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

Stabilizing the unstable: In textbooks barbituric acid is always drawn in its keto tautomeric form, which is indeed preferred in solution and in most polymorphic phases. However, phase IV, obtained by grinding, consists of molecules in the enol form, as shown by neutron powder diffraction. This phase is found to be the most stable one at room temperature; the “unstable” enol tautomer is stabilized by a higher number of hydrogen bonds.