ON DYNAMIC ISOMERISM. 219 
Precisely similar changes are observed in the case of isonitroso-p- 
chlorobenzyl cyanide, 
/Nul 
CLC,H,.C(ON) : NOH 2 CLC,H,.C(ON) € ‘ (2), 
which crystallises from alcohol or water in colourless needles, but passes 
into a yellowish-green isomeride when kept or when crystallised from 
petroleum. The sodium salt also exists in a colourless and a yellow form.' 
In a somewhat similar manner Tanret ? by crystallising from alcohol 
at a high temperature isolated a low-rotatory form of glucose isomeric 
with that which separates at ordinary temperatures. 
Piutti and Abati have recently prepared a white and a yellow 
modification of p-methoxyphenylphthalimide,? which they regard as 
merely dimorphous ; but the difference in colour and the behaviour on 
heating indicate that the two forms are probably dynamic isomerides, and 
may be formulated as the symmetrical and unsymmetrical imide, 
CO C,H 
C.H, aie N.C,H,.OMe 2 CO 2) we C : N.C,H,.0Me 
(colourless) (yellow). 
From boiling alcohol the yellow form separates, but the white form is 
stable in contact with benzene at ordinary temperatures. In the case of 
the reduced compound, p-methoxyphenyltetrahydrophthalimide, 
co O,H 
C,H, cay, N.O;H;.OMe 2 CO Xie *: © : N.C,H,.0Me, 
the white form crystallises from all solvents below 70°, and the yellow 
form above 70°; and this appears to be a fairly definite transition 
temperature. 
Polymorphism.-—If isomeric change is rapid the crystallisation of. 
dynamic isomerides obeys the same laws that govern the crystallisation 
of polymorphous substances, and it becomes very difficult to distinguish 
the two phenomena. In at least two important cases dynamic isomerides 
have been first described as mere polymorphs, and it is probable that in 
other cases the polymorphism is due to differences in molecular structure. 
A rearrangement of the molecules in the crystal can scarcely produce any 
marked alteration in colour, and a difference so striking as that between 
yellow and red mercuric iodide must be associated with some difference in 
molecular structure. 
6. Physical Methods of Studying Dynamic Isomerism. Mutarotation.— 
(1) In some cases it is possible to determine whether a liquid substance is 
a single compound or a mixture of isomerides in equilibrium by calculating 
the physical constants of the possible isomerides and noting whether the 
substance gives a value agreeing with one of these or intermediate in 
magnitude. Unfortunately the constants that can be calculated are few 
in number, and do not vary much in isomeric compounds ; moreover, the 
accumulation of negative groups gives rise to abnormally high optical 
constants, and the observed values often lie right outside the calculated 
1 Zimmermann, J. pr. Chem. 1902, ii. 66, 353. 
2 CLR, 1895, 120, 1060. 3 Ber. 1903, 36, 1000. 
