The Thermodynamically Stable Form of Solid Barbituric Acid: The Enol Tautomer.

Barbituric acid, which has been known since 1863, is drawn in textbooks always as the keto tautomeric form 1. Indeed, this is the most stable form in the gas phase and in solution. Also in the solid state, the keto tautomer is observed in the metastable phase I, the commercial phase II, and a high-temperature phase III, as well as in its dihydrates. In contrast, we now observe that the recently discovered tautomeric polymorph IV consists of molecules in the enol form 2, and that this polymorph is actually the thermodynamically stable phase at ambient conditions.