Gas–solid reactions between the different polymorphic modifications of barbituric acid and amines

Abstract

The two known crystalline polymorphic forms of barbituric acid (C₄H₄N₂O₃) (form I and form II) and the dihydrate form [(C₄H₄N₂O₃)·2H₂O] have been reacted with vapours of ammonia, methylamine, and dimethylamine and the crystalline products investigated by means of single crystal and powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and infrared spectroscopy. It has been shown that form I, II and the dihydrate form of barbituric acid react with ammonia leading to the same crystalline ammonium barbiturate salt NH₄(C₄H₃N₂O₃) 1a, while the gas–solid reactions of form II with methylamine, and dimethylamine yield the corresponding crystalline salts, CH₃NH₃(C₄H₃N₂O₃) 1b and (CH₃)₂NH₂(C₄H₃N₂O₃) 1c. Thermal desorption of the bases at ca. 200 °C leads to formation of a new crystal form of barbituric acid, form III, as confirmed by H¹-NMR spectroscopy and by chemical behaviour. De-hydration of the dihydrate form has also been investigated showing that it releases water to yield exclusively crystals of form II.