Kinetics of hydrolysis of barbituric acid derivatives

Abstract

The neutral and alkaline hydrolyses of barbituric acid and some of its substituted derivatives were followed spectrophotometrically. The rate-pH profiles for all of the 5,5-disubstituted barbiturates were similar. A different profile was observed for barbituric acid because of its comparatively low pKa₁'. The rate-pH profile of metharbital showed no curvature at high pH values, since the 1-methyl substituent prevents the second possible proton dissociation. All profiles could be explained by hydroxyl-ion attack on the undissociated and monoanion forms of the barbiturate, whereas the dianions are unreactive. The reactivity of the barbiturates was correlated with the Newman rule of six as a measure of steric influence on hydrolysis. A previously unmentioned, reversible system between the barbiturate and its ring-opened malonuric acid derivative was observed and may be used to postulate that the fraction decomposing via 1,6 ring opening is a function of pH rather than a direct consequence of a unique degradation pathway for the ionic form of the barbiturate. Ionic strength effects in the alkaline hydrolyses of amobarbital and phenobarbital increase the rate of barbiturate degradation by the attack of hydroxyl ion on the monoanion and/or its kinetic equivalent. The Arrhenius parameters for all compounds were determined.