern name being of course azobenzene, 
C 6 H 5 . N 2 . C 6 H 6 . He considered that the substance was formed 
by the replacement of one atom of nitrogen for one atom of 
hydrogen in benzene, but Zinin showed that ' azoxybenzide ' 
was always formed in this reaction, and this on distillation 
yielded ' azobenzide '.f Zinin also, by the reduction of this 
substance with hydrogen sulphide and treatment of the pro- 
duct with sulphuric acid, obtained the sulphate of benzidine, 
which he considered was formed by the direct reduction of 
' azobenzide '. Hofmann, however,:): showed that in this reduc- 
tion, hydrazobenzene, C 6 H 6 .NH.NH.C 6 H 5 , was first formed, 
which, in presence of sulphuric acid, underwent molecular 
change into the isomeric benzidine, NH 2 .C 6 H 4 .C 6 H 4 .NH 2 , 
and also proved by a vapour density determination that ' azo- 
benzide' must have a formula double that which had been 
previously given to it. 

* Annalen, 1834, 12, 311. t Ghent., 1845, 36, 93. 

| Jahresber., 1863, 424. 

-* C 6 H 6 .NH 2 . 
CH 5 .N C 6 H 5 .NH 


The azo-compounds occupy an intermediate position between 
the nitro-compounds and the corresponding amines ; thus in 
the case of nitrobenzene we have 

C 6 H 5 .N0 2 

Nitrobenzene. Azoxybenzene. 

C 6 H 5 .N C 6 H 6 .NH 


Azobenzene. Hydrazobenzene. Aniline. 

1. Azozy-componnds. These are obtained by the reduction 
of nitro- or nitroso-compounds with methyl- or ethyl-alcoholic 

4C 6 H 6 .N0 2 + 3CH 3 .ONa = 2(C 6 H 5 .N) 2 + 3H.C0 2 Na + 3H 2 0. 
Other reducing agents which may be employed are sodium 
amalgam and alcohol, zinc dust and alcoholic ammonia, and 
arsenious acid in alkaline solution. 

Further, azoxy-compounds are obtained by oxidizing amino- 
and azo-compounds with alkaline potassium permanganate 
or 1'erricyanide and by the oxidation of /3-phenylhydroxyl- 
amine, C 6 H 5 . NH.OH , in the air. 

By treatment with energetic reducing agents, azoxy-com- 
pounds yield products of various degrees of reduction ; thus 
with iron filings azo-compounds are formed, with ammonium 
sulphide, hydrazo-compounds, and, finally, acid reducing agents 
furnish amino-compounds ; the acid used in the latter case 
obviously serves to bring about a molecular change in the 
hydrazo-compound first formed. 

The simplest member of the series, azoxybenzene, is pre- 
pared as follows. 10 parts of sodium are dissolved in 100 
parts of methyl alcohol, 15 parts of nitrobenzene added, and 
the whole heated for 3 hours on a boiling- water bath in an 
apparatus connected with an inverted condenser. The alcohol 
is then distilled off, the residue of sodium formate and azoxy- 
benzene extracted with water, and the azoxybenzene allowed 
to crystallize out ; the yield is about 90 to 92 per cent, of the 
theoretical.* Azoxybenzene is insoluble in water, but crystal- 
lizes from alcohol in long, yellow, rhombic needles melting at 

* Ber., 1882, 15, 865 ; 1883, 16, 81. 


36. On heating with concentrated sulphuric acid it undergoes 
isomeric change and is converted into hydroxyazobenzene 

X N.C 6 H 5 N.C 6 H 6 

0< | -> II 

X N.C 6 H 5 N.C 6 H 4 .OH* 

2. Azo-compounds. As already indicated these are obtained 
by the reduction of nitro- or azoxy-compounds. The reducing 
agents used are, in the case of nitro-compounds, (1) zinc dust 
and alcoholic potash; (2) sodium amalgam and alcohol; (3) 
stannous chloride dissolved in caustic soda, and, in the case of 
azoxy-compounds, iron filings (p. 81). 

Other methods of formation are : (1) by the oxidation of 
hydrazo- or amino-compounds by potassium permanganate in 
alkaline solution f or by potassium f erricyanide ; (2) by the 
interaction of nitrosobenzene and aniline, 

C 6 H 6 . NO + NH 2 . C 6 H 6 = C 6 H 6 . N 2 . C 6 H 6 + H 2 O,t 

of nitrosobenzene and phenylhydrazine, or of nitrosobenzene 
and s-diphenylhydrazine.|| Azobenzene is also formed when 
phenylhydrazine is treated with bleaching powder solution,^" or 
when diazo-compounds are treated with the same reagent ; thus 
diazotized sulphanilic acid yields 2 : 2'-dinitroazobenzene-4 : 4'- 
disulphonic acid, SO 3 H.C 6 H 3 (NO 2 ).N : N.C 6 H 3 (NO 2 ) .S0 3 H ; 
some 4 : 6-dichloro-2-nitroaniline is formed at the same time, 
and this substance is also produced when diazobenzene chloride 
is treated with bleaching powder. 

When the sodium m>-diazo-oxide derived from diazosul- 
phanilic acid is used, the calcium salt of ^-nitroaminobenzene- 
sulphonic acid, N0 2 . NH.C 6 H 4 . S0 3 H : is formed.** 

An interesting synthetic method for the production of these 
substances is that due to Meldola ;ff trinitroacetylaminophenol 

* Ber., 1880, 13, 525 ; Annalen, 1882, 215, 218. 
t Annalen, 1867, 142, 364. 

t Baeyer, Ber., 1874, 7, 1638 ; Bamberger, Ber., 1893, 26, 473, 483. 
Mills, Trans., 1895, 67, 925. 
|] Bamberger, Ber., 1900, 33, 3508. 
T Ber., 1897, 3O, 284. 

** Zincke, Ber., 1895, 28, 2948 ; Zincke and Kuchenbecker, Annalen 
1903, 330, 1 ; see also Lenz, Annalen, 1903, 330, 370. 
ft Trans., 1906, 89, 1943. 


is condensed with phenylhydrazine, forming a hydrazo-com- 
pound which passes by oxidation into the azo-compound thus 


The azo-compounds are usually strongly coloured owing to 
the presence of the chromophoric group .N' : N.* They are 
readily acted on by sulphuric or nitric acids, chlorine, &c. 

Azobenzene is prepared from azoxybenzene by distilling a 
mixture of one part of the latter with three parts of iron filings 
from a small retort. Care must be taken that the materials 
are quite dry. The reddish distillate is crystallized from 
light petroleum and forms red plates melting at 68 and boil- 
ing at 293. 

Complex azo-compounds are also obtained by treating diazo- 
salts with potassium ferrocyamde.f 

3. Aminoazo-compounds. These are formed by the follow- 
ing reactions J : 

(1) Intramolecular rearrangement of diazoamines 

C 9 H 5 . N 2 . NH.C 6 H 5 = C 6 H 6 . N 2 . C 6 H 4 . NH 2 . 

(2) Reduction of ^-nitroazo-derivatives by an alkaline 

(3) Combination of a diazotized monoacetyldiamine with 
an amine or phenol and subsequent hydrolysis of the acetyl- 
derivative. || 

(4) The action of nitrosobenzene on monoacetyldiamines 
and hydrolysis of the acetyl-derivative.lf 

(5) The alkaline reduction of nitroamines.** 

* See, however, Baly and Tuck, Trans., 1906, 89, 982. 

t Griess, Annalen, 1866, 137, 39 ; Ber., 1876, 9, 132 ; Ehrenpreis, Bull. 
Acad. Sci. Cracow, 1906, 265. 

1 Meldola and Eynon, Trans., 1905, 87, 1. 

Meldola, Trans., 1883, 43, 425. 

I] Nietzki, Ber., 1884, 17, 343. f Mills, Trans., 1895, 67, 928. 

** Haarhaus, Annalen, 1865, 135, 164; Mixter, Amer. Chem.J.,1883,5, 
283 ; Nietzki, Ber., 1884, 17, 345 ; Graff, Annalen, 1885, 229. 341 ; 
Noelting and Binder, Ber., 1887, 0, 3016 ; Meldola and Andrews, Trans., 
1896, 69, 10 ; Noelting and Fourneaux, Ber., 1897, 30, 2938. 

G 2 


(6) Combination of a monodiazo-chromate, prepared from 
a diamine by diazotizing only one amino-group and precipi- 
tating with sodium dichromate, with an amine or phenol, f 

(7) Interaction of certain amines (the naphthylamines and 
2 : 4-tolylenediamine, for example) and the para-diazoimides.J 

The usual method of preparation, however, is to allow a 
diazo-salt to react with amine in presence of sodium acetate ; 
thus diazobenzene chloride unites with a solution of a-naphthyl- 
amine hydrochloride to form benzeneazo-a-naphthylamine 

C 6 H 5 . N 2 C1 + C 10 H 7 . NH 2 . HC1 + 2C 2 H 3 O 2 Na 

= C 6 H 5 . N 2 . C 10 H 6 . NH 2 + 2NaCl + 2C 2 H 4 2 . 

It is a fairly general rule that the diazo-group enters the 
aminic nucleus in the para-position with respect to the amino- 
group when this is unoccupied, but otherwise it enters the 

In the following formulae the position of the entering diazo- 
group is shown by the asterisk. 

NH 2 NH 2 NH 2 


In the case of diamines combination only takes place with 
the meta- derivative; thusm-phenylenediamine andm-tolylene- 


form azo-dyes, the diazo-group entering the para-position 

t Meldola and Eynon, loc. cit. 

J Morgan and Micklethwait, Trans., 1907, 91, 1512. 


with respect to an amino-group. It is even possible for a 
second diazo-complex to combine at the carbon atom between 
the amino-groups. The question of the influence of substitu- 
tion in such diamines on the formation of aminoazo-compounds 
has been made the subject of comprehensive researches by 
Morgan, who has arrived at the following conclusions * : 

(1) The mono-substituted meta-diamines and the di-substi- 
tuted meta-diamines, containing one free para-position with 
respect to an amino-group, react readily with diazo-salts 
to furnish para-aminoazo-dyestuffs,f and this reaction takes 
place with equal readiness both with the primary meta- 
diamines of this type and with their completely alkylated 

derivatives. J 

(2) The di-para-substituted primary meta-diamines 



react with diazo-salts to form ortho-aminoazo-derivatives, but 
the reaction takes place much less readily than with those 
diamines having one free para-position, and the yield of azo- 
product is frequently very small. 

(3) The nature of the substituents X and Y exerts some 
influence on the course of the azo-condensation, for when they 
are methyl-groups the base (4 : 6-diamino-m-xylene) reacts 
with diazotized aniline and its homologues, but when both 
substituents are halogen atoms (chlorine, bromine, or iodine) 
the condensation does not occur with these simple diazo-salts, 
but only with those derived from the nitroanilines. When only 
one methyl-group is replaced by chlorine or bromine, reaction 
with diazotized aniline and >-toluidine still occurs, but the 
yield of o-aminoazo-derivative is extremely small. || 

(4) The presence of a nitro-group in one of the two substi- 
tuted para-positions facilitates the azo-condensation, particu- 
larly when the diazo-salt also contains a substituent nitro- 

* Trans., 1907, 91, 370. t Trans., 1900, 77, 1205 ; 1902, 81, 89. 

Trans., 1902, 81, 656. Trans., 1902, 81, 1379; 1905, 87, 935. 

Trans., 1902, 81, 1379 ; 1905, 87, 937. IT Trans., 1905, 87, 940. 


(5) The progressive alkylation of the di-para-substituted 
meta-diamines rapidly reduces their capacity for forming 
azo-derivatives. The symmetrically and unsymmetrically 
dimethylated diamines give mixtures of diazoamines and 
aminoazo-compounds/* whilst the trimethylated diamines 
readily furnish diazoamines and show scarcely any tendency 
to form azo-derivatives. Finally, the interaction of the 
di-para-substituted m-diamines and diazo-salts is entirely 
prevented by the complete alkylation of these bases, f 

The aminoazo-compounds are usually crystalline and 
soluble in alcohol. They are yellow to red or brown in colour, 
and many of them are used as dyestuffs. 

The simplest member, aminoazobenzene, was introduced 
into commerce in 1863 by Simpson, Maule, and Nicholson 
under the name of ' aniline yellow ' ; at the present day it is 
only used as an intermediate product for the manufacture of 
induline, &c., but its sulphonic acids find extensive application 
as ' fast yellow '. 

4- Hydroxyazo-componnds. These are obtained by the 
action of a diazo-salt on phenols and their derivatives in 
alkaline solution,! thus 

C 6 H 6 . N 2 C1 + C 6 H 6 . OH = C 6 H 5 . N 2 . C 6 H 4 . OH + HC1. 

The same rules apply here with regard to the position 
taken by the diazo-complex as in the case of the aminoazo- 
compounds, namely, that the para-position to the hydroxyl- 
group, if free, is occupied. Otherwise the ortho-position is 
taken. In the case of ^3-naphthol, the diazo-group enters 
the a-position. 

It has been found, however, that by the action of diazo- 
benzene or ^>-diazotoluene on phenol, small amounts of the 
corresponding o-hydroxyazo-compounds are formed. 

* Trans., 1905, 87, 946 ; 1906, 80, 1057 ; 1907, 91, 368. 

t Trans., 1902, 81, 656. 

| In acid solution, diphenyl ether is produced, 

C 6 H 5 . N 2 . S0 4 H + C 6 H 5 . OH = (C 6 H 5 ) 2 + H.S0 4 -f N a 
(Hofmeister, Annalen, 1871, 159, 191), to the extent of a 4 per cent, 

Bamberger, Ber., 1900, 33, 3188. 


With two molecules of a diazo-compound, the bisazo- 
derivative is formed 


and with three molecules the trisazo-compound 



a-Naphthol yields with one molecule of a diazo-salt, first, 
the monoazo-compound 


and with two molecules the bisazo-compound 


Resorcinol, like phenol and m-phenylenediamine, can also 
combine with two molecules of a diazo-compound ; thus with 
one molecule it yields 


with two f 2 and with three 

H \J H 

N 2 R N 2 R 

* Grandmougin and Freimann, Ber., 1907,40, 2662; Heller and Nfitzel, 
J. pr. Chem., 1907 [ii], 76, 58. 

t Orndoff and Ray, Ber., 1907, 40, 3211. 


For a more detailed description of the amino- and hydroxy- 
azo-dyestuffs a larger work must be consulted.* 

5. Rate of formation of amino- and hydroxyazo-com- 
ponnds. This has been measured by Goldschmidt and his 
pupils. The case of the formation of methyl-orange from 
>-diazobenzenesulphonic acid and dimethylaniline hydro- 
chloride was studied first, the method of procedure being to 
withdraw samples of the mixture at given periods of time, 
and after acidifying, to determine the quantity of diazo- 
compound present by measuring the nitrogen evolved on 

The conclusions arrived at were : (1) In the combination of 
the hydrochloride of a tertiary amine with diazobenzene- 
sulphonic acid, it is the base liberated from the hydrochloride 
by hydrolysis that acts with the diazo-compound. (2) Excess 
of hydrochloric acid retards the combination. (3) The con- 
centration of the hydrochloride or of the diazo-compound 
has no influence on the reaction. (4) The combination pro- 
ceeds at the same rate when other acids of the same strength 
are used (hydrobromic, nitric, &c.), but much more quickly 
when weak acids such as acetic, the chloroacetic acids, formic, 
propionic, levulinic, or lactic acids are employed. Investiga- 
tions were also made as to the influence of the base used; 
comparisons of the velocity of formation of the azo-compound 
between dimethyl-, diethyl-, and dipropyl-aniline show that 
the replacement of methyl by ethyl lowers, and that of ethyl 
by propyl increases, the velocity. Dimethyl- and diethyl-m- 
toluidine combine more rapidly, and dimethyl- and diethyl- 
m-chloroanilines more slowly, than the corresponding un- 
substituted compounds. In the formation of hydroxyazo- 
compounds from phenols and diazo-salts in alkaline solution 
the active agents are the free phenol liberated by the hydro- 
lytic action of the water on the alkali salt and the syn- 
diazo-compound (for an explanation of this see p. 123). An 
excess of alkali retards the combination, and the more con- 

* See Billow, Chemische Technologie der Azo-Farbstoffe, 1897; Pauli, 
Synthese der Azo-Farbstoffe, 1904 ; Cain and Thorpe, Synthetic Dyesiuffs, 
1905, p. 47. 


centrated the solution of the phenol and diazo-salt the more 
time is required for the combination. Similar results were 
obtained when sodium diazo-oxide (see p. 99) was used instead 
of the diazo-salt. 

6- Constitution of the hydroxyazo-compounds. The earlier 
assumptions that the hydroxyazo-compounds possessed the 
formula R.N:N.R.OH were somewhat shaken by the 
discovery that, by the action of phenylhydrazine on a- and 
/3-naphthaquinones, compounds resulted which were identical 
with those obtained by treating a- and ^-naphthol with diazo- 
salts,* and it was found that phenylhydrazine and benzo- 
quinoneoxime yielded a condensation product which was 
identical with p-hydroxyazobenzene, thus 

C 6 H 5 .N 2 .C 6 H 4 .OH C 6 H 4 .NH.N;C 6 H 4 :0 

p-Hydroxyazobenzene. Benzoquinoneplienylhydrazoiie. 

The question was attacked for many years chiefly from the 
purely chemical side. Goldschmidt and his pupils, f McPher- 
Ron,J and Hewitt, expressed the opinion that para-hydroxy- 
azo-compounds were true phenols, and ortho-compounds 
quinones, whilst Jacobson,|| Meldola,1f and Nietzki and 
Kostanecki ** adhered to the view that both series of com- 
pounds possessed the phenolic constitution. 

On the other hand, Farmer and Hantzsch,f f from determin- 
ations of electrical conductivity, and Mohlau and Kegel,J{ from 
the reactions with carbinols, expressed the opinion that both 
ortho- and para-hydroxyazo-compounds in the free state were 
really quinones, the metallic salts, however, being phenolic in 

The views generally held about this time, therefore (1900- 

* Zincke, Ber., 1884, 17, 3026; 1887, 20, 3171 ; compare also Lieber- 
mann, Per., 1883, 16, 2858. 

t Ber., 1890, 23, 487 ; 1891, 24, 2300; 1892, 25, 1324. 

| Ber., 1895, 28, 2414 ; Amer. Chem. J., 1899, 22, 364 ; Auvers, Ber., 
1896, 29, 2361 ; 1900, 33, 1302. 

Trans., 1900, 77, 99, 712. || Ber.. 1888, 21, 414. 

1 Trans., 1888, 53, 460; 1889, 55, 114, 603 ; 1891. 59, 710 ; 1893, 63, 
923 ; 1894, 65, 834. 

** Ber., 1890, 23, 3263 ; 1891, 24, 1592, 3977. 

tt Ber., 1899, 32, 3089. %\ Ber., 1900, 33, 2858. 


1903), were that the metallic salts and alkyl ethers of all 
hydroxyazo-compounds, as well as the acyl-derivatives of 
2>-hydroxyazo-compounds, were true azo- (phenolic) com- 

It was considered, further, that the free ortho-compounds 
were quinonoid in constitution, as were also the acid additive 
products of both series of compounds, for example 


Opinions as to the constitution of the free para-hydroxy- 
compounds were, however, divided, as has been shown. 

From this period onwards a number of researches rather 
tend to show that the phenolic constitution for both the free 
ortho- and para-compounds is to be adopted. 

Thus the properties of m-hydroxyazophenol were found by 
Jacobson and Honigsberger to agree closely with those of the 
para-derivative,* and these chemists concluded that the free 
para-compounds, as well as their additive compounds with 
acids, were to be regarded as azo (phenolic) in constitution. 

This view was confirmed by the researches of Borsche,f 
and Goldschmidt and Low-Beer showed that the earlier 
opinion of Goldschmidt as to the quinonoid character of the 
ortho-compounds had been based on incorrect data, and con- 
cluded that all hydroxyazo-compounds possessed the azo 
(phenolic) structure. 

Most of the later work has confirmed this conclusion,! 
although Tuck, from an examination of the absorption spectra, 
inclines to the view that the free ortho-compounds are quino- 
noid, whilst the para-compounds, their hydrochlorides, and the 
hydrochlorides of the ortho-compounds, are phenolic in 

* Ber., 1903, 36, 4093. 

t Annalen, 1904, 334, 143; 1905, 340, 85; 1907, 357, 171. 

\ Hewitt and Mitchell, Proc., 1905, 21, 298; Mitchell, Trans., 1905, 
87, 1229 ; Willstatter and Veraguth, er. t 1907, 40, 1432 ; Auvers, Ber., 
1907, 40, 2154. 

Trans., 1907, 91, 449; compare also Hewitt and Mitchell, Trans., 
1907, 91, 1251. 


7. Mixed azo-componnds. These are represented by the 
general formula A.N : N.B where A is an aliphatic and B an 
aromatic group. 

The simplest member of this class, namely, benzeneazo- 
methane, is best prepared by treating phenylhydrazine with 
formaldehyde.* It is a yellow, volatile oil, boiling at 150 
with decomposition. 

The first representative of this class, however, prepared 
directly from a diazo-com pound was obtained by V. Meyer 
and Ambuhl ; f by the action of diazobenzene nitrate on sodium 
nitroethane they prepared benzeneazonitroethane 

C e H 6 .N 2 .C 2 H 4 .N0 2 . 

This is probably not a true azo-compound, but a hydrazone- 
derivative having the constitution 

C 6 H 5 .N 2 H : C 2 H 3 .N0 2 . 

Shortly afterwards Friese J described benzeneazonitro- 
methane as being obtained by treating sodium nitromethane 
with diazobenzene nitrate, but Bamberger has shown that 
Friese's compound was nitroformazyl, produced according to 
the equation 

CH 3 . N0 2 -f 2C 6 H 6 . N 2 . OH = N0 2 . 

This behaviour of nitromethane is exceptional, as formazyl- 
derivatives are not produced with the higher homologues of 
nitromethane ; under certain conditions, however, the simple 
compound nitroformaldehydrazone, NO 2 . CH : N 2 H.C 6 H 5 , is 

By the action of diazo-salts on the sodium-derivative of 
acetoacetic ester V. Meyer obtained a compound which was 
first considered to be a true mixed azo-compound, thus 

CH 3 . CO.CH(C0 2 Et) . N 2 . C 6 H 6 ,|| 

but later he considered that, owing to its supposed insolubility 
in alkali, the substance possessed a hydrazone structure 
CH 3 . CO.C(C0 2 Et) : N.NH.C 6 H 5 .1[ 

* Baly and Tuck, Trans., 1906, 89, 986 ; see also Tafel, Ber., 1885, 18, 1742. 
t Ber., 1875, 8, 751, 1053. J Ber., 1875, 8, 1078. 

Ber., 1894, 27, 155 ; 1900, 33, 2043 ; compare also Oddo and Ampola, 
Gazzetta, 1893, 23, i. 257, /3 naphthylazonitroehane. 
|| Ber., 1876, 9, 384 ; 1878, 11, 1418 ; 1884, 17, 1928. 
IT Ber., 1888, 21, 12. 


He adhered to this opinion, although he found that the com- 
pound was really soluble in alkali.* Kjellin also adopted 
this view, and noticed that apparently two isomeric condensa- 
tion products were formed ; these he considered to be stereo- 
isomeric hydrazones, thus 

CH 3 . CO.C.C0 2 Et CH 3 . CO.C.COJEt 


C 6 H 5 . NH.N N.NH.C 6 H 5 f 

Billow, however, showed that not only was the compound 
very readily soluble in alkali, but that it could not be acetyl- 
ated and was not acted on by benzoyl chloride or methyl iodide, 
so that the original theory of V. Meyer was correct, and the 
compound must be regarded as the true azo-derivative 

CH 3 . CO.CH(C0 2 Et).N : N.C 6 H 5 .J 

Biilow obtained the same compound from diazobenzene 
chloride and sodium benzene-iso-diazo-oxide. 

By the action of a diazo-salt on the sodium-derivative of 
ethyl methylacetoacetate, the acetyl-group is eliminated and 
the compound was written CH 3 . CH(CO 2 Et).N:N.C 6 H 5 ,|| 
the corresponding acid being CH 3 . CH(C0 2 H).N 2 . C 6 H 5 . 
This, however, was found to be identical with the condensa- 
tion product formed by the action of phenylhydrazine on 
pyroracemic acid, CH 3 . C(C0 2 H) : N.NH.C & H 5< H 

In order to decide which of these formulae was correct 
Japp and Klingemann** treated the so-called benzeneazo- 
acetone ff with sodium ethoxide and ethyl chloroacetate, and 
reduced the corresponding acid. The substance obtained was 
anilinoacetic acid, proving that the . CH 2 . CO 2 H group had 
combined with the nitrogen atom attached to the phenyl- 
group. The second formula for benzeneazoacetone is there- 

* Ber., 1888, 21, 2121. 

t Ber., 1897, 30, 1965; compare also Favrel, Compt. rend., 1901, 132 ; 
983 ; 1898, 127, 116. 
t Ber., 1899, 32, 197. 
Ber., 1898, 31, 3122. 

|| Japp and Klingemann, Ber., 1887, 20, 2942, 3284, 3398. 
IF E. Fischer and Jourdan, Ber., 1883, 16, 2241. 
** Trans., 1888, 53, 521, 538. 
tt V. Meyer andMiinzer, Ber., 1878, 11, 1417, 

CH 8 . CO.CH 2 . N 2 . C 6 H 5 or CH 8 . CO.CH : N.NH.C 6 H 5 . 


fore the correct one, and the compound is really a mono- 
hydrazone of pyruvic aldehyde. This was also confirmed by 
the fact that it yields an osazone with phenylhydrazine. 

This conclusion is, however, not to be applied universally, 
for when diazobenzene chloride acts on the sodium derivative 
of acetoacetaldehyde, benzeneazoacetaldehyde, 
CH 3 . CO.CH(CHO).N 2 . C 6 H 5 , 

is formed, and this, with phenylhydrazine, yields the corre- 
sponding hydrazone 

CH 3 . CO.CH(CH:N.NH.C 6 H 5 ).N 2 . C 6 H 5 .f 

When diazobenzene chloride acts on acetonedicarboxylic 
acid in presence of sodium acetate, the bishydrazone of 
mesoxalaldehyde, CO(CH:N.NH.C 6 H 5 ) 2 , is obtained.! If, 
however, the ethyl ester of acetonedicarboxylic acid is sub- 
jected to the action of >-nitrodiazobenzene, a compound is 
obtained which reacts partly as a hydrazone and partly as an 
azo-derivative. This is therefore to be regarded as contain- 
ing a labile hydrogen atom, indicated by a star in the 


N0 2 .C 6 H 4 .N.N.C.C0 2 Et 

CO.CH 2 .C0 2 Et. 

When a hydrazone such as is described above, in which the 
group C 6 H 5 .NH.N:C: is combined with H, CO 2 H, or COR, 
each of the latter groups can be replaced by the action of 
diazobenzene in alkaline or acetic acid solution. Thus, by 
the action of diazobenzene on malonic ester, V. Meyer and 
Miinzer|| also obtained a condensation product which they 
regarded as 

t Claisen and Beyer, Ber., 1888, 21, 1697 ; compare also Ber., 1892, 
25, 3190. 

I v. Pechrnann and Jenisch, Ber., 1891, 24, 3255 ; v. Pechmann and 
Vanino, Ber., 1892, 25, 3190. 

Billow and HSpfner, Ber. y 1901, 34, 71 ; compare also Billow and 
Hailer, Ber., 1902, 35, 915. 

II Loc. cit. 


but which is now considered to be the phenylhydrazone of 
malonic ester 

If the acid itself is used, formazylcarboxylic acid and formazyl 
result t 

C 6 H 6 . NH.N : C<g&- g 6 ^ C 6 H 5 . NH.N : C /JJ* C H 

Formazylcarboxylic acid. Formazyl. 

The latter is also formed by the action of diazobenzene on 
the ethyl hydrogen salt of phenylhydrazonemalonic acid. 

By the further action of diazobenzene on formazyl, or its 
carboxylic acid, formazylazobenzene 

is obtained, as well as by allowing diazobenzene and acet- 
aldehyde to interact in alkaline solution. J 

These compounds are usually dark red, crystalline sub- 
stances, and on reduction they give colourless hydrazones. 

Formazylcarboxylic acid is also obtained by the action of 
diazobenzene on ethyl acetoacetate under certain conditions ; 
the first product of the reaction being the hydrazone 

f! "FT 
6 tl 

By the action of diazobenzene on ethyl oxalacetate the 
hydrazone, C 6 H 6 .NH.N:C (C0 2 Et).CO.C0 2 Et, is first formed, 
and with a further molecule of diazobenzene, ethyl di- 
phenylformazylformate, C 6 H 6 .N:N.C(N.NH.C 6 H 5 ).C0 2 Et, is 

Further confirmation of the hydrazone constitution of such 
condensation products is afforded by the action of diazo- 

* Compare also Favrel, Compt. rend., 1899, 128, 829 ; 1901, 132, 1336. 
t Compare also Busch. and Wolbring, J. pr. Chem., 1905 [ii], 71, 366. 
t Bamberger and Miiller, Ber., 1894, 27, 147. 
v. Pechmann, Ber., 1892, 25, 3175. 
|| Bamberger, Ber., 1892, 25, 3201, 3539, 3547. 

IT Rabischong, Bull. Soc. chim., 1904 [iii], 31, 76, 83 ; compare also 
Favrel, Compt. rend., 1899, 128, 318. 


benzene on ethyl cyanoacetate whereby the phenylhydra- 
zone of the latter is produced.* 

The question of the constitution of the mixed azo-compounds 
cannot, however, be said to be finally settled by fixed rules. 
Some appear to be tautomeric substances, others true hydra- 
zones, and many undoubtedly true azo-compounds. In all 
probability the course of the reaction depends on the constitu- 
tion of the diazo-compound used, the discussion of which is 
postponed until later (see p. H2).f 

* Kriickeberg, J. pr. Chem., 1892 [ii], 46, 579, 47, 591, 49, 321; 
compare also Uhlmann, ibid. 51, 217 ; Marquardt, ibid. 52, 160 ; Favrel, 
Compt. rend., 1900, 131, 190; 1907, 145, 194. 

t For mixed bisazo-compounds, see Duval, Compt. rend., 1907,144, 1222. 



BY adding a cold saturated solution of diazobenzene nitrate 
to a large excess of concentrated aqueous potassium hydroxide, 
evaporating the resulting yellow liquid, and extracting with 
alcohol, Griess obtained a substance containing potassium, to 
which he gave the formula C 6 H 5 N 2 . OK. 

When this compound was treated with acetic acid a yellow 
oil was obtained which Griess regarded as free diazobenzene, 
C 6 H 4 N 2 . 

The potassium compound was examined by Curtius,* who 
found, however, that it contained only two-thirds of the 
nitrogen required by the above formula. Also when diazo- 
benzene sulphate was neutralized with barium hydroxide and 
the mixture extracted with ether, a yellow substance was 
obtained which melted at 3. This contained only two atoms 
of nitrogen to three benzene nuclei, although no nitrogen had 
been evolved as gas. 

Griess's product was shown later by Bamberger to consist 
chiefly of the ^so-salt. 

A very important addition to the chemistry of the metallic 
diazo-derivatives was made by Schraube and Schmidt in 
1894.J- These chemists found that when a 10 per cent, 
solution of >-nitrodiazobenzene chloride was quickly added to 
an 18 per cent, solution of sodium hydroxide at 50-60, golden- 
yellow plates separated which no longer combined with 
/3-naphthol, and which they considered to be sodium _p-nitro- 
phenylnitrosoamine, N0 2 . C 6 H 4 . NNa.NO. 

When an ice-cold, aqueous solution of this was treated with 

* Ber., 1890, 23, 3035. t Ber., 1894, 27, 514. 


acetic acid, a pale yellow precipitate was obtained which was 
regarded as the free ^-nitrophenylnitrosoamine, 

N0 2 .C 6 H 4 .NH.NO; 

this, like the sodium salt, did not couple with alkaline 
jS-naphthol. When hydrochloric acid was substituted for acetic 
acid, jo-nitrodiazobenzene chloride was slowly regenerated. 

On treatment with methyl iodide, the sodium salt gave the 
nitrosoamine of ^-nitromonomethylamline 

NO a .C 6 H 4 .NMe.NO. 

Schraube and Schmidt also investigated the properties of 
Griess's potassium salt, and showed that it differed from the 
compound described by them in that it coupled with alkaline 
/S-naphthol. On being heated with concentrated aqueous 
potassium hydroxide, however, it was converted into the 
potassium salt of phenylnitrosoamine, C 6 H 5 . NK.NO, which 
no longer combined with /3-naphthol, and with methyl iodide 
gave the nitrosoamine of methylaniline. This potassium salt, 
on neutralization with acetic acid, gave a colourless oil which 
combined with /?-naphthol solution, and on adding an excess of 
acetic acid to the oil, a solution of diazobenzene acetate was 

From this work Schraube and Sonmidt drew the following 
conclusions : 

(1) The alkali salts of diazobenzene can exist in two forms, 
namely, the diazo-metallic salts, C 6 H 5 . N : N.OR, and the 
nitrosoamines, C ft H 5 . NR.NO (R denoting the metal). 

(2) Free phenylnitrosoamine, C 6 H 5 . NH.NO, does not exist, 
as a solution of its non-combining sodium salt, when acidified 
with acetic acid, immediately gives an azo-dyestuff with 

(3) jo-Nitrodiazobenzene appears to exist only in the 
nitrosoamine form, and its alkali salt does not exist in the 
' oxime ' condition. 

These conclusions, as will be shown, do not truly represent 
the course of the reactions (see p. 98). 

In a paper published shortly after the appearance of 
Schraube and Schmidt's work, Bamberger stated that he 
had earlier discovered a derivative of /3-naphthylamine cor- 



responding to the formula C 10 H 7 .NH.NO, which did not 
couple with /3-naphthol, but did so after treatment with a 
mineral acid.* 

He considered that the influence of the latter was to effect 
a transformation into the isomeric diazo-compound, thus 
C 10 H 7 .NH.NO -> C 10 H 7 .N:N.OH. 

Confirmation of this view was afforded by von Pechmann 
and Frobenius, who stated that the silver salt of p-nitro- 
phenylnitrosoamine, when treated with methyl iodide, yielded 
an oxygen-ether of j9-nitrodiazobenzene 

N0 2 .C 6 H 4 .N:N.O.CH 3 . 

The conclusion is emphasized, therefore, that the hydroxide 
corresponding with these compounds exhibits the phenomenon 
of tautomerism.f That is to say, that the hydroxide can 
act either as 

N0 2 . C 6 H 4 . N ; N.OH or NO 2 . C 6 H 4 . NH.NO. J 

It was thus established that two isomeric forms of the 
metallic diazo-compounds exist ; the modification described by 
Schraube and Schmidt may be called the stable or iso- 
modification, and the labile or normal form is that which 
couples with phenols much more readily than its isomeride. 

Most of the metallic diazo-compounds exist in these two 
modifications, but the presence of negative groups in the 
aromatic nucleus greatly diminishes the stability of the 
normal modification. Although, for the purpose of defining 
these isomeric compounds, it has been necessary to mention 
the constitutions which were assigned to them at the time of 
their discovery, the subsequent developments of the views on 
this subject have been so extensive, and the discussion so 
prolonged, that an account of this must be postponed. We 

* Ber., 1894, 27, 679. 

t Ber., 1894, 27, 672 ; compare also Bamberger, ibid., 679. 

j Although Hantzsch would not accept the views of Bamberger and 
von Pechmann, yet he arrived at this conclusion from his own work 
some years later (p. 151). 

The presence of alkali has a great effect on the combining power 
of the two isomerides : Schraube and Schmidt had a large excess of alkali 
present when they noticed that the stable form did not combine, but 
when less is used it does combine, although much more slowly than the 
labile form. 


shall therefore proceed to a description of some of the more 
important compounds of this class. 

Potassium benzenediazo-oxide (normal, labile, or syn-soli). 
10 c.c. of a 15 per cent, solution of diazobenzene chloride 
are dropped slowly into a mixture of 140 grams of potassium 
hydroxide and 60 grams of water cooled to 5. The tempera- 
ture is allowed to rise to 15-20, whereby the potash becomes 
completely dissolved, and the precipitated potassium benzene- 
diazo-oxide is collected. This is pressed on porous porcelain. 
One gram of the crude product is now dissolved in 3 c.c. of 
absolute alcohol at 5, the solution quickly filtered, and 8-10 
times its volume of dry ether added. The salt is obtained in 
this way in snow-white, silky needles, which are very hygro- 
scopic and soon become pink.* 

s^/Ti-Diazo-oxides are also obtained by treating nitroso- 
acylanilides with potassium hydroxide,! 

Ar.N(NO)Ac + 2KOH = Ar.N : N.OK + KOAc + H 2 0, 
and by the reduction of salts of diazoic acid, 

Ar.N 2 O.OK + 2H = H 2 O + Ar.N : N.OK. 

When the normal salts are treated with acids the correspond- 
ing hydroxides are not formed, but the yellow, explosive 
diazo-anhydrides are produced (see p. 148). 

Potassium beuzenediazo-oxide (iso-, stable, or anti-salt). 
This is obtained by heating the strongly alkaline solution of 
the diazo-chloride to 130-140 until the product no longer 
combines with /S-naphthol (compare p. 98). 

Another method of preparing the iso-metallic compounds 
consists in treating o- or ^9-hydroxybenzylated nitrosoaryl- 
derivatives of the type NO.NAr.CH 2 . C 6 H 4 . OH with very 
dilute aqueous potassium hydroxide 

OH.C 6 H 4 . CH 2 . N(C 6 H 5 ).NO + KOH 

= OH.C 6 H 4 . CH 2 .OH + C 6 H 5 . NK.NO. 

The potassium tso-diazo-oxide is formed together with 
hydroxybenzyl alcohoLJ 

* Bamberger, Ber., 1896, 29, 461. 
t Bamberger, Ber., 1894, 27, 915. 
| Bamberger and Muller, Annalen, 1900, 313, 97. 

H 2, 


iso-Diazo-oxides are also formed by the reduction of iso- 
diazoic acids, and by heating secondary nitrosoamines with 
potassium hydroxide.* 

When the potassium ^so-diazo-oxides are treated with acetic 
or mineral acids, the corresponding hydroxides are obtained, 
which, with alkalis, regenerate the metallic iso-salt. 

The hydroxides of m>-diazobenzene, ^so-diazo-p-toluene, 
p-chloro- and p-bromo-^so-diazobenzene, a- and /3-^so-diazo- 
naphthalene, and potassium ^so-diazobenzenesulphonate are 
all colourless, whilst the hydroxides of o- and _p-nitro-^so- 
diazobenzene are yellow, f 

^so-Diazobenzene hydroxide is a colourless oil which is 
readily soluble in ether. It is very unstable. 

The hydroxides of the remaining substances mentioned 
above are white crystalline solids. J 

The action of alkalis on diazo-salts sometimes, however, 
proceeds differently. Thus 2:4:6-tribromodiazobenzene gives 

rise to 3:5-dibromo-o-benzoquinonediazide 



(see p. 67), and o-diazotoluene furnishes indazole 

C 6 H/ 

(see p. 31). 

Diazobenzene hydroxide (diazonium hydroxide). This 
hydroxide does not correspond with either of the two fore- 
going potassium salts according to Hantzsch, although 
Bamberger regards it as the hydroxide derived from the 
labile salt (see p. 144). 

For the preparation, 0-7 gram of pure diazobenzene chloride 
is dissolved in about 50c.c. of ice-cold water, and about 

* Bamberger, Ber., 1894, 27, 1179; Ber., 1900, 33, 1957. 
t Bamberger, Ber., 1896, 29, 446. 
| Bamberger, Ber., 1896, 29, 1383. 
Bamberger, Annalen, 1899, 305, 289. 


0-8 gram (the theoretical amount is 0*62) of freshly-precipi- 
tated moist silver oxide mixed with ice, added, and the whole 
shaken for five minutes. The filtrate consists of a practically 
pure solution of the hydroxide. 

This solution has a strongly alkaline reaction, and combines 
instantly with /3-naphthol. The pure solution is colourless. 
The hydroxide is also obtained by treating a solution o;f the 
diazo-sulphate with the calculated amount of baryta.* The 
solution is very unstable, even at it decomposes and be- 
comes dark coloured. 

Other diazo-hydroxides are prepared in a similar manner. 

Reduction of the metallic diazo-oxides. Both series of 
diazo-oxides, when treated with sodium amalgam in presence 
of excess of sodium hydroxide, yield the corresponding 

Oxidation of the metallic diazo-componnds. Aromatic 
diazoic-acids. When an alkaline solution of diazobenzene is 
oxidized by potassium ferricyanide, a substance is obtained 
to which the name benzenediazoic acid is given 

C 6 H 5 .N 2 2 H. 

It is formed in white leaflets, melting at 46-46-5, sparingly 
soluble in water, but readily so in organic solvents or alkalis. 
It forms well-defined salts.! 

Potassium permanganate may also be used as the oxidizing 

A better yield is obtained by oxidizing potassium benzene- 
iso-diazo-oxide with potassium ferricyanide.|| The compound 
may also be prepared by treating diazobenzene perbromide 
with sodium hydroxide,^ or by the action of nitrogen pen- 
toxide on aniline in ethereal solution at 20.*"* 

When benzenediazoic acid is slowly heated, or when it is 

* Hantzsch, Ber., 1898, 31, 340. t Hantzsch, Ber., 1898, 31, 340. 
I Bamberger and Storch, Ber.., 1893, 26, 471 ; Bamberger, ibid., 1894, 
27, 359. 

Bamberger and Landsteiner, Ber., 1893, 26, 482. 
|| Bamberger, Ber., 1894, 27, 914. 
IT Bamberger, Ber., 1894, 27, 1273. 
** Bamberger, Ber., 1894, 27, 584. 


treated with mineral acids, it undergoes intramolecular change 
with formation of a mixture of o- and ^-nitroaniline. 

When heated with potassium hydroxide to about 290 it is 
decomposed into aniline and nitric or nitrous acid. By gentle 
reduction with zinc and acetic acid, diazobenzene is formed, 
and with sodium amalgam, phenylhydrazine is produced.* 

On account of the conversion into nitroaniline, Bamberger 
regarded benzenediazoic acid as the simplest aromatic nitra- 
mine, or phenylnitramine, C 6 H 5 .NH.N0 2 , and represented 
the change into nitroaniline as follows 
NH.N0 2 NH 

The proof of this constitution was found in the study of the 
action of hypochlorite on the diazoic acid, for a chloro- 
derivative was obtained which gave the characteristic re- 
actions of a chloroimide, and underwent molecular change 
even more readily than the parent compound, forming p-chloro- 

The constitution of the chloro-compound is therefore 
C 6 H 5 .NCl.N0 2; t 

Benzenediazoic acid forms two methyl esters ; with methyl 
iodide the sodium salt gives the a-ester, C 6 H 5 . NMe.NO 2 , and 
the silver salt yields the /3-ester, C 6 H 6 .N:NO 2 Me. 

Benzenediazoic acid is therefore, as Bamberger had shown 
to be the case with diazobenzene hydroxide, a tautomeric 
substance, thus 

C 6 H 5 .NH.N0 2 ^ C 6 H 5 .N:N0 2 H. 

This conclusion, after a considerable amount of discussion,! 
was confirmed by Hantzsch, who showed that the compound 
reacted as a pseudo-acid. 

* Ber., 1894, 27, 365. f Ber., 1894, 27, 361. 

I Bamberger, Ber., 1894, 27,2601 ; 1897, 30, 1248 ; Annalen, 1900, 311, 
99; Hantzsch, Ber., 1894, 27, 1729; 1898, 31, 177 ; 1899, 32, 1722. 
Ber., 1902, 35, 258. 



1. Preparation. The amines of the aliphatic series do 
not react with nitrous acid as do those of the aromatic series. 
Only in certain cases is there a departure from the usual 
reaction of substitution of the amino- by the hydroxyl-group, 
and then the product has not, as might be expected, a compo- 
sition similar to that of an aromatic diazo-salt, but the nitrogen 
atoms are each linked to the aliphatic nucleus, thus 


The reason of this difference in behaviour will be explained 
in the discussion of the constitution of the aromatic diazo-salts 
(see p. 167). 

The first number of the series was obtained by SchifF and 
Meissen in 1881, who prepared diazocamphor from camphor- 
imide.f This diazo-compound was also obtained by Angeli by 
the action of nitrous acid on aminocamphor.J 

The principal worker in this field of research is, however, 
Curtius, who, a little later, succeeded in diazotizing the ethyl 
ester of glycocoll, or ethyl aminoacetate, a reaction which 
proceeds in two stages ; the first stage is the formation of the 
nitrite of the aliphatic amine, 

HC1.NH 2 . CH 2 . C0 2 Et + NaN0 2 

= NaCl + HN0 2 . NH 2 . CH 2 . C0 2 Et, 

and this then loses water with production of the diazo- 

HNO 2 . NH 2 . CH 2 . C0 2 Et = 2H 2 O + N 2 : CH.CO 2 Et. 

* Compare also Cuiiius^azoverbindungen der Fettreihe, 1886; Demetre 
Vladesco, Sur les composts diazoi'ques de la serie grasse, 1891. 
t Gazzetta, 1881, 11, 171. 

t See also Angeli, Ber., 1904, 37, 2080, footnote. 
Ber., 1883, 16, 2230; J. pr. Chem., 1888 [ii], 38, 401. 


The free fatty acids do not yield diazo-derivatives, as these are 
immediately decomposed, so that the esters, amides, &c., must 
be used. The constitution of diazoacetic ester is proved by (1) 
the ready substitution of the two atoms of nitrogen by two 
atoms of iodine, yielding di-iodoacetic ester, 

/ N 


| X N+I,=| +N 2 
CO 2 Et COgEt 

and (2) the reduction to ammonia and glycocoll 

CH< || 

CH 2 .NH 2 

X N+3H 2 = | +NH 3 

C0 2 Et C0 2 Et 

Similarly diazosuccinic acid yields ammonia and aspartic acid 

CO 2 Et . C< || CO,H . CH.NH 

CH 2 .C0 2 H CH 2 .C0 2 H 

The preparation of diazoacetic ester is carried out as follows* : 
Five grams of sodium acetate are dissolved in two litres of 
water in a ten-litre separating funnel ; to this solution one 
kilo of the finely powdered hydrochloride of ethyl aminoacetatef 
is added, and then 750 grams of sodium nitrite. The mixture 
is shaken until the temperature has fallen to about 0. Five 
c.c. of ten per cent, sulphuric acid and half a litre of ether are 
then added and the whole again well shaken. During this 
period the gradual solution of the still undissolved salts cools 
the mixture and prevents the reaction from becoming too 
violent. As soon as the action slackens, the ethereal solution 
of ethyl diazoacetate is run off, fresh ether added, and ten 
per cent, sulphuric acid run in from time to time in small 
quantities until red fumes are evolved. The ethereal solution 
is then run off, added to that already obtained, washed with 
small quantities of dilute sodium carbonate solution until the 
washings assume a deep yellow colour and have an alkaline 

* Silberrad, Trans., 1902, 81, 600. 

t For the preparation of this compound see Hantzsch and Silberrad, 
Ber., 1900, 33, 70. 


reaction. The ethereal solution is dried by shaking with fused 
calcium chloride, and freed from ether on the water-bath. 
The yield amounts to 770 grams, or 947 per cent, of the 
theoretical quantity. 

2. Properties of diazoacetic esters. The esters of diazo- 
acetic acid are liquids which solidify at very low temperatures. 
They are citron -yellow, and possess a characteristic odour. On 
being warmed to 100 the colour changes to a deep orange j 
but, on cooling, the original .colour reappears. The esters boil 
without decomposition under very low pressures, and even at 
the ordinary pressure by quick distillation over the free flame 
the greater part of the liquid passes over unchanged ; the rest 
decomposes with slight detonation, and forms .a thick, white 

The ethyl ester is extraordinarily volatile, and rapidly 
vaporizes in a vacuum over sulphuric acid. 

The esters distil mostly unchanged with steam, the volatility 
increasing with the weight of the ester radical, whilst the 
solubility in water at the same time decreases. 

The diazo -compounds of the fatty esters are miscible in all 
proportions with alcohol, ether, benzene, light petroleum, &c. 

Methyl diazoacetate, N 2 : OH.C0 2 . CH 3 , boils at 129 
under a pressure of 721 mm. Its sp. gr. is 1'139 at 21. It is 
moderately soluble in water, and has a neutral reaction. 

Ethyl diazoacetate, N 2 :CH.CO 2 . C 2 H 5 , crystallizes in a 
mixture of ether and solid carbon dioxide to a crystalline 
mass, which melts at -24. It boils at 143-144 under 721 mm. 
pressure, and its sp. gr. is 1-073 at 22. On gentle warming it 
takes fire and burns quickly with a luminous flame. It does 
not explode by concussion, but on adding concentrated sul- 
phuric acid a violent explosion occurs ; this also takes place on 
heating it with certain organic nitro-compounds such as 

The ester is sparingly soluble in water, and has a neutral 
reaction. By heating diazoacetic ester near its boiling-point 
nitrogen is evolved, and finally fumaric ester remains 

2 N 2 : CH.C0 2 Et = 2N 2 + C 2 H 2 (C0 2 Et) 2 . 


As an intermediate product in the reaction there is formed 
azinsuccinic ester * 

CH.C0 2 Et 

Amyl diazoacetate, N 2 : CH.CO 2 . C 6 H n , boils at 160 under 
a pressure of 721 mm. It is insoluble in water, and has a 
neutral reaction. 

When diazoacetic esters are mixed with aqueous potassium 
hydroxide or baryta water the corresponding metallic salts 
are formed. These are stable, however, only in cold, aqueous 
solution, and Curtius was unable to isolate them or to prepare 
the free acid by treatment with mineral or organic acids. 
Thiele, however, by another method, succeeded in preparing 
the pure sodium salt (see p. 109). 

With concentrated aqueous ammonia, diazoacetic esters 
yield diazoacetamide, N 2 ; CH.CO.NH 2 , which crystallizes from 
warm alcohol or water in large, gold-yellow, transparent 
prisms. These crystals melt at 114 with copious evolution 
of gas. 

3. Reactions of the aliphatic diazo-compounds, The 
reactions of the fatty diazo-compounds are very similar to those 
of the aromatic ; thus, with water, nitrogen is evolved, and the 
corresponding hydroxyester produced. 

The reaction in the case of ethyl diazoacetate has been 
quantitatively studied by Fraenkel.f As in the case of the 
diazo-compounds of the aromatic series the reaction is 
unimolecular, and the rate is proportional to the concentration 
of the hydrogen ions, these exerting a catalytic influence. 

The presence of neutral salts destroys the regularity of the 
decomposition and introduces secondary reactions. 

The other decompositions of diazoacetic ester are as follows 

With alcohol it yields ethylglycollic ester. 

With picric acid it yields picrylglycollic ester. 

With benzaldehyde it yields benzoylglycollic ester. 

* Curtius, Ber., 1885, 18, 1302. 

t Zeitsch. physikal. Chem., 1907, 60, 202. 


With hydrochloric acid it yields monochloroacetic ester. 
With iodine it yields di-iodoacetic ester. 
A concentrated solution of hydrofluoric acid yields with 
diazoacetic ester, diglycollic ester 

2CH : N 2 . C0 2 R + H 2 = O* ' ** + 2N 

On reduction, diazoacetic esters yield the original amino- 
compound, a hydrazine being formed as intermediate product, 

N 2 : CH.C0 2 R + 2H 2 = NH 2 . NH 2 . CH 2 . C0 2 R. 

A very singular reaction takes place with concentrated 
aqueous alkalis. An acid is produced having the same com- 
position as diazoacetic acid, but possessing a greater molecular 
weight. This was considered by its discoverer * to be triazo- 
acetic acid, composed of three molecules of diazoacetic acid, 
but it was later shown that the substance was really bis- 
diazoacetic acid 

With potassium sulphite, diazoacetic ester gives the potas- 
sium salt of ethyl sulphohydrazimethylenecarboxylic acid 

C0 2 Et.CH< | 

x N.S0 3 Kt 

When ethyl diazoacetate is treated with potassium or 
sodium ethoxide, the corresponding salt 


of ethyl iso-diazoacetate 

is obtained. This ester is an oil which does not dissolve in 

* Curtius, J. pr. Chem., 1889 [ii], 39, 116. 

t Hantzsch and Silberrad, Ser., 1900, 33, 58; compare also Curtius, 
Darapsky, and Miiller, Ser., 1907, 40, 1176, 1194. 
I Von Pechmann, Ber., 1895, 28, 1847. 


water, and, unlike the diazoacetate, does not form an additive 
compound with sulphites.* 

The simplest diazo-compound of the aliphatic series, namely, 


was prepared by von Pechmann in 1894.f This substance, 
which is a yellow gas, is obtained by warming nitrosoacyl- 
derivatives of methylamine, CH 3 .NAc.NO, with aqueous or 
alcoholic alkalis. One part by volume of nitrosourethane 
(from 1 Ibo 5-c.c.), together with 30-50 e.c. of pure ether, and 
1-2 parts by volume of 25 per cent, methyl-alcoholic potassium 
hydroxide, are warmed in a small flask fitted with a reflux 
condenser on the water-bath. J The mixture becomes yellow, 
and a yellow gas is evolved. The heating is continued until 
the solution becomes colourless. 

The distillate consists of an ethereal solution of diazo- 
methane, the yield of which is about 50 per cent, of the 

Diazomethane is also obtained by the interaction of 
hydroxylamine and dichloromethylamine 

CH 3 . NC1 2 + H 2 N.OH = 2HC1 + H 2 + CH 2 N 2 . 

The disulphonic acid of diazomethane 

/ N 

(S0 3 H) 2 C< || 

X N 

is obtained in a remarkable manner. When potassium cyanide 
is treated with an aqueous solution of potassium bisulphite in 
presence of potassium hydroxide, and the product acidified, 
aminomethanedisulphonjc acid results, which, on treatment 
with nitrous acid, furnishes the corresponding diazo- 

* Hantzsch and Lehmann, Set:, 1901, 34, 2506. 

t Ber., 1894, 27, 1888. J Ber., 1895, 28, 855. 

Bamberger and Renauld, Ber., 1895, 28, 1682 ; compare also Thiele 
and Meyer, Ber., 1896, 29, 961. 

|| Von Pechmann and Manck, Ber., 1895, 28, 2374; von Pechmann, 
Ber., 1896, 29, 2161. 


Phenyldiazomethane is obtained by decomposing potassium 
benzyldiazo-oxide (p. 110) with water * 

CH . CH . N : N.OK = CH . CH || + HO 

It is a dark red-brown oil which has only a faint odour, and 
is slightly volatile. It decomposes when rapidly heated. 
When distilled under the ordinary pressure it is mostly 
decomposed with formation of stilbene 

2C 6 H 5 . CH : N a = C 6 H 5 . CH : CH.C 6 H 6 + 2N 2 . 

When warmed with water it yields benzyl alcohol, and in its 
other reactions it resembles diazomethane. 

Another interesting method of obtaining aliphatic diazo- 
compounds is that due to Traube, which consists in treating 
the sodium or lead salt of iso-nitraminoacetic acid, 

H0 2 N*.CH 2 .C0 2 H, 

with sodium amalgam at 0. Reduction takes place, and the 
sodium salt of diazoacetic acid is produced.f A metallic salt 
of diazoacetic acid was thus isolated in the pure state for the 
first time. 

By treating aminoguanidine nitrate with nitrous acid it 
was thought that the corresponding diazoguanidine nitrate 
was formed, J but this was later shown to be a derivative of 

NH 2 .C(NH).N 

4. Metallic diazo-componnds of the aliphatic series. 

The compounds of this class were obtained by Hantzsch and 
Lehmann || by treating nitrosoalkylurethanes with concen- 
trated potassium hydroxide solution or ethereal potassium 
ethoxide, thus 

* Hantzsch and Lehmann, Ber., 1902, 35, 897. 
t Ber., 1896, 29, 667. 

t Thiele, Annalen, 1892, 270, 1; E. P. 2194 of 1892; Thiele and 
Osborne, Ber., 1897, 30, 2867 ; Annalen, 1899, 305, 64. 
Hantzsch and Vagt, Annalen, 1901, 314, 339. 
|| Ber., 1902, 35, 897. 


CH 3 . N<^ pQ Tji, 

. N : N.OK + EtOH + KEtCO 

These salts are highly unstable ; with water they decompose 
with explosive violence. The metallic salt obtained from 
nitrosomethylurethane forms diazomethane, and that derived 
from nitrosobenzylmethane gives phenyldiazomethane. 

Potassium methyldiazo-oxide, CH 3 . N :N.OK + H 2 O } separ- 
ates in white crystals when nitrosomethylurethane is gradually 
added to a very concentrated aqueous solution of potassium 
hydroxide at 0. As excess of alkali is used, the reaction 
proceeds according to the equation 

= K 2 CO a + C 2 H 5 . OH + CH 3 . N : N.OK, H 2 O. 

Potassium benzyldiazo-oxide, C 6 H 5 . CH 2 . N : N.OK + H 2 0, 
is obtained in a similar manner from nitrosobenzylurethane. 

On decomposition with water it yields, as chief products, 
phenyldiazomethane and potassium hydroxide, and as second- 
ary products, benzyl alcohol and nitrogen, thus 


* 66 
\C R H B .CH,. OH + N 

It is found that only esters of a-amino-acids yield diazo- 
esters ; /3- and y-amino-esters, on the other hand, do not form 
diazo-compounds, and an a/S-diamino-ester therefore yields 
an a-diazo-/3-hydroxy-ester. 

Further, only fatty compounds in which the ammo-group, 
carbonyl, and at least one hydrogen atom are attached to the 
same carbon atom, yield diazo-compounds with nitrous acid;* 
thus, for example, diazoacetophenone is obtained by adding 
sodium nitrite solution to an aqueous solution of the hydro- 
chloride of aminoacetophenone, and then dropping acetic acid 
into the cold solution. A solid substance separates, which is 

* Curtius and Miiller, Ber., 1904, 37, 1261 ; compare also Angeli, Ber., 
1904, 37, 2080. 


washed with sodium carbonate solution and crystallized from 
light petroleum. Yellow needles are obtained, possessing the 

,N * 

C 6 H 5 .CO.CH<(|| 
N N 

5. Diazoamino-compouiids of the aliphatic series. The 

simplest representative of the aliphatic diazoamino series, 
namely, diazoaminomethane, is obtained by the action of 
magnesium methyl iodide on methylazoimide f and decom- 
position of the resulting compound with water. { 
Its formation is represented by the equations 

CH 3 . Mgl + CH 3 . N 3 = CH 3 . N : N.N(CH 3 ).MgI 

CH 8 . N : N.N(CH 3 ).MgI + H 2 O 

= CH 3 .N : N.NH.CH 3 + MgI(OH). 

Diazoaminomethane is a colourless liquid, boiling at 92, 
which solidifies when immersed in a mixture of solid carbon 
dioxide and ether; the solid melts at 12. When heated 
quickly it explodes, and it decomposes with acids according to 
the equation 

N 3 (CH 3 ) 2 H -f- 2HC1 = CH 3 C1 + N 2 + NH 2 . CH 3 , HC1. 

* Angeli, Ser., 1893, 26, 1715 ; Gazzetta, 1895, 25, ii, 494. 
t Prepared by treating sodium azoimide with methyl sulphate (Dimroth 
and Wislicenus, Ber., 1905, 38, 1573). 
I Dimroth, Ber., 1906, 39, 3905. 



ON account of the extraordinary controversy which has 
raged round the subject of the constitution of the diazo- 
compounds, it has appeared more advisable to deal with this 
question separately and more fully than would otherwise 
have been possible. 

As will have been gathered from the account already given, 
the first question calling for attention is that of the constitution 
of the diazo-salts, and then naturally follows that of the two 
classes of isomeric metallic diazo-derivatives. 

1. Constitution of the diazo-salts according to Griess. 

The first attempt to assign a constitutional formula to a 
diazo-salt was made by Griess, who gave to diazobenzene 
nitrate the formula C 6 H 4 N 2 , HNO 3 * 

Griess considered that in a diazo-compound two atoms of 
hydrogen of the benzene nucleus were substituted by two 
atoms of (monoatomic) nitrogen. 

In 1859 Wurtz f suggested that each atom of nitrogen was 
tervalent, and that a bivalent group N 2 " was present. 

Erlenmeyer J and Butleroff developed this idea and gave 
to diazobenzene the formula 


which Griess adopted. || 

The idea that two hydrogen atoms of the benzene nucleus 
were substituted by nitrogen was still present. 

Griess also, in the same year, considered that a possible 
formula for diazobenzene nitrate was C 6 H 4 :N|N, HN0 3 . 

* Phil. Trans., 1864, 154, 667. 

t Eepert. de Chimie pure, 1858-9, 1, 348. 

| Zeitsch.f. Chem., 1861, 176 ; 1863, 678. 

Zeitsch.f. Chem., 1863, 511. || Ber., 1874, 7, 1618. 


About this time Griess discovered that the diazoamino- 
compound obtained from aniline and bromodiazobenzene 
nitrate was identical with that prepared from bromoaniline 
and diazobenzene nitrate, and therefore put forward for 
diazoaminobenzene the symmetrical formula 

C 6 H 4 : NH.NH.NH : C 6 H 4 . 

It is interesting to notice in passing that in the aliphatic 
series the diazo-group is actually united with two valencies 
of carbon, thus 

diazomethane CH 2 <f || 

2. Constitution of diazo-compounds according to Kekule. 
In 1886 Kekule' advanced the view that diazo-compounds 
contained the group .N : N., and considered that the behaviour 
of these compounds was not in accord with Griess's idea that 
two atoms of hydrogen in the benzene nucleus were displaced;* 
thus the formation of diazobenzene nitrate proceeded, according 
to Kekule', as follows 

C 6 H 6 .NH 2; HN0 3 - C 6 H 5 .N:N.N0 3 . 

Kekul^'s opinion that, in diazobenzene nitrate, there were 
five, and not four, atoms of hydrogen attached to the benzene 
nucleus was proved by the fact that penta-substituted deriva- 
tives of aniline could be converted into the corresponding diazo- 
salt without suffering any loss of their substituents.f 
The formulae which Kekule introduced were thus 
Free diazobenzene . . . C 6 H 5 . N : N.OH 
Diazobenzene sulphate . . C 6 H 5 . N : N.HSO 4 
Diazobenzene platinichloride . (C 6 H 6 . N : N.Cl) 2 PtCl 4 
Potassium salt . * . C r H 5 . N : N.OK 
Diazoaminobenzene . . CJEL . N : N.NH.CJEL 


Diazobenzene perbromide . C 6 H 5 . N : NBr, Br 2 
or . C 6 H 6 .NBr.NBr 2 

Whereas Griess regarded the diazobenzene salts as additive 

* Lehrbuch der organischen Chemie, II. 717. 

t Langfurth and Spiegelberg, Annalen, 1878, 191, 205 ; 1879, 197, 
305 ; compare also ibid., 1874, 174, 355 ; 1880, 215, 103. 



compounds of diazobenzene and acids, Kekule' looked upon 
diazobenzene as playing the part of a base analogous to 

Further, he explained the difference in stability between 
a diazo-salt such as diazobenzene chloride and an azo-com- 

C 6 H 6 .N:NC1 C 6 H 5 .N:N.C 6 H 5 

as being due to each nitrogen atom being united with a phenyl 
group in the latter, whilst in the former the union of chlorine 
and nitrogen rendered the compound similar to chloride of 
nitrogen. Diazoaminobenzene was formed, according to 
Kekule', by the union of the acidic part of the diazo-salt with 
a hydrogen atom of aniline, and the residues of both uniting 

The compound thus was an anilide, and hence was called 
diazobenzene anilide. 

This conception of the constitution of diazoaminobenzene is 
supported by the fact that, like many hydrazones, it forms 
metallic compounds, the hydrogen of the NH group being 

Diazobenzene perbromide was considered by Kekule' to be 
either an additive compound of diazobenzene bromide with 
one molecule of bromine (1) or a compound of formula (2) 
C 6 H 5 . N : NBr, Br 2 C 6 H 6 . NBr.NBr 2 

(1) (2) 

the former of which was regarded as the correct one. 

The constitution of diazobenzene imide was correctly written 
by Kekule' as 

and he pointed out that although free diazobenzene (or diazo- 
benzene hydroxide), C 6 H 6 . N : N.OH, was very unstable, yet 
certain other diazo-compounds could exist in the free state, 
such as, for example, the diazophenols. The reason of this was, 
that whilst one nitrogen of the diazo-group was attached to 


the benzene ring, the other was united to the oxygen atom of 
the phenol group, thus 


C 6 H/> 

This view was confirmed by the fact that the diazo- 
derivatives of phenol ethers for example, anisole behave as 
derivatives of diazobenzene and not of diazophenol. Thus 
salts of nitrodiazoanisole with mineral acids are easily obtain- 
able, whilst those of diazophenol are not ; further, free diazo- 
anisole does not exist. 

The diazosulphonic acids possessed a similar constitution to 
the diazophenols ; thus diazotized sulphanilic acid was 

S0 8 
C 8 H/> 

Kekule' also pointed out that the diamines could be divided 
into three classes according to their behaviour on diazotiza- 
tion, namely, (1) those in which only one amino-group was diazo- 
tized, (2) those in which both amino-groups were diazotized, or 
(3) those in which one amino-group was diazotized and the 
other took part in the reaction.* 

Many examples of these three classes have already been 
described in the foregoing pages, The difference in stability 
between the aromatic diazo- and azo-compounds was explained 
by Kekule' to lie in the fact that whilst both contained the 
group C 6 H 5 .N:, in the latter series it was attached to a 
benzene radical, whilst in the former a halogen or acid radical 
was united with it. 

Kekule"s theory of the constitution of diazo-compounds was 
universally adopted, and held its own for thirty years, until, 
in fact, the discovery of the isomeric metallic salts, giving, as 
it did, an immense impetus to the study of their constitution, 
led to the abandonment of Kekule" s theory in favour of that 
of Blomstrand. 

* Compare Holleman, Zeitsch.f. Chem., 1865, 557 ; Hofmann, Annalen, 
1860, 115, 251. 

I 3 


3. Constitution of diazo-salts according to Blomstrand. 
An entirely novel view of the constitution of the diazo-salts 
was published by Blomstrand in 1869.* This chemist ex- 
plained the formation of diazoaminobenzene and of diazo- 
benzene nitrate in the following way. 

In order to obtain a diazo-compound from an amine and one 
molecule of nitrous acid (HONO), three atoms of hydrogen 
must be present in order to become replaced by an atom of 
nitrogen. In the preparation of diazoaminobenzene from an 
alcoholic solution of aniline and nitrous acid, two molecules of 
aniline are required to furnish these three hydrogen atoms; 
consequently a simple diazo-compound is not the final pro- 

C 6 H 5 .NH:Hi C fl H 5 .NHR 

C 6 H 5 .N;HHj C 6 H 5 .N|N 

If, however, the starting-point is a salt of aniline, which, 
according to the ammonium theory of Berzelius, is a salt of 
a substituted ammonium (C 6 H 5 .NH 3 ), the three necessary 
hydrogen atoms are now present. Further, in the latter is 
present a quinquevalent nitrogen atom, whilst in diazoamino- 
benzene the two atoms of nitrogen are in the tervalent 

The formation of diazobenzene nitrate is therefore to be 
regarded as follows 

v in v 

C 6 H 6 . N.O.N0 2 + HO.N : = C 6 H 5 . N.O.N0 2 + 2H 2 O. 

H 3 N 

The three atoms of hydrogen attached to the quinquevalent 
nitrogen atom in aniline nitrate have thus been replaced by 
a tervalent nitrogen atom. 

This theory of the constitution of the diazo-salts was also 
put forward independently by Strecker in 1871 f and by 
Erlenmeyer in 1874 J without the knowledge of Blomstrand's 
paper or of each other. 

* Chemie der Jetztzeit, 1869, No. 4, 272. 

t Ber., 1871, 4, 786. } Ber., 1874, 7, 1110. 

?. The Chemie der Jetztzeit seems to be a very obscure publication. No 
copy exists in the Patent Office library or that of the Chemical Society. 


That these two chemists had been anticipated was shown 
in a paper by Blomstrand in 1875,* who thus established his 
claim to priority. 

Blomstrand pointed out the superiority of his formula to 
that of Kekule' in that no change in the valency of the aniline- 
nitrogen was postulated, a change for which there is no justi- 

v in 

Kekute C 6 H 5 .NH 3 C1 -> C 6 H 5 .N:NC1 

v v 

Blomstrand C 6 H 6 .NH 3 C1 -^ C 6 H 5 .NC1 


He also explained the instability of the diazo-salts by 
referring to the unusual replacement of three atoms of 
hydrogen by one of nitrogen in an ammonium salt. 

Blomstrand agreed with Erlenmeyer in adopting the formula 
C 6 H 6 . NBr : . NBr 2 for diazobenzene perbromide. 

In later papers Blomstrand developed his theory more fully 
in the light of recent work and suggested the names: 
' Azoammonium ' compounds for the diazo-salts; 'azo '-com- 
pounds for not only the stable compounds such as 

C 6 H 6 .N:N.C 6 H 6 , 

but even for potassium diazobenzene sulphonate, 
C 6 H 5 .N:N.S0 3 K; 

' diazo '-compounds for the aliphatic diazo-compounds of 
Curtius, and ' iso-azo ' compounds for the labile isomerides of 
Hantzsch (see later) and the labile forms of metallic salts and 

Blomstrand regarded the unstable azoammonium compounds 
as readily undergoing change into the azo-compounds under 
the influence of reagents such as phenols, &c. ; for example 

* Ber., 1875, 8, 51. 

t Acta Reg. Soc. Physiogr. Lund., 6, 1 ; J. pr. Chem., 1896 pi], 53, 



HI + C 6 H 5 . OK = KC1 + R.N : N.C 6 H 4 . OH 

and ]| | + K 2 S0 3 = KC1 + R.N : N.S0 3 K. 

He pointed out that a sharp distinction must be drawn 
between the quinquevalent salt-forming nitrogen atom in the 
azoammonium compounds, and the tervalent non-salt-forming 
nitrogen atom in the azo-compounds. 

The chief reason why Blomstrand's theory of the constitu- 
tion of diazo-salts (azoammonium salts) was not accepted was 
due to the objection of E. Fischer, who showed that this 
constitution did not explain the formation of phenylhydra- 
zine, discovered by him in 1875, by simple reduction of a 

Fischer pointed out that when diazobenzene nitrate was 
treated with an equimolecular quantity of potassium sulphite, 
a yellow crystalline salt, C 6 H 5 . N 2 . S0 3 K, was formed, but when 
an excess of potassium bisulphite was used, a colourless salt, 
C 6 H 6 . N 2 H 2 . S0 3 K, was obtained. The latter, potassium 
phenylhydrazine sulphonate, had already been prepared by 
Strecker and Romer in 1871.f 

The former salt had all the properties of a diazo-compound, 
and on treatment with zinc dust and acetic acid passed into 
the latter, which was, therefore, a product of reduction. It 
had no longer the properties of the diazo-compound but was 
converted into this by gentle oxidation. 

On treatment with hydrochloric acid, the sulphonic acid 
group was eliminated, and there resulted the hydrochloride of 
phenylhydrazine. Strecker and Romer formulated these com- 
pounds as follows 

C 6 H 6 . NH.S0 3 K C 6 H 5 . N.S0 3 K 

Jn I 

Potassium phenylhydrazine Potassium benzenediazo- 

eulphonate. sulphonate. 

(The latter formula, it will be noticed, differs from that 
suggested by Blomstrand.) 

* Ber. t 1877, 10, 1331. + Ber., 1871, 4, 784. 


Fischer pointed out that in order to explain the formation 
of phenylhydrazine from a diazobenzene salt according to the 
Blomstrand theory, it would be necessary to assume the 
change of quinquevalent nitrogen into tervalent by the addi- 
tion of hydrogen, and also the change of tervalent into quin- 
quevalent nitrogen by the withdrawal of hydrogen, 

C 6 H 5 .N.C1 m v 


+ 4H ^ C 6 H 5 .NH.NH 3 C1 


a procedure which was extremely improbable. 

On the other hand, if we assume that no change of valency 
occurs when phenylhydrazine is formed, we have 

C 6 H 5 .N.C1 C 6 H 5 .NH 3 C1 

giving us for free phenylhydrazine the formula 

C 6 H 5 .NH 2 


Fischer proved, however, that this formula could not repre- 
sent the constitution of phenylhydrazine * in the following 
way: phenylhydrazine and ethyl bromide unite to form the 

C 6 H 5 .N 2 H 2 (C 2 H s )<Bf 5 

which is also produced by the addition of ethyl bromide to 
phenylethylhydrazine. As the latter substance, however, is 
derived from ethylaniline by the substitution of the remaining 
JV-hydrogen atom by the group NH, and thus possesses the 

therefore the compound of phenylethylhydrazine and ethyl 
bromide must contain the complex > N.NH 2 , and consequently 
phenylhydrazine itself must be C 6 H 5 . NH.NH 2 . Fischer thus 

* Annalen, 1877, 190, 67. 


concluded that its formation from diazobenzene chloride could 
only be explained by the aid of Kekule"s formula 

C 6 H 6 . N : NCI -> C 6 H 5 . NH.NH 3 C1. 
When the whole question of the constitution of the diazo- 
compounds was undergoing renewed investigation many years 
later, Blomstrand explained that the formation of phenyl- 
hydrazine might be expressed as follows 

+ 4H = K + HQ Ha = R.NH.NH 3 C1 

and pointed out that it was impossible to postulate a double 
linking between the two nitrogen atoms as shown in the 


C 6 H 6 .NH 



when complete reduction had taken place.* 

The objections of Fischer, however, as has been said, were 
taken as final, and it was only in 1895 that the formula of 
Blomstrand was again adopted. 

It seems suitable, at this stage, to postpone further inquiry 
as to the constitution of the diazo-salts until we have con- 
sidered a little more fully that of the free diazobenzene, for 
the two are very closely connected. 

4. Constitution of diazobenzene hydroxide to 1894. In 

chapter xiii we have seen that when diazo-compounds are con- 
densed with various substances with the formation of mixed 
azo-compounds, the resulting substances were regarded in 
some cases as true azo-compounds and in others as hydrazones. 
It was therefore argued by the workers in this field that the 
original diazo-compound (or rather the hydroxide which might 
be supposed to be formed) might also be represented by a 
tautomeric formula 


the former of which would give rise to true azo-compounds, 
and the latter to the hydrazones. f 

* Chem., 1896 [ii], 53, 176. 

t V. Meyer, Ber., 1888, 21, 15 ; Japp and Klingemann, Ber., 1891, 24, 
2264 ; von Pechmann, Ber., 1891, 24, 3255 ; Bamberger, Ber., 1891, 24, 
3264; 1893,26,495. 


It has also been shown that Bamberger's discovery of the 
diazoic acids lent support to this view* (p. 102). 

Further confirmation was adduced by the experiments of 
von Pechmann f on the action of acetic anhydride on an 
alkaline solution of diazobenzene. He found that an acety- 
lated nitrosoamine was formed which was identical with the 
nitrosoamine prepared by O. Fischer J by the action of nitrous 
acid on acetanilide. 

Von Pechmann therefore concluded that primary amines 
by successive treatment with (1) nitrous acid, (2) acetic 
anhydride, or (1) acetic anhydride, (2) nitrous acid, yielded 
the same product, and that free diazobenzene was to be regarded 
as the anilide of nitrous acid 

OH.NO C 6 H 5 .NH.NO 

Nitrous acid. Anilide of nitrous acid. 

In confirmation of this view von Pechmann showed that 
nitrosoanilides (prepared from an anilide and nitrous acid) 
actually coupled with primary amines and phenols, yielding 
diazoamino- and hydroxyazo-compounds respectively, the 
acetyl group being at the same time split off. 

The discovery by von Pechmann and Frobenius that the 
methyl ether prepared from the silver salt of ^-nitrodiazo- 
benzene, to which reference has already been made (p. 98), was 
isomeric with that obtained from the sodium salt of Schraube 
and Schmidt, gave emphasis to the view of the tautomeric 
nature of diazobenzene 

C 6 H 6 . N : N.OH or C 6 H 5 . NH.NO 
N0 2 . C 6 H 4 . N : N.OAg NO 2 . C 6 H 4 . NNa.NO 

Silver salt of P. and F. Sodium salt of S. and S. 

(See, however, p. 147.) 

Further work bearing on this point was immediately pub- 
lished by Bamberger.|| By treating a /3-diazonaphthalene 
solution with concentrated aqueous sodium hydroxide, an iso- 

* Hantzsch regarded benzenediazoic acid as 
C 6 H 5 . N-N.OH 

(Ber., 1894, 27, 1730). 

t Ber., 1894, 27, 651. t Per., 1877, 10, 959. 

Ber. t 1894, 27, 672. || Ber., 1894, 27, 679. 


meric substance was obtained which was called /3-^so-diazo- 
naphthalene, C 10 H 7 .NH.NO, and which did not form an 
azo-compound with alkaline phenols. When, however, it was 
subjected to the action of a mineral acid, molecular change 
took place rapidly, and /3-diazonaphthalene was re-generated 
C 10 H 7 . NH.NO -* C 10 H 7 . N : N.OH. 

Bamberger drew the following conclusions as to the mecha- 
nism of diazotization. The properties of benzenediazoic acid, 
especially its transformation to o-nitroaniline, led him to 
suggest that the first stage in the process of nitration of a 
primary amine was the formation of a diazoic acid, for he had 
succeeded in obtaining benzenediazoic acid by the action of 
nitrogen pentoxide on aniline 

C 6 H 5 . NH 2 + N 2 6 -> C 6 H 5 . NH.N0 2 . 

Similarly, by the action of nitrous acid on a primary amine, 
the first product was the nitrosoamine (iso-diazo-compound), 

C 6 H 6 .NH 2 +NA - C 6 H 6 .NH.NO; 
the ordinary form of diazobenzene, C 6 H 6 . N : N.OH, would 
then result by molecular change from this. 

Bamberger found, in confirmation of this view, that under 
certain conditions, many primary amines yielded the iso- 
diazo-compound as first product.* 

Further, these isomeric forms of diazobenzene give (accord- 
ing to Bamberger) metallic salts, 

C 6 H 6 .NK.NO C 6 H 6 . N : N.OK 

Potassium salt of Potassium salt of 

iso-diazobenzene. diazobenzene. 

of which the m>-salt (like m>-diazobenzene) does not couple 
with phenols, and is transformed into the normal diazo-salt 
by mineral acids (see, however, p. 144). 

* Ber., 1894, 27, 1948. 




1. Constitution of the diazo-componnds according to 
Hantzsch. An important contribution to the current ideas 
was next made by Hantzsch.* 

He introduced the theory that the constitution of the 
isomeric diazo-compounds was exactly analogous to that of 
the isomeric oximes, according to which the former existed as 
stereoisomeric substances of the general formulae 
C 6 H 5 .N C 6 H 6 .N 

xl J,.x 

syn. anti. 

Hantzsch pointed out that the development of the chemistry 
of the isomeric diazo-compounds had undergone a precisely 
similar course to that of the isomeric oximes ; to the isomeric 
diazo-compounds had been assigned the formulae 

C 6 H 5 . N :N.OH and C 6 H 6 . NH.NO, 

just as, after the discovery of ' iso-benzaldoxime ', its constitu- 
tion was very generally regarded as being structurally different 
from that of the normal oxime, thus 

C 6 H 5 .CH:N.OH 

C 6 H 5 .CH.NH 


Stable oxime. Labile oxime. 

He showed that the formulae advocated by Schraube and 
Schmidt, and confirmed by Bamberger, for the metallic diazo- 
compounds for example, 

C 6 H 6 . NK.NO C 6 H 5 . N : N.OK 

iso-salt. Normal salt. 

required that, in the change from normal to iso-salt, the 

* Ber., 1894, 27, 1702. 


potassium should wander from the oxygen, for which it has 
an enormous affinity, to the nitrogen which has little attrac- 
tion for it. As this transformation takes place, in the case 
of ^?-nitrodiazobenzene, at the ordinary temperature and in 
aqueous solution, the current theory of its mechanism could 
not be accepted. Further, all substances which exhibit 
tautomerism in their salts, such as nitrous, cyanic, hydro- 
cyanic, and sulphurous acids, would also have to be considered 
as displaying structural isomerism in these, but no structural 
isomeric salts had been found the origin of which isomerism 
lay in a dissociable group (H or Me) which could alter its 
position in the molecule. Hence Hantzsch concluded that 
such isomerism must be steric and not structural. The fact 
that structurally isomeric alkyl derivatives had been obtained 
could not be used as a proof of the structural difference of the 
original substances; Schraube and Schmidt had concluded 
that because an JV-ether (I) was formed from the m>-diazo- 
benzene salts and alkyl iodide the original salt had the 
composition (II) 

C 6 H 6 . N(Alk).NO C 6 H 6 . NH.NO 

but this could not be maintained, for if the m>-diazobenzene 
salts were nitrosoamines, their alkyl derivatives ought, by 
alkaline hydrolysis, to yield the corresponding nitrosoamines ; 
thus C 6 H 5 .NAlk.NO should give C 6 H 6 . NH.NO, but Bam- 
berger had shown that the normal metallic diazo-salt and not 
the iso-salt was formed in each case.* 

The production of an N- ether from the potassium salt, and 
an 0-ether from the silver salt (p. 121) of >-nitrodiazobenzene, 
found an analogy in the case of the oximes. 

Hantzsch, therefore, was of the opinion that there was no 
proof of the structural isomerism of the free phenylnitroso- 
amines with the true diazo-compounds, and stated that if 
a derivative of diazobenzene in which the dissociable hydrogen 
or metallic atom was replaced by a non-dissociable group 
should exist in two isomeric forms, of which the one showed 

* Hantzsch himself showed later, however (see p. 147), that in this 
reaction the iso-salts are actually produced. 


the reactions of a true diazo-compound (for example, coupling 
with /3-naphthol) and the other those of the iso-diazo-com- 
pounds, such isomerides would be structurally identical, and 
their difference would be due to stereoisomerism. 

As an example of such isomerism, Hantzsch * described a 
series of new diazoamino-compounds which he regarded as 
stereoisomeric with those already known; these were, how- 
ever, soon shown by Bamberger f not to have any existence, 
as the substances described by Hantzsch were identical with 
the bisdiazobenzeneanilides of von Pechmann.J 

A second example was given, namely, a new (syn) form of 
potassium benzenediazosulphonate which, as will be shown, 
gave rise to a long controversy as to its nature. 

For the purpose of determining which diazo-com pounds 
belong to the syn- and which to the cmtfi-series, Hantzsch 
took for example those compounds which were considered to 
form anhydrides, such as diazosulphanilic acid, diazophenol, &c. 
At that time these were supposed to have the constitution 

C 6 H 4 .N N0 2 .C 6 H 4 .N 

SO 3 . N O 

Anhydride of diazobenzene- Nitrodiazophenol. 

eulphonic acid. 

If we now imagine the ring to be broken by addition of 

* Ber., 1894, 27, 1857. t Ber., 1894, 27, 2596. 

I Ber., 1894, 27, 703. This is an exceedingly good example of the 
importance of carrying out complete and exhaustive analyses in organic 
research. For ' benzene-st/w-diazoanilide ' 

C 6 H 6 .N 

C 6 H 5 .NH.N 
Hantzsch gave the following numbers : 

Found. : C = 72-7, H = 6.0, N = 20-8, 
C 12 H n N 3 requires C = 734, H = 5-6, N = 21-3. 
The compound was really bisdiazobenzeneanilide, 

C 6 H 5 . N : N N. C 6 H 5 ) N : N C 6 H 5 or C ]S H 1B N 6 , 

which requires C = 71-8, H = 5-0, N = 23 2, for which von Pechmann 
found C = 71-8, 72-3 ; H = 5-0, 5-8 ; N = 23-5 ; and Bamberger, 
N = 23-2, 2347 per cent. 

For ' jp-toluene-stt-diazotoluide ' Hantzsch gave 

Found. : N = 19-5, 
C 14 H 5 N 3 requires N = 18-7, 

whereas the substance really possessed the formula C 21 H 21 N 5 , which 
requires N = 204 per cent. 


water, salt-formation, &c., it is reasonable to suppose that the 
group attached to the nitrogen atom should retain its position, 

C 6 H 4 .N C 6 H 4 - -N 

S0 3 . N S0 3 H HO.N 

but as all these cyclic compounds readily couple with 
/S-naphthol, it is to be concluded that ordinary diazo-com- 
pounds have a similar configuration, or, in other words, those 
diazo-compounds which combine readily with 0-naphthol 
belong to the s^/ii-series, whilst those combining with diffi- 
culty or not at all are cwfa'-compounds.* 

Hantzsch explained the fact that many diazo-compounds, 
when left for some time in alkaline solution, lose their power 
of coupling with jS-naphthol by saying that the s^/Ti-compound 
(alkali-labile) was transformed into the anti-compound (alkali- 
stable) as follows 

C 6 H 4 . N C 6 H 4 N C 6 H 4 . N 

| || +H 2 - | || +KOH -* | || 

SO 3 . N S0 3 H HO.N S0 3 K N.OK 

syn. anti. 

Another method of determining the configuration of the 
isomeric diazo-compounds was drawn from the analogy to the 
oximes. In this class of compounds intramolecular change 
proceeds only in the case of the s^Ti-compounds, so that only 
the diazo-compounds belonging to the same series could 
decompose according to the equation 

C 6 H 6 .N 2 .X = C () H 5 .X + N 2 . 
The diazo-compounds which correspond to this condition 

* Hantzsch at a later date accepted the formulae 


and : C 6 H 4 (N0 2 ) : N 2 for these compounds, so that this particular 
argument cannot be maintained. This constitution of the quinonediazides 
(diazophenols) was adduced by L. Wolff (Annalen, 1900, 312, 119 etseq.) 
from the fact that compounds containing undoubtedly the grouping 


had properties quite different from the former. 


are the normal compounds, so these were to be regarded, 
according to Hantzsch, as s2/n-compounds. 

The decomposition thus takes place as follows : 

C 6 H 6 |.N C 6 H 5 


On the other hand, the omfi-compounds have a tendency to 
decompose into two residues, each containing a nitrogen atom, 

C 6 H 5 .N 

-||-> -* C 6 H 5 .N: + :NX 


In this way Hantzsch explained the formation of nitroso- 
benzene by the oxidation of the iso-diazo-compounds 

C 6 H 5 .N 


= C 6 H 5 . 

A third method of distinguishing between the syn- and 
an ^compounds was to be found in their difference in 
explosibility, the normal or s^'ft-diazo-compounds being much 
more explosive than the a7ii-compounds. 

It is to be mentioned here that Hantzsch regarded diazo- 
benzene chloride as a s^/Ti-compound, 


but soon adopted another view (see p. 133). 

Hantzsch's main conclusions were, therefore, that the 
ordinary normal diazo-compounds were sy^i-diazo-compounds, 
and the so-called ^so-diazo-compounds (nitrosoamine formula) 
were cmi-diazo-compounds. Bamberger,* in criticizing these 
views of Hantzsch, denied that it was possible to draw a 
parallel between the stereoisomeric oximes and the isomeric 
diazo-compounds. He based his objections on the very great 
difference between the latter, a difference which was greatly 
in contrast to the very small one existing between the oximes, 
and which could not be due to stereoisomerism. Moreover, 

* Ber., 1894, 27, 2582. 


the isomeric oximes gave corresponding isomeric ethers, but 
from the diazo- and ^so-diazo-silver salts only the one normal 
ether was obtained with methyl iodide. 

He maintained that the difference between the normal and 
the iso-diazo-compounds was best explained by the presence 
of the labile hydrogen atom (*) 

C 6 H 5 . N : N.OH* C 6 H 6 . NH*.NO 

Normal. iso. 

Further, the objection of Hantzsch that in the normal and 
iso-potassium benzenediazo-oxides 

C 6 H 5 . N : N.OK and C 6 H 6 . NK.NO 

Normal. iso. 

it was unlikely that the potassium atom should leave the 
oxygen and become attached to the nitrogen atom was met 
by Bamberger by the reminder that many compounds con- 
taining the imino-group for example, azoimide and benzimin- 
azole readily dissolve in alkali, and he pointed out, also, that 
the positive group C 6 H 5 . N 2 in the salt C 6 H 5 . N 2 . OK greatly 
lessened the affinity of potassium for the oxygen atom, just as 
in potassium oxide, K.O.K., one potassium atom is easily 

As was briefly indicated above (p. 124), it was obviously of 
great importance in connexion with Hantzsch's stereochemical 
theory to adduce evidence of the existence of isomerides other 
than those capable of being explained by the presence of 
a labile hydrogen atom which could give rise to isomerism as 
shown by the formulae 

C 6 H 5 . N : N.OH and C 6 H 5 . NH.NO. 

Hantzsch's first example of stereoisomeric diazoamino- 
benzenes was, as already mentioned, shown to be based on an 
error, and considerable interest, therefore, was attached to the 
other example of stereoisomeric benzenediazosulphonates. 

Isomeric "benzenediazosulphonates. f It has been already 
explained (p. 55) that by treating diazobenzene chloride with 
neutral potassium sulphite, potassium benzenediazosulphonate 
is formed ( c Strecker's salt '). In i 894 Hantzsch J described a 

t These must not be confused with the salts of diazobenzenesulphonic 
acid prepared by diazotizing sulphanilic acid. 
I Ber., 1894, 27, 1726. 


new isomeride which he obtained by pouring diazobenzene 
nitrate solution into an ice-cold solution of neutral potassium 
sulphite containing an excess of potassium carbonate. Orange 
plates separated which were readily soluble in water and con- 
tained one molecule of water of crystallization. The substance 
was very unstable, and coupled readily with j3-naphthol, and 
Hantzsch assigned to it the ^^configuration 

C 6 H 5 .N 

S0 3 K.N 

The clear dark-yellow aqueous solution of this new sub- 
stance gradually became paler on standing, and crystals of 
the Strecker salt separated out. This was much more stable 
than the sT/n-compound, and gave no colour reaction at all 
with alkaline phenol solution. It was, therefore, the anti- 

Bamberger considered* that the isomeric benzenediazor 
sulphonates of Hantzsch 

C 6 H 5 .N ,C 6 H 6 .N 


syn. anti. 

might just as well be represented as 

C 6 H 5 . N : N.O.S0 2 K and C e H 5 . N : N.SO 3 K 

Potassium diazobenzene Potassium benzenediazo- 

sulphite. sulphonate. 

in view of the fact that Hantzsch's new salt gave all the re- 
actions of a sulphite, and he maintained that Hantzsch had 
not proved that the two were stereoisomerides. 

He showed also that Strecker's salt, on acidification, did 
not pass into the diazo-salt as did iso-diazo-compounds. 

The same view of the constitution of these salts was 
expressed by Glaus, f 

In reply to this .criticism, Hantzsch J showed that both 
modifications gave the same ions, (C 6 H 5 . N 2 S0 3 ) and K, in 
solution, so that they must both have the structure 

* Ber., 1894, 27, 2586, 2930. t J. pr. CHem., 1894 [ii], 50, 239. 
J Ber., 1894, 27, 2099, 3527. 


S0 3 K.N N.S0 3 K 


C 6 H 5 . N 2 . S0 3 K ; he pointed out, further, that the sulphite 
reaction showed only that the new salt decomposed readily 
with separation of sulphurous acid, just as does the compound 
Hg(S0 3 K) 2 , which in aqueous solution gives the three ions 

Hg(S0 3 ) 2) K, K. 

He maintained that a diazo-sulphite, C 6 H 5 . N 2 . 0.SO 2 K, must 
give three ions in solution and not two, although Ostwald 
had informed Bamberger* that only two ions, namely, 

(C 6 H 5 N 2 S0 3 ) and K, 
were to be expected according to analogy. 

Up to this point we may summarize the evidence as proving 
the existence of two isomeric compounds, C 6 H 5 . N 2 . (S0 3 K). 
As we have seen, Hantzsch supposed that only one consti- 
tutional formula was possible for these, and that the 
substances were therefore stereoisomeric. Other formulae 
have, however, been advocated, but the discussion of these 
must be postponed until the constitution of the diazo-salts 
(chlorides, &c.) have been more fully dealt with (p. 140). 

Isomeric diazo-cyanides. In 1895 Hantzsch and Schultze 
succeeded in preparing isomeric diazo-cyanides by the action 
of potassium cyanide on jp-chloro- and >-nitro-diazobenzene 
chloride solution.f 

A little more than the theoretical quantity of potassium 
cyanide is added to the hydrochloric acid solution of the 
diazo-salt, and care must be taken that sufficient hydrochloric 
acid is present to ensure an acid reaction at the end of the 

At a low temperature (below 5) the primary, syn, or labile 
compound is obtained, whilst at higher temperatures the 
secondary, anti, or stable compound is produced. 

Both compounds are coloured, crystalline, and almost in- 
soluble in water, and the labile variety passes into the stable 
form slowly in the solid state but quickly in alcoholic 
solution. The labile isomerides couple with /3-naphthol, and 
are explosive, whilst the stable do not possess these properties. 

The two compounds behave very differently when treated 

* Ber., 1894, 27, 2934. t Ber., 1895, 28, 666. 


with copper powder; the ST/TI- compounds derived from both 
^?-chloro- and >-nitroaniline yield ^-chlorobenzonitrile and 
jp-nitrobenzonitrile respectively, whilst the cm^-compounds 
are entirely without action. Further, the s^-compounds 
yield azo-dyestuffs with R salt, but the cmfa'-compounds 
do not. These are substantial chemical differences in the 
behaviour of these substances which are not usually met 
with in stereoisomeric compounds (see p. 127). 

Hantzsch regarded the*se differences, however, as merely 
showing that the one compound was more stable than the 
other, and considered the existence of these substances to be 
a proof of his stereochemical theory, formulating them as 
C1.C 6 H 4 .N C1.C 6 H 4 .N 


Labile (syn). Stable (anti). 

As in the case of the isomeric diazosulphonates, we have 
first to discuss the later development of the theory of the 
constitution of the diazo-salts with mineral acids before 
describing the criticism to which these two series of 
isomerides have been subjected. 

K 2 



1. Constitution of the diazo-compounds according to 
Bamberger. In 1894 Bamberger * stated that, in the diazo- 
salts, the radical (C 6 H 5 . N 2 ) is strongly positive, and that even 
negatively substituted diazo-salts, such as bromo- and nitro- 
diazobenzene nitrates, showed a neutral action towards litmus 
or Congo-red, and were not hydrolytically dissociated in 
solution. He suggested, therefore, that the formula 

C 6 H 6 .N!N.X 

was worth consideration. Shortly afterwards,! he pointed out 
that there were no compounds known in which tervalent 
nitrogen was combined with a negative complex, such as 
N0 3 , Cl, &e., to form a salt. Hence he concluded that the 
nitrogen atom united with such groups in the diazo-salts must 
be quinquevalent, and therefore that Kekule^s formula, 

C 6 H 5 .N:NC1, 
which was commonly accepted, could not be correct. 

He adopted instead of this, or the one just referred to, the 
old formula suggested by Blomstrand (p. 116), C 6 H 5 . NCI : N, 
for the diazo-salts with mineral acids. J 

When the negative group was withdrawn from this (by 
formation of the hydroxide, for example) the nitrogen atom 
to which it was attached became tervalent, thus 

C 6 H 5 .N(OH)iN -> C 6 H 5 .N:N.OH. 

Diazobenzene. iso-diazobenzene. 

It will be noticed that Bamberger here proposed a formula for 

the iso-compounds differing from the nitrosoamine formula, and 

a convincing proof of the correctness of this formula for the 

iso-diazo-compounds was apparently given by the discovery 

* Ber., 1894, 27, 3417. t Ber., 1895, 28, 242. 

J Ber., 1895, 28, 444. 


that these were formed by the action of hydroxylamine on 
the nitroso-compounds, thus 

C 6 H 6 . NO + H 2 N.OH = C 6 H 5 . N : N.OH + H 2 O * 
but Hantzsch f showed that, in reality, the normal compound 
was formed as follows 

C 6 H 5 . NO + H.NH.OH = C 6 H 5 . 

C 6 H 5 .N.OH C 6 H 5 .N 

-> ||+H 2 Ot 


Bamberger's view of the constitution of the diazobenzene 
salts was not at first accepted by Hantzsch, who maintained 
that diazobenzene chloride in the dry state possessed the 

C 6 H 5 .N 


and when dissolved in water was to be regarded as the 
hydrochloride of s2/n-diazobenzene hydroxide 

C 6 H 6 .N 

HO.N, HC1 

Very soon, however, Hantzsch gave up the latter idea and 
adopted Blomstrand's formula for the diazo-salts. He pre- 
ferred also to call these ' diazoninm ' salts, from their analogy 
to the ammonium salts.|| 

Bamberger had shown that the diazo-salts had, like the 
alkali salts, a neutral reaction in solution. That the diazo-salts 
are electrolytically dissociated in solution had indeed been 
demonstrated by Goldschmidt in 1890, who also found that 
they form two ions;^[ and Hantzsch now made a careful 
comparison of the electrical conductivities of various diazo- 
salts and salts of the alkali metals, and was able to show that 
the degree of ionization is about the same in the two cases ; 

* Bamberger, Ber., 1895, 28, 1218. t Ber., 1905, 38, 2056. 

I Compare also Angeli, Ber., 1904, 37, 2390. 
Ber., 1895, 28, 676. || Ber., 1895, 28, 1734. 

1 Ber., 1890, 23, 3220. 


that is to say, diazonium salts are dissociated almost to the 
same extent as the corresponding potassium or ammonium 

Further analogy was illustrated by Hantzsch's discovery of 
various double salts of the diazo-salts with cobalt nitrite, 
mercuric chloride, and mercuric cyanide. 

Having thus developed the idea of c diazonium ' as a com- 
plex alkali metal, Hantzsch agreed with Bamberger in regard- 
ing the salts of ' diazonium' with acids as possessing the 
constitution which had been attributed to them by Blom- 
strand, Erlenmeyer, and Strecker, namely, C 6 H 5 . NCI : N. 

The metallic salts, cyanides, sulphonates, &c., belonged to 
the ordinary ( diazo ' form ; for example, C 6 H 5 . N :N.OK could, 
as we have seen, exist in two stereoisomeric modifications. 

The cyclic diazo-compounds were divided into two groups. 
On the one hand, Hantzsch assigned to the diazo-compound 
prepared from sulphanilic acid the formula which had been 
already given to it by Strecker, namely, 

C 6 H 

and the diazo-phenols and naphthols, which are anhydrides, he 
regarded as possessing the formulae 

N N 

C 6 H/V and C 10 H/\O 


To this pronouncement of Hantzsch, Bamberger replied 
that the diazonium radical is not directly comparable with an 
alkali metal, as its behaviour varies with the substituting 
group in the aromatic nucleus. Moreover, the electrical con- 
ductivity experiments of Goldschmidt and Hantzsch were not 
a trustworthy basis upon which to speculate as to the nature 
of diazonium.* 

* Ber., 1896, 29, 446, 564, 608. 


Hantzsch now elaborated his arguments in favour of re- 
garding the diazonium radical as a ' compound alkali metal ' 
of the same strength as ammonium or potassium. He pointed 
out that the alkali salts of all strong monobasic acids, such as 
hydrochloric, sulphuric, &c., all undergo very extensive electro- 
lytic dissociation in solutions of moderate dilution, but are 
not hydrolytically dissociated. The dissociation and mole- 
cular conductivity are only very slightly increased by further 
dilution, and the increase ceases at a point of very moderate 
dilution ; the salts of silver and thallium behave similarly, as 
do also salts of complex ammonium bases, such as mono-, di-, 
tri-, and tetra-alkylammonium and phenyltrimethylam- 
monium, but not phenylammonium, the ion of the aniline 
salts. Now the diazonium salts behave in exactly a similar 
manner,* so that there was strong presumptive evidence that 
diazonium was constituted similarly to the complex 
ammoniums, f 

2. Relation between diazonium compounds and normal 
or s?/ft-diazo-coinpounds. Hantzsch's theory that the syn- 
diazo-compounds are those which ' couple ' with phenols, &c., 
to yield azo-dyestuffs led him to explain that these syn-com- 
pounds were formed as intermediate products in the ordinary 
reactions of the diazo-salts. The coupling process was thus 

CH 5 R C5H R 

N i N + I = N=N 


The formation of the diazo-metallic salts, &c., was expressed 
as follows 

C 6 H 6 OK S0 3 K C ! N 

C 6 H 6 OK (SO.K), (CN) 

;N+ &c.= 





+ KN0 3 ,&c. 
Ber., 1895, 28, 1737. t Ber., 1895, 28, 1740; 1898, 31, 1612. 


and the decomposition of the diazonium salts was explained 

OH C 6 H 5 \ /OH C 6 H 5 -OH* 
NiN+| _> Naar if _^ 5 N 

H Cl H C1H 

Diazosulphanilic acid, when treated with one molecule of 
alkali, passes into the mono- and with two molecules of alkali 
into the di-alkali salt, thus 

C 6 H 4 .N ; N S0 3 Na.C 6 H 4 . N S0 3 Na.C 6 H 4 . N 

S0 2 HO.N NaO.N 

This view of the intermediate formation of syti-diazo-com- 
pounds in the reactions of the diazonium salts received con- 
firmation from the experiments of Hantzsch and Gerilowski,f 
who showed that whereas free diazosulphanilic acid is fairly 
stable in aqueous solution, the primary alkali salt obtained 
by the action of one molecule of alkali, under the same condi- 
tions, loses practically all its nitrogen, thus 

S0 3 Na.C 6 H 4 .N S0 3 Na.C 6 H 4 N 

HO! - in + B 

It is evident that the nature of the decomposition of the 
diazo-salts is different from that of the ammonium salts, for 
if the two resembled each other, one would expect that as the 
group attached to the diazonium radical becomes less nega- 
tive the stability of the salt would decrease just as 
NH 4 C1, (NH 4 ) 2 C0 3 , and NH 4 . OH decrease in stability. 

This is, however, not the case, as solutions of diazonium 
carbonates are comparatively stable. Moreover, one would 
expect also that diazonium halogen salts, if they decomposed 
in aqueous solution analogously to the ammonium salts, would 
yield halogen-substituted benzenes and not phenols. 

Hantzsch explained this difference by assuming the inter- 

* Per., 1895, 28, 1734 ; 1900, 33, 2517. 
t Ber., 1896, 29, 1063. 


mediate formation of s^/Ti-diazobenzene hydroxide, which then 
could decompose into nitrogen and phenol 

C 6 H 5 .N 

C 6 H.NCliN + H 

CH 6 N 

+ JJ 

The action of alcohol was explained in this way : 
(1) Formation of ethers 

Ar OEt 


Ar OEt 

1 I 


NiN + 



-* NiN 

A, ! 




(2) Formation of hydrocarbons 

Ar H Ar H 

NiN + 



H 4 

3. Double salts of diazoninm halides and metallic salts. 
Double salts of diazo-halides with platinum and gold 
chlorides have been known since the days of Griess and a 
large number of others have since been prepared. 

Hantzsch has further shown * that two kinds exist, namely, 
colourless, stable, diazonium halogen double salts and coloured, 
labile, S2/n-diazo-halogen double salts. 

For example we have 

C 6 H 6 . N.C1, HgCl 2 C 6 H 6 . N, Cu 2 Cl 2 

I " d oft 

Certain of the diazo-halides can also unite with halogen 
acids to form additive compounds of formulae 

Ar.N 2 Cl, HC1 and 3Ar.N 2 . Cl, HCl,f 
the constitution of the former being represented by Hantzsch 



Ber., 1895, 28, 1736. 

t Hantzsch, Ber., 1897, 30, 1153. 


and the latter being regarded as compounds of two molecules 
of the neutral and one of the acid salt. 

4. Diazonium halides and s^-diazo-halides. Hantzsch 
found that all diazo-bromides, iodides, and thiocyanates which 
belong to the same series as colourless diazonium chlorides 
and salts of oxygen acids are coloured when in the solid state ; 
with increase of colour is noticed an increase in the explosi- 
bility. Thus we have 

Diazo-chloride. Diaz o -bromide. Diazo-thiocyauate. 

Colourless. Slightly coloured. Strongly coloured. 

Hardly explosive. Slightly explosive. Very explosive. 


Intensely coloured. 
Extremely explosive. 

As diazonium salts should be, like the corresponding alkali 
and ammonium salts, colourless, and as, on the other hand, syn- 
diazo-halides, from analogy to the coloured s^-diazo-cyanides, 
should be coloured, and also, as being compounds of the type 
of nitrogen iodide, would be expected to be explosive, 
Hantzsch * concluded that the properties of the above series 
of compounds were only to be explained by the assumption 
that they consist of an equilibrium mixture f of colourless 
diazonium halides and coloured s^/n-diazo-halides, thus 

Ar.N(Br, SON, I) Ar.N 


(Br, SON, I) N 

the chlorides belonging entirely to the diazonium series, and 
the cyanides to the si/Ti-series. 

The proportion of s^/Ti-diazo-compound in the mixture 
becomes less with a lowered temperature,}: for at 60 many 
diazo-halides are nearly colourless and become more intensely 
coloured with rise of temperature. In the colourless aqueous 
solutions, of course, the 62/7i-compound has become entirely 
transformed into the diazonium isomeride. 

5. Diazoninm perhalides. Griess found that two bromine 
atoms in diazobenzene perbromide are more loosely combined 

* Ber., 1897, 33, 2179. t Ber., 1900, 33, 2179. 

J Euler, Ber. t 1895, 31, 4168. 


than the third; the compound was therefore regarded as 
having the constitution C 6 H 5 . N : NBr, Br 2 . 

Kekule' looked upon this as a tribromohydrazine, 

C 6 H 6 .NBr.NBr 2 , 
and Erlenmeyer wrote it as 

C 6 H 5 .NBr 


NBr 2 

Hantzsch has prepared a large number of these perhalides;* 

ArN 2 . Cl 2 Br ArN 2 . Br s ArN 2 . 1 3 

ArN 2 .Cl 2 I ArN 2 .Br 2 Cl ArN 2 .I 2 Cl 

ArN 2 .Br 2 I ArN 2 .I 2 Br 

ArN 2 .ClBrI 

and regards them as analogous to potassium tri-iodide, caesium 
tri-iodide, and the trihalides of the quaternary ammonium 
series. The exact arrangement of the three halogen atoms 
in the molecule is not known, and no isomerism such as 
(Ar.Nj.Cl + Brg) and (Ar.N 2 Br + BrCl) exists. 

6. Relation between syn- and cn?i-componnds. The rate 
of isomerization of syn- to an^-diazo-compounds depends 
largely on the substituents present in the aromatic nucleus. 
Methyl groups hinder the rate, whilst halogen atoms increase 
it. Thus the transformation is very difficult to bring about 
in the case of trimethyl- and methoxy-benzenediazo-oxides, 
whilst in the case of the unsubstituted benzenediazo-oxide, 
C 6 H 5 . N 2 . OK, it proceeds quickly above 100. The >-bromo- 
derivative, on the other hand, is isomerized at boiling-point, 
the p-sulpho-derivative, S0 3 K . C 6 H 5 . N 2 . OK, slowly at the 
ordinary temperature, and the tribromo- and p-nitro-deriva- 
tives, C 6 H 2 Br 3 . N 2 . OK and NO 2 .C 6 H 4 .N 2 .OK, instanta- 
neously, so that the syn-salt cannot be isolated. 

In the case of the diazo-sulphonates it is the alkylated 
derivatives which isomerize more quickly than the parent 
substance, whilst the p- and o-halogen substituted derivatives 
of the syn series are relatively stable.* 

* Ber., 1895, 28, 2754. 

t Hantzsch and Schmiedel, Ber., 1894, 27, 3071, 3530. 


Similarly among the s^/n-diazo-cy anides the o- and ^-halogen 
substituted derivatives are fairly stable, and the parent sub- 
stance, C 6 H 5 . N 2 . CN, has not been isolated. 

The presence of the nitro-group greatly increases the rate of 
isomerization in all the above series of s^/ii-compounds. 

7. The isomeric diazosulphonates and diazo-cyanides. 
Having explained Hantzsch's views as to the constitution of 
the diazonium salts, we can now resume the discussion of the 
constitution of the diazo-sulphonates and diazo-cyanides which 
were described on pp. 128, 130. 

The formulae given by Hantzsch to these compounds 
C 6 H 5 .N " C 6 H 6 .N 

S0 3 K.N N.S0 3 K 

Labile salt (syn). Stable salt (anti). 

C 6 H 6 .N C 6 H 5 .N 


Labile (syri). Stable (anti). 

were objected to by Bamberger* and Blomstrand,t who did 
not accept the stereochemical hypothesis. These chemists 
assigned the following formulae to the above substances 
C 6 H 6 . N(SO a K) N C 8 H 6 . N : N.S0 3 K 

Labile (normal). Stable (iso). 

C 6 H 6 . N(CN) i N C 6 H 6 . N : N.CN. 

Labile (normal). Stable (iso). 

the formulae for the two sulphonates having already been 
suggested by V. Meyer and Jacobson,t to which these authors 
have adhered in succeeding editions of their book. 

Bamberger regarded the quinquevalency of nitrogen in the 
diazonium salts as being dependent on the negative character 
of the group with which the diazonium radical was united. 
Thus when these groups were Cl, N0 3 , S0 4 H, the nitrogen 
atom was necessarily quinquevalent. This condition was still 
maintained by the SO 3 K and CN groups, but owing to their 

* Ber., 1895, 28, 242, 447, 834. 

t J. pr. Chem., 1896 [ii], 53, 169 ; 1897, 55, 481. 

J Lehrbuch der org. Chem., II. 303. 


small negative character the labile salts could pass into the 
stable salts (the nitrogen atom becoming tervalent) with 
extreme ease. 

Hantzsch * reiterated his objection to this view of the consti- 
tution of the diazo-sulphonates from the fact that these salts 
are only dissociated into two ions, namely, Ar.N 2 . S0 3 and K, 
whilst if they possessed the diazonium constitution they would ? 
according to him, be expected to yield three ions, namely, 
Ar.N 2 , S0 3 , and K, corresponding to the behaviour of potassium 
sulphite, which gives the ions K, K, and S0 3 .f Further, he 
pointed out that the colour of the normal diazo-sulphonate 
(red) was another argument against Bamberger's view, as 
benzenediazonium salts with colourless anions (as S0 3 ) were 

A further argument against the stereochemical view of the 
isomerism of the diazo-sulphonates was adduced by von Pech- 

He pointed out the fact that both the groups S0 3 H and CN 
themselves could give rise to isomerism. In order to find a 
group free from this objection, von Pechmann J selected the 
diazo-salts of benzenesulphinic acid, C 6 H 5 . N:N.S0 2 . C 6 H 6 , 
which had been prepared by Koenigs. 

In whatever manner this salt was prepared, it was impos- 
sible to discover the existence of an isomeride ; this was also 
true of the j9-nitro-derivative, N0 2 . C 6 H 4 . N:N.SO 2 . C 6 H 5 , and 
von Pechmann concluded that these facts militated against 
Hantzsch's theory. 

Hantzsch and Singer || also prepared a number of these 
additive compounds, but were unable to detect the existence 
of isomerism. The supposed isomerism of the diazo-thio- 
sulphonates 1f was shown by Dybowski and Hantzsch ** to 
have no foundation in fact. ft 

* Ber., 1895, 28, 676. 

t See however Ostwald's opinion, p. 130. 

I Ber., 1895, 28, 861. Ber., 1877, 10, 1531. 

|| Ber., 1897, 30, 312. 

U Troger and Ewers, J. pr. Chem., 1900 [ii], 62, 369. 
** Ber., 1902, 35, 268. 

ft Compare also Hantzsch and Glogauer, Ber., 1897,30, 2548; Hantzsch, 
Ber., 1898, 31, 636. 


With regard to the constitution of the diazo-cyanides, 
Hantzsch * insisted that a diazonium cyanide, Ar.N(CN) N, 
must be similar to an alkali cyanide, but as the normal diazo- 
cyanides were coloured, sparingly soluble in water, and soluble 
in organic solvents, they could not have the constitution attri- 
buted to them by Bamberger and Blomstrand. 

Finally, Hantzsch and Danzigerf succeeded in preparing 
a third series of cyanides by treating a diazonium chloride 
with a suspension of silver cyanide. The insoluble yellow 
S2/7i-diazo-cyanide is also formed in this reaction, but the filtrate 
contains a soluble double cyanide with silver cyanide, which is 
considered by Hantzsch to be a true diazonium derivative. 
These substances are soluble in water and colourless and 
resemble the alkali cyanides, and are therefore diazonium 

The formation of these double diazonium cyanides led 
Hantzsch to the hypothesis that the sparingly soluble syn-di&zo- 
cyanides may exist in solution in a state of equilibrium with 
the isomeric diazonium salt, and a study of the diazo-cyanides 
derived from ^-anisidine confirmed this idea.J p-Methoxy- 
benzenediazonium bromide and chloride with potassium 
cyanide in alcoholic solution yield the s^/Ti-diazo-cyanide 

MeO.C 6 H 4 .N 


an orange-red, insoluble substance, melting at 51, and coupling 
with /?-naphthol. This changes slowly into the anti-salt 
MeO.C 6 H 4 .N 


which is brownish red, melts at 121, and does not couple with 

(Certain s2/7i-cyanides are difficult to convert into the anti- 
modification. Thus that derived from 2:4: 6-tribromoaniline 
must be combined with benzenesulphinic acid to form the 
additive product C 6 H 2 Br 3 . NH.N(CN).S0 2 . C 5 H 6 , which on 
treatment with alkalis yields the cw^-cyanide.) 

* Ber., 1895, 28, 668. t Per., 1897, 30, 2529. 

t Ber., 1900, 33, 2161 ; 1901, 34, 4166. 


When, however, an aqueous solution of ^-methoxybenzene- 
diazonium hydroxide is evaporated with excess of hydrogen 
cyanide at the ordinary temperature, a colourless crystalline 
substance is obtained which has the composition 
MeO . C 6 H 4 . N 2 . CN, HCN, 2 H 2 O. 

This possesses all the properties of a true metallic salt, it is 
very soluble, and its solution is an electrolyte. It couples with 
/S-naphthol, and is converted into the s^n-diazo-cyanide by the 
action of alkaline solutions. 

There can thus be prepared from _p-anisidine three different 
diazo-cyanides, namely 

MeO.C 6 H 4 . N ; N MeO.C 6 H 4 . N MeO.C 6 H 4 . N 


Colourless, soluble Labile, coloured Stable, coloured 

electrolyte. non-electrolyte. non-electrolyte. 

The isolation of these three isomerides was regarded by 
Hantzsch as a very strong proof of his stereochemical theory, 
as Bamberger's theory could only account for two of them.* 

It is, however, highly significant that in the two series of 
isomeric diazo-compounds, the cyanides and the sulphonates, 
both groups attached to the diazo-nucleus, should themselves 
be capable of giving rise to isomerism. As regards the 
cyanides, it has indeed been suggested f that Hantzsch's 
S2/?i-compound has the constitution Ar.N : N.NC, and the anti- 
compound Ar.N : N.CN. (See also p. 141.) 

This view would seem to be confirmed by the observation 
of Hantzsch and SchultzeJ that both series give the same 
(anti) diazobenzenecarboxylic acid, Ar.N:N.C0 2 H, for the 
former compound would be expected to undergo transforma- 
tion into the latter. According to the stereochemical theory 
the labile s^/Ti-compound would pass into the more stable anti- 
cyanide before hydrolysis. Moreover, Hantzsch has offered 
no proof against this obvious view; he contented himself 
with stating that neither of these compounds was an iso- 

* Another way of accounting for these is explained on p. 168. 
t Orton, Trans., 1903, 83, 805. \ Ber. y 1895, 88, 2073. 


8. Constitution of the metallic diazo-oxides. At the 

beginning of 1895 Bamberger's views on the constitution of 
diazo-compounds up to this time had been as follows: The 
diazo-salts were to be represented by the Blomstrand formula 

C 6 H 6 .NCliN. 

The normal, labile diazo-compounds (coupling with phenols) 
had the constitution C 6 H 5 . N : N.OX. 

The tso-diazo-compounds (nitrosoamines, not coupling with 
phenols) were C 6 H 5 .NH.NO or C 6 H 5 .NX.NO, X being a 
metal such as K, Na, &c., but a little later he represented 
them as being divided into two groups, namely, (1) normal 
diazo-compounds (of diazonium type, C 6 H 5 . NCI : N) ; (2) iso- 
diazo-compounds (of azo-type, C 6 H 5 . N : N.OH). 

Bamberger was led to this change of view by his work on 
the interaction of nitrosobenzene and hydroxylamine,* from 
which he supposed that the stable form of diazobenzene 
hydroxide was formed according to the equation 

C 6 H 5 . NO + H 2 : N.OH = C 6 H 5 . N : N.OH + H 2 O. 

but, as we have shown (p. 133), in reality the normal or labile 
modification is produced. 

The controversy existing in the years 1895 to 1897 be- 
tween Hantzsch and Bamberger mainly resolved itself into 
a discussion of the constitution of the metallic diazo-salts. 
On the one hand Hantzsch strove to prove that they were 
stereoisomeric by means of physical measurements (electrical 
conductivity, &c.), whilst, on the other, Bamberger maintained 
that their differing chemical characteristics were sufficient 
evidence that they differed in constitution. 

In 1895 Hantzsch and Gerilowski f prepared a labile form 
of the sodium salt of diazobenzenesulphonic acid, 

NaO.N 2 . C 6 H 4 . S0 3 Na, 4H 2 O, 

the stable isomeride having been already obtained by Bam- 
berger. This new labile form is obtained by treating the 
diazotized sulphanilic acid mixed with water with concen- 
trated aqueous sodium hydroxide at 0. It forms white, silky 

* Ber., 1895, 28, 1218. t Ber., 1895, 28, 2002. 


needles, has a strongly alkaline reaction, and couples instantly 
with /3-naphthol. It becomes changed into the stable iso- 
meride (which contains no water and does not couple with 
/9-naphthol) by heating with water. The labile salt in aqueous 
solution forms three ions, as does the stable salt ; according 
to Hantzsch, if it were a diazonium compound it should form 
four ions. 

In the following year Bamberger asserted that Hantzsch's 
conclusions as to the stereoisomerism of these two salts could 
not be maintained, as they were based on inaccurate observa- 
tions of their behaviour.* 

Further work was, however, done by Hantzsch. The deter- 
mination of the electrical conductivity of the two salts f 
showed that at moderate dilution (v 16 -v 64 ) the conductivity 
was the same in each case. Whilst, however, the increase in 
the conductivity of the stable salt from v 32 to v 1024 corresponds 
with the theory for sodium salts of dibasic acids not hydroly- 
tically dissociated in aqueous solution, J a fact which shows 
that the stable salt is not hydrolysed, the conductivity of the 
labile salt from v 128 increases very rapidly, thus showing that 
the labile salt has become hydrolysed, forming 

NaS0 3 .C 6 H 4 .N 2 .OH and NaOH. 

The solution also has an alkaline reaction, whilst that of the 
stable salt is neutral. The conclusion is, therefore, that both 
diazo-complexes possess acid properties, that of the labile salt 
being the weaker. The difference between the two salts is 
thus only a gradual one, and consequently Hantzsch con- 
sidered that they were stereoisomeric, assigning to them the 

NaS0 3 . C 6 H 4 . N NaS0 3 . C 6 H 4 . N 

NaO.N N.ONa 

Similarly, the cryoscopic researches of Goldschmidt showed 
that both the normal (syri) and the iso (anti) potassium ben- 
zenediazo-oxides possessed the same number of ions in aqueous 

* Ber., 1896, 29, 564. t Ber. t 1896, 29, 743. 

t Zeitsch. physical. Chem., 1894, 13, 222. 
Ber., 1895, 28, 2020. 


solution, and this was likewise considered to be a proof of 
Hantzsch's view of their constitution. 

Bamberger,* on the other hand, maintained his view that 
the two compounds were to be formulated 

C 6 H 6 .N.OK 

III C 6 H 6 .N:N.OK 


Labile (normal). Stable (iso). 

To Hantzsch's criticism that there existed no alkali metal 
the hydroxide of which possessed acid properties, Bamberger 
denied that diazonium was a compound alkali metal, and held 
that the hydroxide was neither comparable with tetramethyl- 
ammonium hydroxide or with potassium hydroxide. 

Bamberger's examples of chemical differences between the 
normal and the ^so-salts, namely, that the '^so-salt was reduced 
by sodium amalgam to phenylhydrazine and the normal not,f 
and that the m>-salt was converted into the normal salt when 
treated with benzoyl chloride, whilst the normal salt gave 
nitrosobenzanilide,J were both shown by Hantzsch to be 
based on error, as he obtained both phenylhydrazine and 
nitrosobenzanilide || in equal amounts in the two cases. 

Later, however, Bamberger If became convinced that a 
diazonium hydroxide could not act as an acid, and gave up 
the diazonium configuration for the normal metallic salts, 
He now regarded the metallic diazo-salts as existing in the 
two forms: (1) normal metallic diazo-salts (or diazotates), 
Ar(N 2 OK), of unknown constitution; the normal diazo- 
hydroxides, however, were, according to him, 

Ar.N : NH Ar.N N.H 

A or Y 

and (2) iso-diazotates, Ar.N:N.OK. 

9. Diazo-ethers. As has been explained (p. 98), von 
Pechmann and Frobenius ** discovered that the sodium salt of 

* Ber., 1896, 29, 457. 

t Ber., 1896, 29, 473. j Ber., 1897, 30, 211. 

Ber., 1897, 30, 339. 

|| Ber., 1897, 30, 621 ; 1899, 32, 1718. 

t Annalen, 1900, 313, 97. ** Ber. t 1894, 27, 672. 


p-nitrobenzenediazo-oxide (tso-compound of Schraube and 
Schmidt) gave with methyl iodide a nitrogen ether, 

N0 2 .C 6 H 4 .N(CH 3 ).NO 
but that the silver salt yielded an oxygen ether 

N0 2 .C 6 H 4 .N:N.O.CH 3 . 

On this account they considered that nitro-iso-diazobenzene 
hydroxide was a tautomeric substance 

N0 2 . C 6 H 4 . NH.NO or N0 2 . C 6 H 4 . N : N.OH. 
They regarded the oxygen ether, therefore, as a normal diazo- 
compound (although it was derived from the -^so-salt), as it 
combined with phenols like diazo-salts, and the nitrogen 
ether as the ^so-compound. 

Hantzsch * regarded the oxygen ether as an an^-compound, 
but experiments by Bambergerf and von Pechmann and 
Frobenius confirmed the resemblance of this compound to 
the normal diazo-salts and its difference from the a^^-com- 
pounds. Moreover, a large number of similar ethers were 
prepared by Bamberger, and these were also found to react 
as normal compounds ; on hydrolysis with alkalis they yielded 
normal metallic derivatives. Shortly after, Bamberger || con- 
sidered that the diazo-ether ought to be regarded as an ^so- 
compound, and Hantzsch and Wechsler 1F found that >-bromo- 
diazobenzene ethyl ether yielded the cm^-oxide on hydrolysis. 

Some time later, as the conflicting views on this subject had 
not been entirely reconciled, Euler ** investigated the matter 
afresh. By careful experiment he found that the product of 
hydrolysis of diazobenzene methyl ether, as well as p-bromo- 
diazobenzene methyl ether, reacted, as did a normal diazo- 
compound, but Hantzsch was able to show that the coupling 
with a-naphthol, on which these experiments were based, was 
due to a secondary reaction, and that, in fact, the iso (anti) 
compounds were produced on hydrolysis. It appears, there- 
fore, that von Pechmann's oxygen ether belongs to the iso- or 

* Ber., 1894, 27, 1865, 2968. t Ber., 1894, 27, 3412. 

| Ber., 1895, 28, 170. Ber., 1895, 28, 225. 

|| Ber., 1895, 28, 829. IF Annalen, 1902, 325, 226. 

** Ber., 1903, 36, 2503. 

ft Hantzsch, Ber., 1903, 36, 3097, 4361 ; 1904, 37, 3030 ; Euler, Ber., 
1903, 36, 3835. 

L 2, 


10. Diazo-anhydrides. In 1896 Bamberger* discovered 
that when normal metallic diazo-salts are treated with cold 
dilute acetic acid, extremely explosive, yellow diazo-anhydrides 
are formed. These cannot be obtained from the iso-salts, 
which yield colourless hydroxides under similar conditions, 
and this difference was considered by Bamberger to be 
another proof of the structural difference of the two. 

The diazo-anhydrides may also be prepared in some cases 
by treating a diazonium salt with a normal metallic diazo- 
salt. They couple slowly with phenols, yield oxygen ethers 
with the alcohols, and react explosively with benzene, yielding 
diphenyl derivatives. With alkalis they yield the correspond- 
ing normal salt, and mineral acids convert them into diazonium 

With amines, diazoamino-compounds are obtained; with 
ammonia, bisdiazoamino-compounds ; and with bromine, diazo- 

Bamberger was of the opinion that their constitution was 
to be represented by 


1 1 

but Hantzschf considered that they were more probably 
represented by R.N : N.O.N : N.R. He found later f that 
the diazo-anhydrides readily yield s^/Ti-diazo-cyanides on 
treatment with hydrogen cyanide, and pointed out that the 
anhydrides dissolve very slowly in hydrochloric acid to form 
diazonium chlorides facts which confirmed his theory of 
their azo-constitution. 

Bamberger later suggested the formula 
R.N.O.N : N.R 

but this was rejected by Hantzsch on the ground that, the 
S2/?i-diazo-hydroxide being an extremely weak acid, such a 

* Ber., 1896, 29, 446. 

t Ber., 1896, 29, 1074; 1897, 30, 626. 

I Ber. t 1898, 31, 636. Ber., 1898, 31, 2636. 


diazonium diazo-oxide should be instantly decomposed by 

From the fact that the diazo-anhydrides yielded syn- 
diazo-cyanides with hydrogen cyanide and s^/Ti-diazo-sulpho- 
nates with potassium sulphite, Hantzsch adopted the syn- 


N.R R.N 


N N 

11. Diazo-hydroxides. Up to the year 1898, although 
the existence of isomeric metallic diazo-oxides was without 
doubt, the free diazo-hydroxides corresponding to these had 
not been prepared. 

From the great similarity of the diazonium salts to the 
ammonium salts, Hantzsch drew the conclusion that a corre- 
sponding diazonium hydroxide should be capable of existence, 
which would of course make a third isomeric hydroxide, 
having the constitution C 6 H 5 . N(OH) ; N. He succeeded in 
obtaining an aqueous solution of this by treating diazobenzene 
chloride with silver oxide (see p. 100). Determination of the 
electric conductivity of the solution * showed that the affinity 
constant of the base at is seventy times greater than that 
of ammonium hydroxide, and is a little greater than that of 
piperidine. The affinity constants of methoxybenzenedi- 
azonium hydroxide and ^-cumenediazonium hydroxide are 
even greater, and are very close to those of the alkali 

The effect of introducing halogens into the aromatic nucleus 
is shown in the following table : 

k = velocity constant. 

C 6 H 6 .N 2 .OH 0-123 

Br.C 6 H 4 .N 2 .OH 0-0149 

(2:4)Br 2 :C 6 H 3 .N 2 .OH . . . 0.0136 
(2:4:6)Br 3 ;C 6 H 2 .N 2 .OH . . 0-0014 
A comparison of the electrical conductivity experiments 
with the results obtained in the hydrolysis of ethyl acetate 

* Hantzsch and Davidson, Ber., 1896, 31, 1612. 


by benzenediazonium hydroxide indicates that, in 1/128 
j\ r -solution at 0, about 33 per cent, of the base exists in the 
ionized condition. The ionization observed in the hydrolysis 
experiments is greater than that determined by the conduc- 
tivity experiments, and this shows that the electrolytic 
dissociation is entirely due to the reaction 

C 6 H 5 .N 2 .OH ^ C 6 H 5 .N;N + OH 
and not to the electrolysis of a diazonium s^ti-diazo-oxide 
C 6 H 5 .N.O.N 2 .C 6 H 5 


12. Condition of the non-ionized diazonium hydroxide. 

The solution of benzenediazonium hydroxide, when treated 
with alkali hydroxides, generates an appreciable amount of 
heat, and thus behaves as a weak acid. This reaction is also 
indicated by determinations of the electrical conductivity of 
the diazonium hydroxide solutions when treated with one, 
two, or more molecular proportions of sodium hydroxide. 

Hantzsch and Davidson explain this by assuming that the 
non-ionized part of the diazonium hydroxide exists in solution 
in a hydrated form, thus 

C 6 H 6 .N.OH 


which, with alkali hydroxide, loses water, giving the syn- 

C 6 H 5 .N 


and this then furnishes the sodium salt 

C 6 H 5 .N 


Diazonium hydroxides are consequently known only in solu- 
tion, and the existence of ss/Ti-diazo-hydroxides is doubtful. 

13. Constitution of iso (anti) diazo-hydr oxides. In 
1899 Hantzsch enunciated his theory of pseudo-acids, a term 


applied to neutral compounds which, under the influence of 
alkalis, yield stable salts. Thus, for example, phenylnitro- 
methane, C 6 H 5 . CH 2 .NO 2 , is stable, neutral, and a non- 
electrolyte, but with alkalis it changes to the isomeric form 
C 6 H 5 . CH : NO.OH, which forms stable salts, thus 

C 6 H 5 .CH:NO.OK.* 

An examination of the properties of the metallic anti-di&zo- 
oxides showed that the solution obtained by treating them 
with an equivalent amount of hydrochloric acid has a neutral 
reaction, and, conversely, when this solution is treated with 
an equivalent quantity of alkali, the product is neutral. The 
substance obtained, therefore, by treating the diazo-salt with 
acid has the properties of a pseudo-acid,f and is best repre- 
sented as being a primary nitrosoamine 




> R.NH.NO 

Stable nitrosoamine 

an*i-diazo-oxide. Labile (acid). (pseudo-acid). 

In this, it will be noticed, Hantzsch adopts the older view, so 
long combated by him, of the tautomeric form of the iso- 
diazo-hydroxide, with the exception that he adheres to the 
ewtfi-configuration for the labile form. 

Hantzsch and Pohl J claimed to have prepared these anti- 
diazo-hydroxides and also the nitrosoamines in the solid 
condition, and stated that e the nature of. the so-called iso- 
diazo-hydroxides is now definitely elucidated'. This work, 
however, cannot be regarded as having any bearing on the 
point, as one example dealt with by Hantzsch and Pohl, 
namely, the conversion of the metallic anti-s&lt of 2 : 4 : 6-tri- 
bromodiazobenzene into the nitrosoamine, was shown by 
Orton to be quite inaccurate. Orton found that the sub- 
stance described as the nitrosoamine by Hantzsch and Pohl 
was in reality a mixture of the quinonediazide, 

* Ber., 1899, 32, 575. 

t Hantzsch, Schumann, and Engler, Ber., 1899, 32, 1703. 

1 Ber., 1902, 35, 2964. 

Proc. Boy. Soc., 1902, 71, 153 ; Trans., 1903, 83, 796. 


(see p. 67), and a hydroxyazo-compound.* 

This proof, of course, must be held to throw grave doubt 
on the correctness of the other cases mentioned by Hantzsch 
and Pohl, especially as Hantzsch has admitted that 2:4: 6-tri- 
bromophenylnitrosoamine is unstable and cannot be isolated 
free from other substances in an analysable condition. Orton's 
work thus shows that no nitrosoamine is formed under the 
conditions used by Hantzsch. 

* Compare also Hantzsch, Ber., 1903, 36, 2069 ; Orton, Trans., 1905, 



1. Constitution of the coloured diazo-salts of Jacobson. 
In 1895 Jacobson * examined the diazo-salts of >-amino- 
diphenylamine which had been first prepared by Ikuta.f 
These diazo-salts are distinguished by their great stability 
and by their yellow colour, in consequence of which Jacobson 
assigned to them the constitution 

C 6 H 5 .N.C 6 H 4 .N,HX 

Hantzsch investigated the reactions of these compounds, 
pointing out that other coloured diazo-salts were known 
which had undoubtedly the normal constitution, namely, the 
diazo-salts of di-iodobenzene, diazofluoren, diazophenanthrene, 

He showed that Jacobson' s diazo-salts had a neutral reaction 
and were thus similar to the ordinary diazo-salts, whilst 
a compound of the above formula would be expected to 
undergo hydrolytic dissociation and therefore show an acid 
reaction. The salts were therefore considered by Hantzsch to 
possess the constitution C 6 H 5 . NH.C 6 H 4 . NX \ N. 

By the action of potassium hydroxide, however, no corre- 
sponding metallic salt was formed, but an explosive compound 
insoluble in water, having the formula C 12 H 9 N 3 , which was 
evidently an anhydride of the diazo-hydroxide, 

C 6 H 5 .NH.C 6 H 4 .N 2 .OH 
and to which Hantzsch gave the formula 

* Annalen, 1895, 287, 131. t Annalm, 1893, 272, 282. 

corresponding to WolfFs formula* for the quinonediazides 

X N 

0:C 6 H/|| 

X N 

2. Constitution of diazo-salts according to Walther. 

A formula for diazobenzene chloride was proposed by Walther 
in 1895,f but has not hitherto found acceptance. 

Walther, in endeavouring to explain the fact that the same 
product is formed by the interaction of diazobenzene chloride 
and bromoaniline, on the one hand, and bromodiazobenzene 
chloride and aniline, on the other, suggested that nitrous acid 
might be supposed to contain quinquevalent nitrogen, and 
represented the formation of diazobenzene chloride by the 

C 6 H 5 . NH 3 C1 + N^O = C 6 H 5 . NH 2 C1.N^OH 
\H \H 

= C 6 H 6 . NHC1 : NH : + H 2 

this representing diazobenzene chloride only in aqueous solu- 
tion. The hydroxide would hence be C 6 H 5 . N : NH : 0, and 
diazoaminobenzene C 6 H 6 . N : NH : N . C 6 H 5 , thus providing an 
explanation of the fact referred to above. 

3. Constitution of diazo-compounds according to Bruhl. 

The question of the constitution of the diazo-compounds 
has been attacked by Bruhl from the point of view of their 
refractive powers. J 

It was found that the refraction of the N 2 group in the 
diazo-compounds is 8-41, or about 3-4 higher than that of the 
same group in the primary hydrazines. 

In azoxybenzene, the value for the N 2 group is 11-9, whilst 
that calculated on the assumption of a single linking between 
the nitrogen atoms is 7-5, so that Bruhl regards azoxy- 
benzene as a compound of the structure 

: 6 C 6 H 6 .N:N.C 6 H 6 




* Annalen, 1900, 312, 126. 

t J. pr. Chem., 1895 [ii], 51, 528, 581. 

} Zeitsch. physikal. Chem., 1898, 25, 577, 26, 47. 


The normal diazo-oxides are looked upon as being constituted 
similarly to the nitrosoacyl-compounds, and the formation of 
diazobenzene from nitrosoacetanilide is written 

N/l +NaOH = 0^.00^+ 6 5 \N 
\^ H / 

Normal diazo- 

-> C 6 H 5 .N:N.ONa 

Normal diazo-oxide. tso-diazo-oxide. 

Diazonium salts, however, have the Blomstrand formula 
C 6 H 5 .NC1|N. >-Nitrodiazobenzene methyl ether (compare 
p. 147) has the constitution NO 2 . C 6 H 4 . N : N.OMe and is an 

Finally, benzenediazoic acid is regarded as possessing the 
nitroamine constitution 


C 6 H 5 .NH 

4. Constitution of the diazo-compounds according to 
Dobbie and Tinkler. Dobbie and Tinkler* attacked the 
problem of deciding the constitution of the isomeric diazo- 
compounds by observing their ultraviolet absorption spectra. 

The two forms of benzaldoxime had previously been shown 
to exhibit identical spectra, so that isomeric substances, differ- 
ing only as do the benzaldoximes, should also give identical 
spectra, but distinct ones if they were structurally isomeric. 

Diazo-sulphonates. The potassium benzenediazo-sulpho- 
nates were found to give identical spectra, which would be 
expected if the substances had the constitution assigned to 
them by Hantzsch, namely 

C 6 H 5 .N C 6 H 5 .N 

II and |i 

S0 3 K.N N.S0 3 K 

Diazo-cyanides. The diazo-cyanides prepared from p-anisi- 
dine and p-chloroaniline were examined ; both of these pairs 

* Trans., 1905, 87, 273. 


of isomerides gave almost identical spectra, so that here again 
the syn- and a7ii-configuration would account for this. 

The solution of the diazonium cyanide, OMe.C 6 H 4 . N(CN) N, 
gave an entirely different spectrum. This compound is there- 
fore structurally isomeric with the other two. 

Diazo-oxides. The potassium benzenediazo-oxides were 
found to give quite different spectra, and the conclusion is that 
they are structurally isomeric. The nitrosoamine formula for 
the more stable salt would account for this difference, and it was 
found that the spectrum of this salt and that of phenylmethyl- 
nitrosoamine, C 6 H 5 . N(CH 3 ).NO, were in complete agreement, 
a fact which points to the formula C 6 H 5 .NK.NO as the 
correct one for the stable salt. 

It was further discovered that a very dilute solution of the 
labile compound had a spectrum agreeing closely with that of 
diazobenzene chloride, and this appears to indicate that the 
original compound changes into a third modification, having 
the constitution of a true diazonium compound 
C 6 H 6 .N(OK);N. 

Snlphobenzenediazo-oxides. The potassium and sodium 
compounds obtained from diazotized sulphanilic acid by the 
action of caustic alkali were also examined. These gave 
similar results, as in the preceding case ; the spectra were 
different and they are therefore structurally isomeric and not 

Applying the reasoning used by Dobbie and Tinkler in the 
preceding case, these compounds would consequently possess 
the constitution 

KS0 3 . C 6 H 4 . N 2 . OK KSO 3 . C 6 H 4 . NK.NO 

Labile. Stable. 

5. Constitution of the diazo-compounds according to 
Armstrong and Robertson. Armstrong and Robertson, in 
1905,* in discussing the question of the relation of colour to 
constitution, considered that the yellow colour of phenylazo- 
ethane, C 6 H 6 . N : N.C 2 H 5 , is conditioned by the presence of the 
group C 6 H 5 . N : N. alone, the ethyl radical not being known 

* Trans., 1905, 87, 1280. 


as a chromogenic centre in any other case. Arguing from 
this, they concluded that all compounds of the form 

C 6 H 5 .N:N.X 

should be coloured, and consequently that only coloured diazo- 
compounds can be represented by such a formula. Armstrong 
and Robertson call the above compound ' phenyldiazoethane ', 
but it belongs to the azo-group just as much as does azoben- 
zene, C 6 H 5 . N : N.C 6 H 5 . Moreover, there are great chemical 
differences between the coloured diazo-compounds 

C 6 H 5 .N 2 .X 

and the azo-compounds, C 6 H 5 . N 2 . R, where X is an acidic 
group and R is an inert group, and, to take an example, 
according to the above reasoning, if coloured diazo-compounds 
of the formula C 6 H 5 . N : N.X unite readily with phenols, &c., 
to form azo-compounds, one should expect all compounds 
containing the group C 6 H 5 . N : N. to give the same reaction, 
which of course they do not. It is therefore not correct to 
compare the two in the way Armstrong and Robertson have 
done. These authors, from the above reasoning, deny that the 
syn- and cmi-formulae can represent the constitutions of the 
labile and stable metallic diazo-compounds respectively, and 
adopt the nitrosoamine formula R.NK.NO for the latter. 

For the colourless diazo-salts the diazonium formula is 
advocated as being the only alternative one which, at the 
time, could be devised. The isomeric sulphonates and cyanides 
are considered to be represented by the formulae 
R.N.SO,K R.N : N.S0 3 K 




and R.N.CN R.N : N.CN 



In the case of the cyanides, Hantzsch's syn -compound is 
regarded as a mixture of the diazonium salt and the anti-sM. 

In a similar way the labile metallic compounds are assigned 
the diazonium configuration, whilst the stable compounds, 


which, being colourless, could not be written R.N : N.OH 
according to the authors' reasoning, are considered to be 
further hydrated, R.N(OH).NH(OH) or E.NH.N(OH) 2 , and 
these diazo-hydrates, on dehydration, would give rise to the 
isodynamic nitrosoamines, thus 

R.N(OH).NH(OH) -^ R.N.NH 

R.NH.N(OH) 2 -^ R.NH.NO. 

Which of these is the parent substance of the iso-compounds 
is considered to depend on the colour or non-colour of pure 

These views provoked a vigorous criticism by Hantzsch,* 
who pointed out that both coloured and colourless azo-com- 
pounds exist in the aliphatic series, for example, the deep red 
azo-dicarboxylic ester, CO 2 R.N : N.C0 2 R, and the colourless 
azo-iso-butyric acid derivatives, CRMe 2 . N : N.CRMe 2 ,f and in 
the aromatic series, the nitrodiazo-ester 

NO a .C 6 H 4 .N:N.O.CH 8 t 
is quite colourless. 

He maintained, therefore, that the presence of the group 
.N : N. was no reason why a compound should be coloured. 

The diazonium formula for the normal diazo-oxides, and 
Armstrong and Robertson's proposed formulae for the iso- 
diazo-oxides had previously been shown to be unsatisfactory 
and, as the s^-diazo-cyanides and sulphonates are more 
intensely coloured than the cm^-forms, the former could not 
consist of a mixture of the latter with a colourless diazonium 

* Proc., 1905, 21, 289. t Thiele, Annalen, 1896, 290, 1. 

t von Pechmann, Ber., 1894, 27, 672. 



IT is evidently an impossible task to reconcile all the 
conflicting theories of the constitution of diazo- compounds, 
and although some of them may be dismissed at once, others 
must receive careful consideration. 

In spite of the immense amount of work done in this field 
of research by Hantzsch and his pupils, it cannot be said that 
the stereochemical theory is generally accepted there are 
many evidences in chemical literature which point to this 
conclusion. On the other hand, the alternative view of 
structural isomerism has several exponents, but no common 
ground has apparently been reached. 

We shall, therefore, endeavour to sum up the principal 
points connected with the diazo- compounds, which are im- 
portant in arriving at a theory of the constitution of these 

1. Constitution of the diazo-salts (diazonium salts). 
There is fairly general agreement that the formula of 
Blomstrand represents the constitution of the diazo-salts 
better than that proposed by Kekuld The existence of a 
salt-forming nitrogen atom in the diazo-complex makes it 
necessary to assume that one nitrogen at least is quinquevalent. 
Moreover, it would appear most probable that this nitrogen 
atom is the one attached directly to the aromatic nucleus, for 
otherwise we should arrive at the formula C 6 H 5 . N j NCI, 
which, postulating as it does a quadruple linking between 
the two nitrogen atoms, is unlikely. We thus arrive at the 
conclusion that in diazobenzene chloride there is a union 
between the phenjd group and a quinquevalent nitrogen atom, 
which is linked to a univalent chlorine atom and united with 


a second nitrogen atom. This union may be indicated by 
a dotted line thus 

C 6 H 6 N-C1 


It is very important to observe, howevefe that the facts 
referred to do not prove anything more tfoan this ; that is to 
say, they do not indicate the number of bonds between phenyl 
and quinquevalent nitrogen or between quinquevalent and 
tervalent nitrogen. It has, however, been generally considered 
that the union between phenyl and quinquevalent nitrogen is 
one linking, whilst that between the two nitrogen atoms is 
three, thus 

C 6 H 5 -N-C1 

There is, however, another and, in the opinion of the 
author, a better way of arranging these linkings (see p. 163). 

2. The labile and stable isomeric diazo-componnds. In 

studying the properties of these compounds it is impossible 
to avoid the conclusion that the labile or normal compounds 
resemble the diazonium salts most closely. The similar 
behaviour with regard to the formation of azo-compounds, 
the instability, the probability that, in many cases, equilibrium 
mixtures or solid solutions of the two exist, and the re- 
semblance of the two absorption spectra all point to this 

The constitution of the labile salts should therefore be more 
closely allied to that of the diazonium salts than to that of 
the stable salts. Probably for this reason V. Meyer and 
Jacobson, Blomstrand and others, regarded the labile com- 
pounds actually as diazonium compounds. 

It seems, however, probable that Hantzsch's reasoning 
against this view as regards the metallic compounds (p. 135) 
is correct, that is to say, they are not diazonium derivatives. 

In this connexion the views of Bruhl as to the close 
relationship between the constitution of the normal metallic 


diazo- compounds and the nitrosoacyl-compounds are to be 
noted, and it seems likely that the true constitution of the 
former may be similar to that of the latter, although not 
assuming the form assigned to them by Briihl, and that the 
formulae of the nitrosoacyl-compounds may be tautomeric 
with those of the corresponding diazo-acetates. Such a new 
formulation of the nitrosoacyl-compounds, however, cannot be 
suggested up to the present. 

We arrive, therefore, at the conclusion that neither the 
actual diazonium formula C 6 H 5 . N(OK) \ N nor the syn-di&zo- 

C 6 H 5 .N 


really represents the constitutions of these compounds. 

The labile sulphonates are also probably not diazonium 
compounds, but the arguments against the possibility of them 
being sulphites are not so strong, and this is one of the 
reasons why the stereochemical theory is not accepted with 
regard to this case. 

Hantzsch's argument against the sulphite constitution is 
principally that a diazo-sulphite would form three ions whilst 
the sulphonates give rise to only two ions. This argument 
has been recorded in many textbooks without, however, the 
important fact being added that no less an authority than 
Ostwald has stated (p. 130) that a diazonium sulphite would 
give rise only to two ions. Here, therefore, the possibility 
of the normal sulphonates being really sulphites cannot be 
regarded as excluded. 

In the case of the labile diazo-cyanides we are met with 
a similar uncertainty as in the case of the sulphonates, namely, 
the possibility of isomeric change in the added group. So 
long as it is not proved that the labile diazo-cyanides cannot 
be isocyanides it cannot be maintained that they are syn- 

Turning now to the stable or -iso-compounds, most of the 
work done points to the nitrosoamine formula for the metallic 
compounds, C 6 H 5 . NK.NO, and there seems to be a consensus 



of opinion that the -iso-diazo-hydroxides, diazo- cyanides, and 

sulphonates have the azo-constitution 

C 6 H 5 . N : N.OH C 6 H 5 . N : N.CN C 6 H 5 . N : N.S0 3 H. 

From what has been said previously it will be evident that 
the existence of a special cm^-configuration of these compounds 
depends on the simultaneous existence of the corresponding 
s^/Ti-compounds, which, as has been shown, cannot be regarded 
as having been definitely proved to possess this constitution. 



IN 1907 the author of this book put forward a new theory 
of the constitution of diazo-compounds,* which it is con- 
sidered will not only explain the reactions of the diazo- 
compounds more readily than any of its predecessors, but also 
serve to throw light on some phenomena hitherto left unsolved. 
Perhaps the most striking reaction of the diazo-salts (diazonium 
salts) is the readiness with which the whole of the diazo-nitrogen 
is eliminated. There are no examples in the literature of singly- 
linked nitrogen being otherwise than firmly attached to the 
benzene nucleus and requiring energetic treatment for its 

There are, however, cases where nitrogen, when attached to 
the benzene nucleus by two bonds, is most readily eliminated, 
one of the most striking being that of quinonechloroimide 


Here, as in the case of the diazo-salts, the nitrogen is split off 
simply by heating the compound with water to 100. 

This reaction showing the great difference in behaviour of 
nitrogen attached by one and two linkings respectively to the 
aromatic nucleus, is obviously of much importance in arriving 
at a decision as to the manner in which nitrogen is united with 
the aromatic nucleus in diazo-salts. 

It appears almost certain, from this analogy, that an atom 
of nitrogen in these salts is attached to the aromatic nucleus 
by two linkings. This idea at once leads us to a quinonoid 
configuration of the diazo-salts, thus 



* Trans., 1907, 91, 1049. 


which conform to the requirements of these salts in that the 
nitrogen attached to the benzene ring is quinquevalent, 
'and, of course, it explains more satisfactorily than does the 
Blomstrand formula the ready elimination of diazo-nitrogen. 
An obvious criticism, and indeed one which has been privately 
advanced against this formula, is that, with a single linking 
between the tervalent nitrogen atom and the para-carbon 
atom, one should expect that, on reduction, the double linking 
between the nitrogen atom would break and a para-diamine 
result. This objection would be a weighty one were the 
tervalent nitrogen united to a carbon atom simply (as in the 
case of aniline) instead of to the CH group. This fact is of 
much importance, for that a great difference in stability exists 
in the two cases has been proved by E. Buchner. In his 
researches on the action of diazoacetic ester on unsaturated 
acid esters * this chemist found that the group 

always became converted into the group : C : N.NH, and it 
may be concluded therefore that the latter group is more stable 
than the former. 

But we have in the new formula for diazo-salts (I) the 
same group (II) 

C1.N : N 




in which, from the above work by Buchner, we may reason- 
ably conclude that the linking : N.CH is more unstable than 
the linking : C : N, and would be the first to be ruptured in 
any reaction tending to destroy the configuration. 

The first stage, therefore, in such a reaction can be repre- 
sented by 

* Ber., 1894, 27, 868, 877, 879 ; see also Curtius, Ber., 1896, 29, 767. 


C1N : N GIN : N- 

and the quinonoid formation having been thus disturbed, the 
ordinary configuration is resumed when the reaction proceeds 
to the next stage (reduction, formation of azo-compounds, 
elimination of nitrogen, &c.). 

The ordinary reactions of the diazo-salts are thus satisfac- 
torily explained. 

All the work of Hantzsch and his collaborators on the nature 
of the radical ' diazonium ', as has been shown, indicates 
that this acts like a compound alkali metal, the acid radical 
attaching itself to the quinquevalent nitrogen atom. 

It is quite obvious that in this respect no difference can be 
detected between 

V V 

C 6 H 5 .N;N and C 6 H 6 :N:N 

so that Hantzsch's results are equally applicable to the new 
formulation of ' diazonium '. 

We shall now consider some phenomena in diazo-chemistry 
which have hitherto remained unexplained by any of the 
former theories. When we compare ^-phenylenediamine and 

NH 2 ^ ^>NH, NH 2 <( )>-<( >NH, 

we find a great difference in their behaviour towards nitrous 
acid. The former is converted into the tetrazo-compound only 
with difficulty and under special conditions, but the latter 
changes with perfect readiness. According to the Blomstrand 
or Kekule* formula this cannot be explained, but light is 
thrown on the mechanism of the reaction by the following 
considerations. Benzidine, when tetrazotized, becomes 


B ... 1 


but the first stage in diazotizing p-phenylenediamine must 
give a compound of formula 



We now obtain a compound containing an amino-group, 
which is in the para-position with respect to a carbon atom, 
all of whose affinities are satisfied, and therefore it cannot 
link up with a second nitrogen atom. This explains why the 
tetrazotization does not proceed normally. Under the special 
conditions necessary, however (see p. 21), the linking between 
the aminic carbon atom and the tervalent diazo-nitrogen atom 
is broken, thus^ 


and now the amino-group can be diazotized, for its added 
nitrogen atom unites with the corresponding nitrogen atom 
of the first diazo-group 

N - N 

There are also several p-diamines in which only one amino- 
group can be diazotized, thus 

NH 2 NH, 

H n 

Here the same explanation as that given for the case of 
>-phenylenediamine probably holds good, but the para-linking 
may be rendered more stable by the presence of the acidic 
groups, and hence it does not break to allow the diazotization 
of the second amino-group to take place. This is, however 
broken when an azo-compound is formed, so that the second 
amino-group may now be readily diazotized. 


It is evident that, according to this theory, diazo -salts cannot 
be formed where a quinonoid configuration is precluded, so 
that we can now explain why the compounds 
NH 2 H 

NH 2 


do not give diazo-salts, whilst the compounds 


are readily diazotized. 

In this connexion, also, we see that it is impossible for 
aliphatic amines to yield diazo-salts, thus 

CH 2 . NH 2 CH 1| CH 2 . N 2 C1 

| gives | N and not | 

C0 2 Et C0 2 Et COJEt 

This new formula for diazo-salts has received confirmation 
by the work of Morgan and Hird.* 

Isomeric diazo-compounds. The hydroxide corresponding 
with diazobenzene chloride would, on the above formulation, 
have the constitution 

It is evident that the hydroxyl group may migrate to the 
other nitrogen atom now that the quinquevalency of the first 
nitrogen atom is not supported by the presence of an acidic 
group. We thus arrive at the formula 

* Trans., 1907, 91, 1505 ; cpmpare alsp Morgan and Wootton, Trans., 
1907, 91, 13H. 



which, from its close connexion with the previous one, is an 
exceedingly probable one for the normal (s2/7i) diazo- compounds 
(metallic salts, and, supposing that the normal cyanides and 
sulphonates are not isocyanides and sulphites respectively, 
for these also). 

The great resemblance existing between the normal diazo- 
compounds and the diazonium salts is very readily explained 
by this formula. The more energetic means necessary to 
produce the iso-diazo-compounds naturally tend to destroy the 
bicyclic system here shown ; accordingly the change from 
normal to iso-compounds occurs thus 

N(OH)(CN)(SO 3 H) 

.N:N(OH)(CN)(S0 3 H), 

arriving in a very natural manner at the most probable formula 
for the ^-compounds. 

As has been shown, the formula for the iso-metallic salts 
can allow tautomerism to take place, and consequently the 
most stable condition is assumed by these compounds 
C 6 H 5 . N : N.OK - C 6 H 5 . NK.NO. 

Finally, as the stereochemical theory appears thus to be 
rendered unnecessary, it seems best to use the older terms, 
normal and iso, instead of s^/Tfc and anti respectively. 

For the diazo-salts with acids, it is, however, most convenient 
to retain the term ' diazonium ', although in this book, as was 
explained in the introduction, the term ' diazo ' introduced by 
Griess, and also used by the Chemical Society, has been 
retained in order to avoid confusion or misunderstanding before 
the theoretical explanation had been reached. 


Absorption spectra of diazo-compounds, 

Acetoxy-group, replacement of diazo- 
group by, 53. 

Alcohols, action of, on diazo-compounds, 

Amines, diazotization of, 14. 

Aminoazo-compounds, 83 et seq. 

Amino-group, replacement of diazo-group 
by, 52. 

Aminonaphthols, diazotization of, 16. 

Ammonia, action on diazo-compounds, 

Amyl diazoacetate, 106. 

Amyl nitrite, use of, 6. 

Aniline, 53. 

Aurin, 30. 

Azoammonium, 117. 

Azobenzene, 82. 

Azo-compounds, 80 et seq. 

Azo-dyes, discovery of, 2, 4. 

Azogen red, 26. 

Azoimino-group, replacement of diazo- 
group by, 56, 58. 

n-Azonaphthalene, 51. 

Azophor blue D., 26. 

Azophor red P.N., 26. 

Azoxy benzene, 81. 

Azoxy-compounds, 81. 

Barium nitrite, use of, 10. 
Benzeneazoacetaldehyde, 9B. 
Benzeneazoacetone, 92. 
Benzeneazodiphenyl, 51, 60, 62. 
Benzeneazomethane, 91. 
Benzeneazonitroethane, 91. 
Benzenediazoic acid, 102, 122, 155. 
Benzenediazosulphonates, 128, 140, 155. 
Benzidine, 23. 
Benzonitrol, 26. 

Benzoyl chloride, action on diazo-com- 
pounds, 59. 
Bromobenzene, 45. 
/3-Bromonaphthalene, 46. 

Calcium nitrite, use of, 9. 
Chlorobenzene, 43. 

l-Chlorodiazo-/3-naphthalene nitrite, 69. 
Chlorodibromodiazobenzeue bromide, 68. 
Cuprous chloride, r&le of, 44. 
Cyanogen, replacement of diazo-group 

by, 49. 
Cyano-group, replacement of diazo-group 

by, 49. 

Decomposition of diazo-compounds, rate 
of, 35. 

Diamines, 23. 
Diazo, meaning of, 1, 117. 
Diazoacetamide, 106. 
Diazoacetic ester, 104. 
Diazoacetophenone, 110. 
Diazoaminobenzene, 78. 
Diazoaminobenzoic acid, discovery of, 1, 
Diazoamino-compounds, 73 et seq., 78. 
Diazoaminomethane, 111. 
Diazo-anhydrides, 148. 
Diazo-azides, 9. 
Diazobenzene chloride, 7, 17. 
Diazobenzene hydroxide, 100, 120. 
Diazobenzeneimide, 57, 58, 114. 
Diazobenzene nitrate, 6, 11, 27, 28, 70, 


Diazobenzene picrate, 8. 
Diazobenzene sulphate, 7, 27. 
jp-Diazobenzenesulphonic acid,8, 144,156. 
Biazocamphor, 103. 
Diazo-carbonates, 9. 
Diazo-chromates, 8, 9. 
Diazo-compounds, constitution of, 112 et 


Diazo-cyanides, 130, 140, 142, 143, 155. 
Diazodiphenylamine, 153. 
Diazo-ethers, 146. 
Diazo-fluorides, 9. 
Diazo-group, migration of, 76. 
Diazo-halides, 138. 
Diazo-hydroferricyanides, 9. 
Diazo-hydroxides, 149, 150, 167. 
Diazoic ncids, 101. 
Diazomethane, 108. 
Diazomethanedisulphonic acid, 108. 
Diazonaphthalenesulphonic acid, 8, 30. 
Diazo-nitrates, 10. 
Diazo-nitrites, 9. 
Diazonium, 133. 
Diazonium hydroxide, 150. 
Diazo-oxides, 144, 156. 
iso-Diazo-oxides, 100. 
Diazo-perchlorates, 9. 
Diazo-perhalides, 138, 139. 
Diazophenols, 9, 11. See also Quinone- 

p-Diazophenylhydroxylamine chloride, 


E>iazo-picrates, 8. 
Diazoprimuline, 70. 
Diazo-salts, discovery of, 3. 
Diazo-sulphides, 56. See also Thiodia- 


Diazo-thiosulphates, 9. 
Diazotization, 13 et seq., 28. 
p-Diazotoluene nitrate, 11. 
Diazo-tungstates, 9. 



4 :*4'-Dihydroxydiphenyl, 29. 

/3/S-Dinaphthyl, 62. 

3 : 4-Dinitro-o-anisidine, diazotization of, 


Dinitro-p-anisidine, diazotization of, 63. 
2 : 2'-Dinitrodiphenyl, 61. 
Dinitro-p-toluidine, 15. 
Diphenyl, 60. 
Diphenyl ether, 41. 
Dithiosalicylic acid, 50. 

Ethyl diazoacetate, 105, 106, 107. 
Ethyl iso-diazoacetate, 107. 
Explosibility of diazo-compounds, 27. 

Fluorobenzene, 46. 
Formazyl compounds, 94. 

Guaiacol, 31. 

Halogens, replacement of diazo-group 
by, 43. 

Hydrazine, action on diazo-compounds, 

Hydrogen, replacement of diazo-group 
by, 42, 55. 

Hydrogen sulphide, action on diazo- 
compounds, 56. 

Hydroxyazo-compounds, 86 et seq. 

Hydroxydiphenyl, 30, 60. 

lodo-compounds, 46. 

Light, action OB diazo-compounds, 70 et 

Methyl diazoacetate, 105. 

Nitrazol C., 26. 
Nitrobenzene, 51, 52. 
p-Nitro-o-cresol, 31. 
m-Nitrodiazobenzene chloride, 28. 
2>-Nitrodiazobenzene chloride, 14, 26, 28, 

p-Nitrodiazobenzene nitrate, 27. 

Nitroformazyl, 91. 

Nitro-group, replacement of diazo-group 
by, 51. 

Nitro-p-phenylenediamine, 22. 

p-Nitrophenylnitrosoamine, 97. 

Nitrosamine red in paste, 26. 

Nitrosoacetanilide, 77. 

Nitrosoamines, 151. 

Nitrosoanilides, 121, 155. 

Nitrosodiazo-derivatives, 17. 

Nitroso-group, replacement of diazo- 
group by, 52. 

Nitrosulphonic acid, use of, 10. 
Nitrosyl bromide, use of, 10. 
Nitrosyl chloride, use of, 10. 

Oxidation of diazo-compounds, 101. 

Pentabromoaniline, 15. 
Phenols, formation of, 29. 
Phenylcarbimide, 49. 
Phenyldiazomethane, 109. 
Phenylenediamines, action of nitrous 

acid on, 19. 

Phenylethyltriazen, 73. 
Phenylhydrazine, constitution of, 119. 
Phenylmethyltriazen, 73. 
Phenyl sulphide, 50, 51, 62. 
Phenylthiocarbimide, 49. 
Potassium benzenediazo-oxides, 99. 
Potassium benzyldiazo-oxide, 110. 
Potassium methyldiazo-oxide, 110. 

Quinonediazides, 11, 12, 64, 67, 100, 126, 
152. See also Diazophenols. 

Reduction of diazo-salts, 42. 
Refraction of diazo-compounds, 154. 

Sandmeyer's reaction, 43 et seq. 

Sodium diazoacetate, 109. 

Stability of diazo-solutions, 34. 

Strecker's salt, 118, 128. 

Sulphuric acid group, replacement of 
diazo-group by, 55. 

Sulphonic acid group, replacement of 
diazo-group by, 51. 

Sulphur dioxide, action on diazo-com- 
pounds, 54. 

Sulphur, replacement of diazo-group by, 

syn and anti t 123. 

Thermochemistry of diazo-compounds, 

jp-Thiocyanodiazobenzene chloride, 68. 

Thiocyano-group, replacement of diazo- 
group by, 49. 

Thiodiazoles, 12. 

Thiophenols, 50. 

jp-Toluonitrile, 48. 

Triamines, 25. 

Triazolens, 32. 

2:4: 6-Tribromodiazobenzene chloride 
transformation of, 69. 

2:4: 6-Tribromo-l-nitrobenzene, 52. 

Trinitroaniline, 15. 


Abt, 19. 

Altschul, 13, 18. 

Ambtihl, 7, 91. 

Ammelburg, 65. 

Ampola, 91. 

Andresen, 5. 

Andrews, 83. 

Angeli, 44, 103, 110, 111, 

Armstrong, 45, 156, 157. 

Auvers, 89, 90. 

Badische Anilin- und Soda- 
Fabrik, 67, 68. 

Baeyer, 8, 42, 47, 82. 

Baly, 83, 91. 

Bamberger, 5, 11, 17, 18, 
27, 32, 52, 56, 61, 67, 74, 
76, 82, 86, 91, 94, 96 et 
seq., 108, 120, 121,122,125, 
127, 128, 129, 132, 133, 
134, 140, 141, 144 et seq. 

Battegay, 68. 

Bayer & Co., 61. 

Beeson, 41, 42. 

Bennewitz, 10. 

Berger, 61. 

Berlin, 55. 

Bernthsen, 74. 

Berthelot, 27, 28, 70. 

Bevan, 70. 

Beyer, 93. 

Beysen, 15. 

Biehringer, 69. 

Binder, 83. 

Blagden, 47, 52. 

Blomstrand, 116, 117, 120, 
140, 160. 

Bernstein, 51, 62. 

Boehringer & Sons, 18. 

Borgbaus, 20, 21. 

Borsche, 60, 90. 

BromweU, 40. 

Brtihl, 154, 160. 

Buchner, 164. 

Billow, 22, 92, 93. 

Buntrock, 35. 

Burdett, 71. 

Busch, A., 59. 

Busch, M., 94. 

Butleroff, 112. 

Cain, 31, 32, 33, 36, 37, 48, 

Cameron, 11, 39, 40. 

Cantzler, 48. 

Caro, 2, 8, 25. 

Cassella & Co., 14. 

Castellana, 8, 9. 

Cauffman, 40. 

Chamberlain, 41, 42. 

Chattaway, 62. 

Ciusa, 72. 

Claisen, 93. 

Clans, 15, 129. 

Coates, 71. 

Graff, 83. 

Corse, 60. 

Grandmougin, 32, 42, 55, 

Cross, 70. 


Culmann, 42, 61. 

Green, 70. 

Curtius, 4, 96, 103, 106, 107, 

Griess, 1, 2, 3, 4, 6, 8, 12, 

110, 164. 

16, 19, 21, 22, 29, 34, 38, 

D'Angelo, 8, 9. 

39, 43, 55, 56, 57, 58, 60, 

Danziger, 142. 

74, 76, 83, 96, 112, 113, 

Darapsky, 107. 


Dashiell, 41. 

Griffin, 42. 

Davidson, 11, 149, 150. 

Gruhl, 73. 

Dimroth, 73, 111. 

Haarhaus, 83. 

Dobbie, 155, 156. 

Hailer, 93. 

Duval, 95. 

Haller, 39. 

Dybowski, 141. 

Hantzsch, 5, 7, 9, 11, 15, 17, 

East, 53. 

20, 21, 28, 32, 35, 37, 40, 

Eble, 73. 

47, 52, 64, 56, 68, 69, 76, 

Ehrenpreis, 83. 

89, 98, 100, 101, 102, 104, 

Ehrhardt, 48. 

107, 108, 109, 121, 123 et 

Eibner, 42. 

seq., 133 et seq., 141 et 

Ekbom, 54. 

seq., 158, 160, 161, 165, 

Engler, 151. 

Hasse, 39. 

Epstein, 20. 

Hausknecht, 48. 

Erban, 14. 

Hausser, 35. 

Erdmann, 44. 

Hayduck, 38. 

Erlenmeyer, 112, 116, 139. 

Heinichen, 31. 

Euler, 36, 37, 147. 

Heller, 87. 

Ewers, 141. 

Henderson, 51. 

Eynon, 9, 83, 84. 

Hepburn, 9. 

Eyre, 63, 66. 

Hepp, 17. 

Farmer, 89. 

Heusler, 60. 

Favrel, 92, 94, 95. 

Hewitt, 89, 90. 

Feer, 70. 

Hille, 55. 

Feitler, 44. 

Hird, 167. 

Fierz, 57. 

Hirsch, 7, 30, 34, 37, 60. 

Fischer, E., 18, 25, 38, 55, 

Honigsberger, 90. 

58, 92, 118, 119. 

Hopfner, 93. 

Fischer, O., 11, 17, 25, 38. 

Hofmann, 39, 48, 80, 113. 

Forgan, 61. 

Hofmeister, 41, 86. 

Forster, 57. 

Holleman, 115. 

Fourneaux, 83. 

Holm, 73. 

Fraenkel, 106. 

Hopkins, 40. 

Franke, 55. 

Hunt, 1, 2. 

Freese, 56. 

Ikuta, 153. 

Freimann, 87. 

Jacobson, 12, 89, 90, 140, 

Friedlander, 42, 66. 

153, 160. 

Friese, 91. 

Jager, 68. 

Fritsch, 76. 

Jaeger, 8, 17, 47. 

Frobenius,98, 121, 146, 147. 

Japp, 92, 120. 

Gabriel, 48. 

Jenisch, 93. 

Gaess, 65. 

Jochem, 7, 40. 

Gasiorowski, 42, 61. 

Jourdan, 92. 

Gattermann, 5, 44, 45, 48. 

Jungius, 78. 

Gehren, 46. 

Kalle & Co., 31. 

Gerilowski, 136, 144. 

Karres, 25. 

Gerland, 1, 2. 

Kastle, 10. 

Girard, 10. 

Kaufler, 23, 25. 

Glogauer, 141. 

Kegel, 89. 

Glutz, 65. 

Keiser, 10. 

Goldschmidt, 73, 75, 78, 88, 

Kekul<, 67, 113, 114, 115, 

89, 90, 133, 145. 


Goske, 74. 

Kiefer, 41. 

Graebe, 60. 

Kjellin, 92. 



Klascm, 50. 

Noelting, 15, 19, 32, 58, 68, 

Smythe, 68. 

Klingemann, 92, 120. 


Spiegelberg, 113. 

Knoevenagel, 6, 27. 

Notzel, 87. 

Stallberg, 48. 

Koenigs, 54, 141. 

Norris, 60. 

Stein, 70. 

Kolbe, 1, 2. 

Oddo, 28, 34, 37, 46. 91. 

Stenhouse, 10. 

Koninck, 10. 

Oehmichen, 32. 

Stephens, 64. 

Kortright, 40. 

Oliveri-Tortorici, 17. 

Storch, 52, 101 . 

Kostanecki, 89. 

Orloff, 77. 

Streatfeild, 66, 76. 

Kraus, 56, 67. 

Orndorff, 39, 40, 53, 87. 

Strecker, 55, 116, 118, 184. 

Kriickeberg, 95. 

Orton,5, 12, 52,68, 71,143, 

Stiiber, 15. 

Kuchenbecker, 82. 

151, 152. 

Tauber, 19, 20, 23. 

Kuhling, 61. 

Osborne, 109. 

Tafel, 91. 

Ktister, 48. 

Ostwald, 130, 161. 

Thiele, 108, 109. 

Kunz, 11. 

Pabst, 10. 

Tichwinsky, 61. 

Ladenburg, 10, 19. 

Palmer, 39, 41. 

Tinkler, 155, 156. 

Landsteiner, 101. 

Parks, 41. 

Traube, 109. 

Lange, 25. 
Langfurth, 113. 

Pechmann, 4, 56, 75, 93, 94, 
98, 107, 108, 120, 121, 125, 

Troger, 55, 141. 
Tuck, 83, 90, 91. 

Lauth, 18. 

141, 146, 147, 158. 

Uhlmann, 95. 

Lehmann, 108, 109. 

Pelet, 25. 

Ulatowski, 54. 

Lenz, 82. 

Perkin, P. M., 76. 

Ullmann, 44, 45, 61. 

Leuchart, 50. 

Pfitzinger, 42. 

Vagt, 109. 

Licbermann, 89. 

Piria, 1. 

Vanino, 93. 

Limpricht, 10. 

Pohl, 151. 

Vaubel, 77. 

Locher, 60. 

Power, 32. 

Veraguth, 90. 

LOw-Beer, 90. 

Eabischong, 94. 

Vesterling, 55. 

Macintyre, 60. 

Bay, 87. 

Vielle, 27, 28, 70. 

McPherson, 89. 

Eedard, 25. 

Vignon, 27. 

Mai, 42, 53. 

Eenauld, 108. 

Vock, 9, 40. 

Manck, 108. 

Eemsen, 39, 41. 

Vorlander, 61. 

Mann, 50. 

Robertson, 156, 157. 

VotoSek, 44, 45. 

Marquardt, 95. 

Eomer, 55, 118. 

Wacker, 53. 

Martius, 6. 

Ettgheimer, 11. 

Waijss, 42. 

Mebus, 14. 

Euff, 70. 

Walder, 19, 20. 

Meissen, 103. 

Sandmeyer, 5, 43. 

Wallach, 17. 

Meldola, 5, 9, 18, 32, 53, 63, 

Saunders, 46. 

WaUbaum, 15. 

64, 66, 71, 76, 82, 83, 84, 

Salkowski, 81. 

Walter, ,T., 45. 


Schanb, 55. 

Walter, L. E., 50. 

Metcalf, 41. 

Schiff, 58, 103. 

Walther, 154. 

Meyer, C., 108. 

Schleissing, 68. 

Wanklyn, 25. 

Meyer, F., 61. 

Schmiedel, 139. 

Warnecke, 55. 

Meyer, V., 7, 15, 75, 91, 92, 

Schmidt, 5, 96, 97. 

Wechsler, 147. 

93, 120, 140, 160. 

Schmitt, O., 18. 

Wichelhaus, 28. 

Michael, 58. 

Schmitt, E., 3, 8, 9, 11, 46, 

Wiesinger, 55. 

Michaelis, 18. 


Willstatter, 90. 

Micklethwait, 9, 84. 

Schraube, 5, 76, 96, 97. 

Winston, 42. 

Mills, 82, 83. 

Schultze, 130, 143. 

Wislicenus, 111. 

Mitchell, 90. 

Schumann, 28, 151. 

Witt, 10, 32. 

Mitscherlich, 80. 

Schwalbe, 14, 35, 36. 

Wohl, 58. 

Mixter, 83. 

Seidler, 15. 

Wolbring, 94. 

Moale, 42. 

Sheddon, 32. 

Wolff, 126. 

Mohlau, 17, 32, 61, 89. 

Shober, 41. 

Woolcott, 32, 71. 

Morgan, 8, 9, 69, 84, 85, 167. 

Silberrad, 10, 104, 107. 

Wootton, 8, 167. 

Mttller, E., 107, 110. 

Silberstein, 67. 

Wray, 32, 71. 

Miiller, F. H. S., 55. 

Simpson, Maule & Nichol- 

Wroblewski, 32, 38. 

Mtiller, J., 94, 99. 

son, 86. 

Wulz, 74. 

Miinzer, 92, 93. 

Singer, 54, 141. 

Wurtz, 112. 

Muller, 35. 

Smart, 10. 

Zander, 38. 

Nicoll, 36, 37. 

Smith, W., 1. 

Zenisek, 45. 

Niementowski, 61. 

Soc. Chim. des Usines du 

Zincke, 10, 82, 89. 

Nietzki, 21, 83, 89. 

Ehone, 31. 

Zinin, 80. 

Oxford : Horace Hart, Printer to the University. 

Mr. Edward Arnold's List of 

Technical & Scientific Publications 

Extract from the LIVERPOOL DAILY POST: 

" During recent years Mr. Edward Arnold has placed in the hands of 
engineers and others interested in applied science a large number of volumes 
which, independently altogether of their intrinsic merits as scientific works, 
are very fine examples of the printers' and engravers' art, and from their 
appearance alone would be an ornament to any scientific student's library. 
Fortunately for the purchaser, the publisher has shown a wise discrimination 
in the technical books he has added to his list, with the result that the 
contents of the volumes are almost without exception as worthy of perusal 
and study as their appearance is attractive." 

The Dynamical Theory of Sound. By HORACE 

LAMB, D.Sc., LL.D., F.R.S., Professor of Mathematics in the Victoria 
University of Manchester. viii + 304 pages, 86 Illustrations. DemySvo., 
i2s. 6d. net (inland postage 5d.). 

An Introduction to the Theory of Optics. By 

ARTHUR SCHUSTER, Ph.D., Sc.D., F.R.S., Honorary Professor of Physics 
at the University of Manchester. Second Edition (Revised), xvi+352 
pages. Demy 8vo., 153. net (inland postage 5d.). 

The Becquerel Rays and the Properties of 

Radium. By the Hon. R. J. STRUTT, F.R.S., Professor of Physics at the 
Imperial College of Science and Technology. Second Edition (Revised 
and Enlarged). vi + 2i5 pages. Demy 8vo., 8s. 6d. net (inland postage 5d.). 

The Text-Book of Physics. By Dr. R. S. 

WILLOWS, Lecturer at the Sir John Cass Institute. 280 Figures. 

Advanced Examples in Physics. By A. O. 

ALLEN, M.A., B.Sc., Assistant Lecturer in Physics at Leeds University. 
With Answers. Crown 8vo., 2s. (inland postage 5d.). 

Notes on Practical Physics. By A. H. FISON, 

D.Sc., Lecturer in Physics at the Medical Schools of Guy's Hospital 
and London Hospital. Crown 8vo., 35. 6d. 

An Introduction to Practical Physics : for 

Colleges and Schools. By E. H. BARTON, D.Sc., F.R.S.E., Professor 
of Experimental Physics, University College, Nottingham ; and T. P. 
BLACK, M.Sc., Ph.D., Registrar of University College, Nottingham. 
55 Figures. Crown 8vo., 33. 6d. 

Five-Figure Tables of Mathematical Functions. 

ByJ. B. DALE, M. A., Assistant Professor of Mathematics, King's College, 
London. Demy 8vo., 35. 6d. net. 


Mr. Edward Arnolds List of 

Logarithmic and Trigonometric Tables (To 

Five Places of Decimals). By J. B. DALE, M.A. 2s. net. 

Mathematical Drawing. Including the Graphic 

Solution of Equations. By G. M. MINCHIN, M.A., F.R.S., and J. B. 
DALE, M.A. ys. 6d. net (inland postage 4d.). 

Homogeneous Co-ordinates. By W. P. MILNE, 

M.A., D.Sc. , Mathematical Master, Clifton College. Crown 8vo.,5s. net. 

An Introduction to Projective Geometry. By 

L. N. G. FILON, M.A., F.R.S., Assistant Professor of Mathematics, 
University College, London. Crown 8vo. F 73. 6d. 

The Strength and Elasticity of Structural 

Members. By R. J. WOODS, M.E., M.Inst.C.E. Second Edition. 
xii-f 310 pages. Demy 8vo., IDS. 6d. net (inland postage 4d.). 


The Theory of Structures. xii + 276 pages. 

Demy 8vo., los. 6d. net (inland postage 4d.). 

The Field Engineers Handbook. By G. 

CARVETH WELLS, Federated Malay States Government Railways, and 
late of the Grand Trunk Pacific Railway, Canada; and ARDNDEL S. 
CLAY, B.Sc., Bramwell Medallist. With Illustrations and Tables. 
Small 8vo., 73. 6d. net (inland postage 4d.). 

Reinforced Concrete Design. By O. FABER, 

B.Sc., A.M.I.C.E., etc., Chief Engineer to Messrs. Trollope and Colls, 
Ltd. ; and P. G. BOWIE, A.C.G.I., Assistant Engineer to Messrs. Trollope 
and Colls, Ltd. xx+332 pages. With 158 Illustrations. 123. 6d. net 
(inland postage 5d.). 

The Calculus for Engineers. By JOHN PERRY, 

M.E., D.Sc., F.R.S., Professor of Mechanics and Mathematics in the 
Royal College of Science. Tenth Impression. Crown 8vo., 73. 6d. 

The Balancing of Engines. By W. E. DALBY, 

M.A., B.Sc., M.Inst.C.E., M.I.M.E., Professor of Engineering, City and 
Guilds (Engineering) College. Second Edition. xii-f- 283 pages. 
Demy 8vo. , IDS. 6d. net (inland postage 4d.). 

Valves and Valve Gear Mechanisms. By 

W. E. DALBY, M.A., B.Sc., M.Inst.C.E., M.I.M.E. xviii + 366 pages. 
Royal 8vo., 2 is. net (inland postage 5d.). 

Machine Sketches and Designs for Engineering 

Students. By A. CRUICKSHANK, A.M.I.Mech.E., and R. F. McKAY, 
M.Sc. Demy 4to., is. 6d. 

Petrol Engine Construction and Drawing. 

By W. E. DOMMETT, Wh. Ex., A.M.I.A.E., Admiralty Prizeman. 
Demy 8vo., 35. net (inland postage 4d. ). 

Technical and Scientific Publications 3 

Steam Turbine Design. With especial reference 

to the Reaction type, and including chapters on Condensers and 
Propeller Design. By J. MORROW, M.Sc., D.Eng. (Armstrong College, 
Newcastle-on-Tyne). viii + 472 pages. i6s. net (inland postage 6d.). 

Steam Boilers and Boiler Accessories. By W. 

INCHLEY, B.Sc. , Lecturer in Electrical and Mechanical Engineering, 
University College, Nottingham. Fully illustrated. Crown 8vo., cloth. 
8s. 6d. net (inland postage 4d.). 

Heat Engines. By H. A. GARRATT, Assoc.M.Inst. 

C.E., M.I.N.A., Principal of the L.C.C. School of Engineering and 
Navigation, xii + 332 pages, 173 Figures, and a Chart of Properties of 
Steam. Crown 8vo., 6s. (inland postage 4d.). 

Heat and Steam. Notes and Examples on Steam 

Engines and Turbines for Engineers and Engineering Students. By 
Eng.-Lieut. S. G. WHEELER, R.N. viii+224 pages, 85 Figures. Crown 
8vo., 43. 6d. net (inland postage 4d.). 

Hydraulics. For Engineers and Engineering 

Students. By F. C. LEA, M.Sc., A.M.Inst.C.E., Lecturer in Applied 
Mechanics and Engineering Design, City and Guilds (Engineering) College, 
London. Second Edition. 153. net (inland postage 5d.). 

Hydraulics. By R. BUSQUET, Professeur a 1'ficole 

Industrielle de Lyon. Translated by A. H. PEAKE. 73. 6d. net. 

The Practical Design of Motor-Cars. By JAMES 

GUNN. viii + 256 pages. Demy 8vo. 103. (inland postage 4d.). 

Power Gas Producers : their Design and 

Application. By PHILIP W. ROBSON, sometime Vice-Principal of the 
Municipal School of Technology, Manchester. iv+ 247 pages. Demy 
8vo., los. 6d. net (inland postage 4d.). 

The Foundations of Alternate Current Theory. 

By C. V. DRYSDALE, D.Sc. (Lond.), M.I.E.E. xii+ 300 pages. Demy 
8vo., 8s. 6d. net (inland postage 4d). 

Electrical Traction. By ERNEST WILSON, Whit. 

Sch., M.I.E.E., Professor of Electrical Engineering in the Siemens 
Laboratory, King's College, London; and FRANCIS LYDALL, B.A., B.Sc. 
Two volumes, sold separately. Vol. I., Direct Current ; Vol. II., Alter- 
nating Current. 155. net each (inland postage 5d. each). 

A Text-Book of Electrical Engineering. By 

Dr. A. THOMALEN. Translated by G. W. O. HOWE, M.Sc. Second 
Edition, viii + 464 pages. Royal 8vo. , 153. net (inland postage 6d.). 

Alternating Currents. A Text-Book for 

Students of Engineering. By C. G. LAMB, M.A., B.Sc., A.M.I.E.E., 
Clare College, Cambridge. Second Edition. 333 pages. los. 6d. net 
(inland postage 5d.). 

Mr. Edward Arnold's List of 

Electric and Magnetic Circuits. By ELLIS H. 

CRAPPER, M.I.E.E., Head of the Electrical Engineering Department in 
the University College, Sheffield. viii + 38o pages. IDS. 6d. net. 

Applied Electricity. A Text-Book of Electrical 

Engineering for "Second Year" Students. By J. PALEY YORKE. 
Second Edition. xii + 42o pages, ys. 6d. (inland postage 4d.). 

Exercises in Electrical Engineering. By 

T. MATHER, F.R.S.. M.I.E.E., and G. W. O. HOWE, M.Sc., M.I.E.E. 
viii-Hya pages, is. 6d. net. 

Physical Chemistry: its Bearing on Biology 

and Medicine. By J. C. PHILIP, M. A. , Ph.D. , B. Sc., Assistant Professor 
of Chemistry in the Imperial College of Science and Technology. Illus- 
trated. 75. 6d. net (inland postage 4d.). 

Lectures on Theoretical and Physical Chemis- 
try. By Dr. J. H. VAN 'T HOFF, Professor of Chemistry at the University 
of Berlin. Translated by R. A. LEHFELDT, D.Sc. 


Part II. CHEMICAL STATICS. 8s. 6d. net. 


A Text-Book of Physical Chemistry. By R. A. 

LEHFELDT, D.Sc., Professor of Physics at the Transvaal University College, 
Johannesburg. xii + 3o8 pages. Crown 8vo., 73. 6d. (inland postage 4d.). 

Organic Chemistry for Advanced Students. 

By JULIUS B. COHEN, Ph.D., B.Sc., Professor of Organic Chemistry in 
the University of Leeds, and Associate of Owens College, Manchester. 
viii + 632 pages. Demy 8vo., 2is. net (inland postage 6d.). 

Organic Chemistry for Advanced Students. 

Part II. By Prof. J. B. COHEN. Demy 8vo., i6s. net (inland postage 6d.). 

The Chemistry of the Diazo-Compounds. By 

JOHN CANNELL CAIN, D.Sc., Editor of the Publications of the Chemical 
Society. Demy 8vo. , los. 6d. net (inland postage 4d.). 

The Chemical Synthesis of Vital Products and 

the Inter-relations between Organic Compounds. By RAPHAEL MEL- 
DOLA, F.R.S., V.P.C.S., F.I.C., etc. ; Professor of Chemistry in the City 
and Guilds of London Technical College, Finsbury. Vol. I., xvi + 338 
pages. Super royal 8vo., 2 is. net (inland postage 5d.). 

Organic Analysis : Qualitative and Quantita- 
tive. By H. T. CLARKE, B.Sc., A.I.C., Lecturer in Stereo- Chemistry in 
University College, London. With Introduction by Professor J. NORMAN 
COLLIE, Ph.D., LL.D., F.R.S. viii + 264 pages. Crown 8vo., 53. net. 

Elements of Inorganic Chemistry. By the late 

W. A. SHENSTONE, F.R.S. New Edition, revised and partly rewritten 
by R. G. DURRANT, M.A., Assistant Master, Marlborough College, 
cioth, 55. (inland postage 40!.). 

Technical and Scientific Publications 5 

A Course of Practical Chemistry. Being a 

Revised Edition of "A Laboratory Companion for Use with 
Shenstone's 'Inorganic Chemistry.'" By the late W. A. SHEN- 
STONE, F.R.S. xii+ 136 pages. Crown 8vo., cloth, is. 6d. 

Outlines of Inorganic Chemistry. With special 

reference to its Historical Development. By E. B. LUDLAM, D.Sc., 
Head of Chemical Department, Clifton College. With Introductory 
Note by Professor SirW. RAMSAY, K.C.B., F.R.S. Crown 8vo., 45. 6d. 

Outlines of Experimental Chemistry. By E. B. 

LODLAM, D.Sc., and H. PRESTON. Demy 8vo., 2s. 

Service Chemistry. Being a Short Manual of 

Chemistry and Metallurgy and their Application in the Naval and 
Military Services. By VIVIAN B. LEWES, F.I.C., F.C.S.. Professor of 
Chemistry, Royal Naval College, Greenwich ; and J. S. S. BRAME. F.C.S., 
Instructor in Chemistry, Royal Naval College, Greenwich. FOURTH 
EDITION, thoroughly revised. With 65 Diagrams and 6 full-page Plates. 
Demy 8vo., i6s. (inland postage 6d.). 

Exercises in Chemical Calculation. By H. F. 

COWARD, D.Sc., Chief Lecturer in Chemistry. Municipal School of 
Technology, Manchester ; and W. H. PERKINS, M.Sc., Assistant Lecturer 
in Chemistry, University of Leeds. viii + i52 pages. 2s. 6d. net. 

A History of Chemistry. By Dr. HUGO BAUER, 

Royal Technical Institute, Stuttgart. Translated by R. V. STANFORD, 
B.Sc. (Lond.). Crown 8vo., 33. 6d. net (inland postage 4d.). 

Physical Chemistry for Beginners. By Dr. CH. 

M. VAN DEVENTER. With a Preface by Dr. VAN 'T HOFF. Translated 
by R. A. LEHFELDT, D.Sc. Crown 8vo., cloth, 2s. 6d. 

The Principles of Applied Electrochemistry. 

By A. J. ALLMAND, D.Sc. 136 Figures. Demy 8vo., cloth, i8s. net 
(inland postage 6d.). 

Experimental Researches with the Electric 

Furnace. By HENRI MOISSAN. Translated by A. T. DE MOUILPIED, 
M.Sc., Ph.D. xii + 3O7pages. DemySvo., los. 6d. net (inland postage 4d.). 

Electrolytic Preparations. Exercises for use 

in the Laboratory by Chemists and Electro-Chemists. By Dr. KARL 
ELBS, Professor of Organic and Physical Chemistry at the University of 
Giessen. Translated by R. S. HDTTON, M.Sc. 45. 6d. net. 

Introduction to Metallurgical Chemistry for 

Technical Students. By J. H. STANSBIE, B.Sc. (Lond.), F.I.C., Lecturer 
in the Birmingham University Technical School. Second Edition, xii + 
252 pages. Crown 8vo., 43. 6d. (inland postage 4d.). 

On the Calculation of Thermo-Chemical Con- 
stants. By H. STANLEY REDGROVE, B.Sc. (Lond.), F.C.S. iv + io2 pages. 
Demy 8vo., 6s. net (inland postage 4d.). 

Mr. Edward Arnold's List of 

Electroplating. By W. R. BARCLAY, A.M.I.E.E., 

Silver Medallist, City and Guilds of London Institute ; Lecturer on 
Electroplating in the University of Sheffield ; and C. H. HAINSWORTH, 
A.M.I.E.E., Assistant Lecturer in Electrical Engineering in the Uni- 
versity of Sheffield. Crown 8vo., 75. 6d. net (inland postage 4d.). 

Manual of Alcoholic Fermentation and the 

Allied Industries. By CHARLES G. MATTHEWS, F.I.C., F.C.S., etc. 
xvi + 295 pages. Crown 8vo., 75. 6d. net (inland postage 4d.). 

The Chemistry of Breadmaking. By JAMES 

GRANT. M.Sc.Tech., F.I.C., F.C.S., Head of the Fermentation Industries 
Department in the Municipal School of Technology, Manchester ; Ex- 
aminer in Chemical Technology in the Victoria University, Manchester. 
viii+ 224 pages. Illustrated. 

An Introduction to Bacteriological and En- 
zyme Chemistry. By GILBERT J. FOWLER, D.Sc., Lecturer in Bacterio- 
logical Chemistry in the Victoria University of Manchester. Illustrated. 
Crown 8vo., 73. 6d. net. 

Modern Methods of Water Purification. By 

JOHN DON, A.M.Inst.Mech.E., and JOHN CHISHOLM, A.M.Inst.Mech.E. 
xvi + 368 pages. Second Edition. Demy 8vo., 155. net. 

Smoke : A Study of Town Air. By J. B. 

COHEN, F.R.S., Professor of Organic Chemistry in the University of 
Leeds; and A. G. RUSTON, B.A. , B. Sc., Science Tutor in the Depart- 
ment of Agriculture, University of Leeds. 53. net (inland postage 40!.). 

Practical Photo-micrography. By J. EDWIN 

BARNARD, F.R.M.S., Lecturer in Microscopy, King's College, London. 
Illustrated. Demy 8vo., 155. net. 

The Chemistry and Testing of Cement. By 

C. H. DESCH, D.Sc., Ph.D., Lecturer in Metallurgical Chemistry in the 
University of Glasgow. Illustrated. IDS. 6d. net (inland postage 4d.). 

Wood. A Manual of the Natural History and 

Industrial Applications of the Timbers of Commerce. By G. S. 
BOULGER, F.G.S., A.S.I., Professor of Botany and Lecturer on Forestry 
in the City of London College. Second Edition, xi + 348 pages, with 48 
Plates and other Illustrations. Demy 8vo., 125. 6d. net (inland postage 

A Class Book of Botany. By G. P. MUDGE, 

A.R.C.Sc., and A. J. MASLEN, F.L.S. With over 200 Illustrations. 
Crown 8vo., 73. 6d. 

Elementary Botany. By E. DRABBLE, D.Sc., 

Lecturer on Botany at the Northern Polytechnic Institute. 234 pages, 
with 76 Illustrations. Crown 8vo., cloth, 2s. 6d. 

An Experimental Course of Chemistry for 

Agricultural Students. By T. S. DYMOND, F.I.C. 23. 6d. 

Technical and Scientific Publications 7 

The Development of British Forestry. By 

A. C. FORBES, F.H.A.S., Chief Forestry Inspector to the Department 
of Agriculture for Ireland. Illustrated. Demy 8vo.,ios. 6d. net. 

English Estate Forestry. By A. C. FORBES, 

F.H.A.S. x-f 332 pages, Illustrated. Demy 8vo., 123. 6d. net (inland 
postage . 5d.). 

Astronomical Discovery. By HERBERT HALL 

TURNER, D.Sc., F.R.S., Savilian Professor of Astronomy in the 
University of Oxford. xii + 225 pages, with 15 Plates. Demy 8vo., 
cloth, IDS. 6d. net (inland postage sd.). 

An Introduction to the Study of the Protozoa. 

With Special Reference to the Parasitic Forms. By E. A. MINCHIN, 
F.R.S., Professor of Protozoology in the University of London. With 
194 Figures and Bibliography. Demy 8vo., cloth, 2is. net (inland 
postage 6d.). 

The Evolution Theory. By Dr. AUGUST WEIS- 

MANN, Professor of Zoology in the University of Freiburg in Breisgau. 
Translated, with the Author's co-operation, by Professor J. ARTHUR 
THOMSON, and MAKGARET THOMSON. Two vols., xvi + 4i6 and viii + 3g6- 
pages, with more than 130 Illustrations. Royal 8vo., cloth, 323. net. 

The Chances of Death and Other Studies in 

Evolution. By Professor KARL PEARSON, M.A. , F.R.S. 2 vols. With 
Illustrations. Demy 8vo., 255. net (inland postage 6d.). 

Hereditary Characters. By CHARLES WALKER, 

M.Sc.. M.R.C.S., Director of Research in the Glasgow Cancer Hospital. 
Demy 8vo., 8s. 6d. net. 

The Life of the Salmon. With reference more 

especially to the Fish in Scotland. By W. L. CALDERWOOD, F.R.S. E., 
Inspector of Salmon Fisheries for Scotland. Illustrated, ys. 6d. net. 

A Text-Book of Zoology. By G. P. MUDGE, 

A.R.C.Sc. (Lond.), Lecturer on Botany and Zoology at the London 
School of Medicine for Women, and Demonstrator on Biology at the 
London Hospital Medical College. Illustrated. Crown 8vo., ys. 6d. 

House, Garden, and Field. A Collection of 

Short Nature Studies. By L. C. MIALL, F.R.S. viii + 3i6 pages. 
Crown 8vo., 6s. (inland postage 4d.). 

Animal Behaviour. By C. LLOYD MORGAN, LL.D., 

F.R.S., Professor of Psychology in the University of Bristol, viii + 344 
pages. Second Edition, ys. 6d. net (inland postage 5d.). 


Psychology for Teachers. New Edition, entirely 

rewritten, xii + 308 pages. Crown 8vo. , cloth, 43. 6d. 

8 Mr. Edward Arnold's Technical & Scientific Books 


The Geology of Coal and Coal-Mining. By 

WALCOT GIBSON, D.Sc., F.G.S. 352 pages. With Illustrations. 75. 6d. 
net (inland postage 4d.). 

The Geology of Ore Deposits. By H. H. 

THOMAS and D. A. MACALISTER, of the Geological Survey of Great 
Britain. Illustrated. 75. 6d. net (inland postage 4d.). 

The Geology of Building Stones. By J. ALLEN 

HOWE, B.Sc., Curator of the Museum of Practical Geology. Illustrated. 
75. 6d. net (inland postage 4d.). 

The Geology of Water Supply. By H. B. WOOD- 
WARD, F.R.S. Illustrated. Crown 8vo., 75. 6d. net (inland postage 4d.). 

Geology of the Soil and Substrata. By H. B. 

WOODWARD, F.R.S. Crown 8vo. [In the Press. 

A Text-Book of Geology. By P. LAKE, M.A., 

Royal Geographical Society Lecturer in Regional and Physical Geography 
at the University of Cambridge; and R. H. RASTALL, M.A., F.G.S. , 
Demonstrator in Geology in the University of Cambridge. Illustrated. 
Second Edition. Demy 8vo., i6s. net. 

The Dressing of Minerals. By HENRY Louis, 

M.A., Professor of Mining and Lecturer on Surveying, Armstrong College, 
Newcastle-on-Tyne. x + 544 pages. With 416 Illustrations. 

Traverse Tables. With an Introductory 

Chapter on Co-ordinate Surveying. By HENRY Louis, M.A., and 
G. W. CAUNT, M.A. Flexible cloth, 45. 6d. net (inland postage 3d.). 

Oil-Finding : An Introduction to the Geological 

Study of Petroleum. By E. H. CUNNINGHAM CRAIG, B.A., F.G.S., late 
of H.M. Geological Survey. With an Introduction by Sir BOVERTON 
REDWOOD, Bart. 13 Plates, and 18 Illustrations. 8s. 6d. net. 

Winding Engines and 'Winding Appliances : 

Their Design and Economical Working. By G. MCCULLOCH, 
A.M.I.M.E., Inspector of Machinery in the Department of Mines, West 
Australia ; and T. CAMPBELL FUTERS, M.I.M.E. Fully Illustrated. 
Demy 8vo., 2 is. net (inland postage 6d.). 

Mines and Minerals of the British Empire. 

By RALPH S. G. STOKES, xx + 403 pages, 70 Illustrations. Demy 8vo., 
155. net (inland postage 5d.). 

Geological and Topographical Maps : their 

Interpretation and Use. By A. R. DWERRYHOUSE, D.Sc., F.G.S., 
Lecturer in Geology in the Queen's University of Belfast, viii + 133 pages, 
with 90 Figures. 45. 6d. net.