21 research outputs found
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