Mechanism of alkaline hydrolysis of lactones: dipolar aprotic versus protic solvent effects as a diagnostic tool

Abstract

Rate data for alkaline hydrolysis of γ-butyrolactone, γ-valerolactone, phthalide, and 3-phenylphthalide and their open chain analogues, ethyl acetate, isopropyl acetate, methyl benzoate, and benzyl benzoate respectively have been evaluated in the mixed solvents dimethyl sulphoxide (DMSO)–water, ethanol–water, and acetone–water. The lactones are hydrolysed faster than the open chain esters in all the solvent systems, and the rate enhancements are higher in aqueous ethanol than in aqueous DMSO or aqueous acetone. This shows the minor importance of the electrostatic repulsion between the OH^– ion and the ‘lone pair dipole’ of oxygen, the absence of which in the case of lactones was considered to cause their higher reactivity over open chain esters in B_AC2 reactions.