SOLUBILITY BEHAVIOR OF ORGANIC COMPOUNDS 17

this class of compounds we find many examples illustrating the
fact that solubility depends upon the molecular complexity of
the solid. The tartaric acids furnish a typical illustration.

In solution, the racemic tartaric acid may be represented as
C02H- (CHOH)2-C09H; as is indicated by its cryoscopic depres-
sion and its ionization constant; its solubility is controlled,
however, by the molecular complexity of the solid. (See Table VI.)

TABLE V

Solubility ii
a 100 grams

solvent
at 20°.

Substance
M. p.

Water
Ethanol

CH— CO2H

|| maleic acid .....
130°
60 g
51 e

CH—C02H

HC— C02H

|| fumaric acid .....
286° auhl.
O.Gg.
sg.

COoH— CH

TABLE VI

Solubility in
Solubility in

M. p.
100 g. water
100 g. alcohol

(20°)
(25°)

C4H6Oc
^-tartaric acid
370°
139 cr
27 <r

&( j.,-

C4HfO6
Z-tartaric aicd
170°
139 e:
97 <r

(C4HaOr,
•H2O)2 'iZ-tartaric acid. . .
205-200°
20. 6 g.
2£-

(racemic)

Among the di- substituted benzene derivatives, we find very
often that the order of solubility lies in the order of the melting-
points. This is illustrated in the solubilities of the following sub-
stituted benzoic acids.