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 of 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 small 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 N—H...O hydrogen bonding in the monoclinic phase. The thermal motion in the crystals of both phases has been analyzed and their thermal expansion determined. The enthalpies of solution in a number of solvents have been calculated from the solubility measurements of Chattaway & Lambert [(1915), J. Chem. Soc. 107, 1773–1781], which also give the temperature and enthalpy of the enantiotropic `orthorhombic to monoclinic' phase transformation (T<sub>c</sub> = 283 K; ΔH<sub>transf</sub> = 1.54 kJ mol<sup>−1</sup>). The phase-transformation endotherm in a DSC (differential scanning calorimetry) trace from the orthorhombic polymorph occurs only at ∼310 K on heating and there is no corresponding exotherm on cooling; the DSC trace gives ΔH<sub>transf</sub> = 1.62 kJ mol<sup>−1</sup> and ΔH<sub>fus</sub> = 26.9 kJ mol<sup>−1</sup>. This phase transformation is an example of the common type (occurrence ∼95%) where ΔV<sub>transf</sub> = (V<sub>monoclinic</sub> − V<sub>orthorhombic</sub>) is positive. Analogous (but less complete) results have been obtained for the monoclinic and triclinic polymorphs of N-(N′-methylanilino)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 `monoclinic to triclinic' phase transformation also has ΔV_transf = (V_triclinic − V_monoclinic) positive. The solubility (and other) measurements of Chattaway & Lambert (1915) gave T_c = 328.43 K and ΔH_transf = 4.17 kJ mol⁻¹. A DSC trace for the monoclinic crystals shows a broad endotherm at ∼372–376 K on heating, but there is no corresponding exotherm on cooling; ΔH_transf = 3.6 kJ mol⁻¹ and ΔH_fus = 21.7 kJ mol⁻¹. These two compounds provide further examples of molecular crystals with a large hysteresis in their first-order enantiotropic solid-state phase transformations, the transformation 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 nucleation processes.

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