Polymorphism in substituted barbituric acids

Polymorphism of phenobarbitone: the crystal structure of 5-ethyl-5-phenylbarbituric acid monohydrate

“…However, this preparation proved to be unstable, and did not last long enough for single-crystal measurements to be made. Form XIII was originally reported as a polymorph of phenobarbitone by Cleverley & Williams (1959), and called by them form V. It is unusual among drug barbituric acids in showing absorption bands in the infrared near 3500 cm -1, which were attributed to some degree of enolization in the pyrimidine ring by Cleverley & Williams (1959), and by Mesley et al (1968). Mesley et al also considered the possibility that these absorption bands resulted from the presence of water of crystallization, but rejected this hypothesis in favour of enolization.…”

“…Thus, this form should strictly be excluded from the list of known polymorphs of phenobarbitone. It seems likely that form XII of phenobarbitone (Mesley et al, 1968) and form III of thialbarbitone (Kemithal, 5-alkyl, 5-AZ-cyclohexenyl barbituric acid) (Cleverley & Williams, 1959), both of which also show O-H stretching bands in the infrared, are also hydrates.…”

“…Fischer, 1932;Fischer & K6fler, 1932;Brandsffttter, 1942;Cleverley & Williams 1959;Brandst~tter-Kuhnert & Aepkers, 1961). Of the large number of barbituric acids studied, the 5-ethyl-5-phenyl derivative has been found to exhibit the greatest number of polymorphs.…”

The polymorphism of phenobarbitone

“…The less stable form, corresponding to form V of Cleverley & Williams (1959) is here designated form XI11 and was obtained as a precipitate by addition of hydrochloric acid to an alkaline solution of phenobarbitone. It could also be prepared by grinding form I1 in water, and this suggests that it may be hydrated; this possibility is considered below.…”

Polymorphism of phenobarbitone. II. The crystal structure of modification III

“…167°C) is described. Powder diffraction patterns, infrared spectra and descriptions of the crystal morphology have previously been given by Mesley et al (1968), Cleverley & Williams (1959), (1961).…”