' f

100 QUALITATIVE ORGANIC ANALYSIS

to the amine nitrogen. Groups which when substituted into the
molecule lower the basicity of a base or increase the acidity of an
f ;f acid are often spoken of as negative groups. It is not essential

that the negative group be directly joined to the amine group.
Aniline is a weak base but substitution by the nitro group decreases
the basicity still farther. Meta and p-nitraniline are only feebly
basic but there is no doubt but that they fall in solubility Group III.
A nitro group in the ortho position, however, exerts a still greater
effect and we find o-nitraniline and 2, 4-dinitraniline to be almost
insoluble in dilute acids. Halogens exert an effect similar to, but
less powerful than, the nitro group. The substitution of three
halogen atoms into aniline yields a compound that is only feebly
basic.

The union between carbon and nitrogen is fairly stable towards
hydrolysis; negative substitution, however, leads to instability.

R—NH2 1 heat

1 + alkali-------» No reaction.

Ar-NHo J

/NEW1) heat /OHO)

C6H4< + alkali-------> C6H4< +NH3

XN02(2or4) XN02(2or4)

Nitro groups exert a similar effect upon the labilization of
halogen, the effect being greatest in the ortho position.

/Cl

j ale. sol'n No reaction unless at

+ alkali + heat --------------> very high temperature

under pressure.

•ONa

-N02
2NaOH + heat -> I I + NaCl + H20

The union between carbon and carbon is generally very stable
and is ruptured only by high-temperature reactions. We have
already observed, page 43, however, that in the structure

0 O

II 1!

-C-CH2-C-