Thermal analytical and infrared spectroscopic investigations on polymorphic organic compounds—I

Seven polymorphic compounds are described, for each of which at least one enantiotropic relationship can be shown. 2-Chloro-ethylamine hydrochloride is a classical example, with two highly reproducible transformation points. Compounds which do not normally occur in their highest-melting form include 3,4-dibenzyloxyphenylacetic acid (3 modifications), 2-chlorotriethylamine hydrochloride (2 modifications) and 3,5-dimethoxbenzamide (3 modifications).N-Benzylphthal-imide and 2-chloro-4-nitrobenzoic acid occur naturally as mixtures of two modifications, the lower-melting of which is metastable at room temperature and the higher-melting is stable, with transformation points between 10 and 20° C. Only 2,4-dinitrophenyl-2,4-dinitrobenzoate (4 modifications) occurs naturally as the pure highest-melting modification, which is stable at room temperature.