Polymorphism of benzylthiouracil, an active pharmaceutical ingredient against hyperthyroidism.

The crystal structures of dimorphic benzylthiouracil, a drug against hyperthyroidism, have been redetermined and the atom coordinates of the two independent molecules of form I have been obtained for the first time. The dimorphism convincingly demonstrates the conformational versatility of the benzylthiouracil molecule. It has been established through calorimetric studies that the low-temperature form II transforms endothermically (ΔH = 5.6(1.5) J g) into form I at 405.4(1.0) K. The high-temperature form I melts at 496.8(1.0) K (ΔH = 152.6(4.0) J g). Crystallographic and thermal expansion studies show that form II is denser than form I, leading to the conclusion that the slope of the II-I equilibrium curve in the pressure-temperature phase diagram is positive. It follows that this dimorphism corresponds to a case of overall enantiotropic behaviour, which implies that both solid phases possess their own stable phase region irrespective of the pressure. Moreover, form II is clearly the stable polymorph under ambient conditions.