Kinetics and mechanism of hydantoic ring opening. The alkaline hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids

Abstract

The kinetics of hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids were studied in aqueous solution over the pH range 8–14 at 25°. In strongly alkaline media (pH > 11.5), an ionisation equilibrium gives rise to an unreactive anion. The pK_a values determined both spectrophotometrically and kinetically for the unsubstituted derivative are in good agreement. In mildly alkaline media (pH < 11.5), the observed rates have a first-order dependence on hydroxide ion concentration and no general catalysis is detected. The value of the Hammett parameter (ρ 0.8), the low deuterium isotope solvent effect (k_H2O/k_D2Oca. 0.7), and the markedly negative values of the entropies of activation (–30 cal mol^–1 K^–1 < ΔS^‡ < –20 cal mol^–1 K^–1) are rationalized in terms of a rate-determining hydroxide ion attack which is in agreement with the pK_a value of the leaving group of 3-arylhydantoins.