Xr. He et al., Conformational color polymorphism and control of crystallization of 5-methyl-2-[(4-methy1-2-nitrophenyl)amino]-3-thiophenecarbonitrile, J PHARM SCI, 90(3), 2001, pp. 371-388
5-Methyl-2-[(4-methyl-2-nitrophenyl)amino]-3-thiophenecarbonitrile is an ex
ample of conformational and color polymorphism. The compound crystalizes in
red (R), dark red (DR), light red (LR), and orange (O) modifications. Ther
e are two specific goals for this study. One is to characterize the complex
thermodynamic relationship among these four known forms, and the other is
to use the knowledge of the thermodynamic relationship to control the cryst
allization of these forms. The different forms were characterized by X-ray
powder diffractometry as well as Fourier-transform infrared (FT-IR) and Ram
an spectroscopy; their complex thermodynamic relationships were determined
by thermal analysis, solubility measurements, and slurry conversion studies
. According to the solubility results, all forms are enantiotropically rela
ted: R is the thermodynamically most stable form above 60 degreesC, O is th
e most stable form between room temperature and 60 degreesC, LR is the most
stable form below - 15 degreesC, and DR is metastable throughout the entir
e temperature range. DR, LR, and O have very similar free energy at ambient
temperature, which is the reason for the complex transition behavior. Fina
lly, a schematic energy-temperature diagram was constructed that combines a
ll experimental data in a comprehensive thermodynamic picture and provides
insights into how to control the crystallization of the individual forms. (
C) 2001 Wiley-Liss, Inc.