Prevalent polymorphism in benzophenones

We report here the crystal structures of dimorphs of 4-hydroxybenzophenone, C₁₃H₁₀O₂, and 4-(dimethylamino)benzophenone, C₁₅H₁₅NO, as well as trimorphs of 4,4′-dimethylbenzophenone, C₁₅H₁₄O. The polymorphs were isolated from slow-evaporation conditions or from cocrystallization attempts. The main differences between the polymorphs involve differences in packing rather than differences in conformation, owing to the limited conformational freedom of the three molecules. 4-Hydroxybenzophenone is the exception, exhibiting almost identical packing arrangements in the two polymorphs, with the only major changes being in the interplanar orientations. The lattice energies of the respective polymorphs of the three compounds reported here are all within 1 kcal mol⁻¹ of each other. The existence of nine further polymorphic benzophenone derivatives in the literature suggests that there is a good deal of polymorphic space in this class of compounds.