POLYMORPHISM 551 



/>-bromo and of 2 : 4 dibromoacetanilide is the stable form ; the 

 compounds therefore do not show a transition point between 

 these limits. 



The number of enantiotropic organic compounds is not 

 large and it is only recently that the discovery of two suitable 

 examples in phthalylphenylhydrazide and phthalylmethyl- 

 phenylhydrazide have enabled several of van't Hofif's conclu- 

 sions for the first time to be put to the test. 



These compounds, 



C «<cS>- N <c,H. and C « H <cS>- N <c!h, 



are easily obtained by heating together at about ioo° equivalent 

 amounts of phthalic anhydride and the corresponding hydrazine 

 when water is eliminated. The practical determination of 

 transition points depends in general on — 



(a) Obvious changes in crystalline form or colour as one 

 modification transforms into the other. 



(b) The change in volume to be observed when one modifica- 

 tion changes into the other. 



(c) The difference in vapour pressure — or solution pressure 

 — shown by the two modifications. 



(d) The difference in energy content shown by evolution or 

 absorption of heat when transformation takes place. 



The properties available, in the case of the phthalyl- 

 hydrazides, for the determination of their transition points are 

 the relative solubilities, the colour and crystalline form, which 

 are markedly different in the pairs of enantiomers, and, in one 

 case, the volume change which accompanies transformation. 



Phthalylphenylhydrazide crystallises in bright yellow mono- 

 clinic crystals of short and compact habit and in very pale 

 yellow orthorhombic plates . The pale yellow form when heated 

 on a water bath for a short time in presence of a small quantity 

 of any solvent transforms completely into the bright yellow 

 form, which is consequently easily obtained pure. The pale 

 yellowform is less readilyobtained quite free from its enantiomer. 

 This can be effected, however, by boiling a concentrated solution 

 in alcohol sufficiently long to remove all nuclei and then cooling 

 rapidly. Some time as a rule elapses before crystallisation 

 begins, but on shaking till crystals appear the pale yellow form 

 usually separates free from the other in small almost colourless 



