STRUCTURAL, THERMODYNAMIC AND KINETIC (HYSTERESIS) ASPECTS OF THE ENANTIOTROPIC FIRST-ORDER PHASE-TRANSFORMATIONS OF N-ANILINOPHTHALIMIDE AND N-(N'-METHYLANILINO)PHTHALIMIDE

The crystal structures of the orthorhombic and monoclinic polymorphs o f N-anilinophthalimide (m.p. of monoclinic polymorph 457 K) have been determined by X-ray diffraction at 293 K and were found to have only s mall differences between the molecular conformations in the two phases , but quite different molecular arrangements. There is very weak N-H . .. O hydrogen bonding in the orthorhombic phase and weak NH NH ... O h ydrogen bonding in the monoclinic phase. The thermal motion in the cry stals of both phases has been analyzed and their thermal expansion det ermined. The enthalpies of solution in a number of solvents have been calculated from the solubility measurements of Chattaway & Lambert [(1 915),J. Chem. Sec. 107, 1773-1781], which also give the temperature an d enthalpy of the enantiotropic 'orthorhombic to monoclinic' phase tra nsformation (T-c = 283 K; Delta H-transf = 1.54 kJ mol(-1)). The phase -transformation endotherm in a DSC (differential scanning calorimetry) trace from the orthorhombic polymorph occurs only at similar to 310 K on heating and there is no corresponding exotherm on cooling; the DSC trace gives Delta H-transf = 1.62 kJ mol(-1) and Delta H-fus = 26.9 k J mol(-1). This phase transformation is an example of the common type (occurrence similar to 95%) where Delta V-transf = (V-monoclinic - V-o rthorhombic) is positive. Analogous (but less complete) results have b een obtained for the monoclinic and triclinic polymorphs of N-(N'-meth ylanilino)phthalimide (m.p. of triclinic polymorph 398 K). There were only minor differences between the molecular conformations in the two phases, but the molecular arrangements were quite different. This 'mon oclinic to triclinic' phase transformation also has Delta V-transf = ( V-triclinic - V-monoclinic) positive. The solubility (and other) measu rements of Chattaway & Lambert (1915) gave T-c = 328.43 K and Delta H- transf = 4.17 kJ mol(-1). A DSC trace for the monoclinic crystals show s a broad endotherm at similar to 372-376 K on heating, but there is n o corresponding exotherm on cooling; Delta H-transf = 3.6 kJ mol(-1) a nd Delta H-fus = 21.7kJ mol(-1). These two compounds provide further e xamples of molecular crystals with a large hysteresis in their first-o rder enantiotropic solid-state phase transformations, the transformati on to the high-temperature phase occurring well above T-c and the low- temperature phase not being recovered on cooling below T-c. Presumably the hysteresis must be ascribed to as-yet unknown features of the nuc leation processes.