The Mechanism of the Acid-Catalyzed Ring Opening of Epoxides - A Reinterpretative Review

Abstract

Summarizing all available evidence we may conclude that the acid-catalyzed ring opening of primary and secondary epoxides proceeds with Walden inversion by a S_N² (or overall A-2) mechanism. A fractional positive charge at the reacting carbon atom may be postulated without taking recourse to the “borderline” S_N² mechanism, for which clear evidence seems nonexistent. The entropy of activation and the solvent isotope effect merit reinterpretation as well. More work is needed to establish the mechanisms operating in the hydrolysis of tertiary and mono-aryl substituted epoxides.