Intermediates in the reaction path for the unsymmetrically acid-catalysed hydrolysis of carboxylic esters with electronegative substituents

Abstract

The mechanism for the exceptional acid-catalysed A-B_AC³ hydrolysis is considered in detail on the basis of the estimated free energy levels of transition states and intermediates on the possible reaction paths for the hydrolyses of ethyl trichloroacetate and methyl acetate by the acid-catalysed A_AC² and AB_AC³ and the neutral B_AC³ mechanisms. It is concluded that the first step of the B_AC³ and A-B_AC³ reactions to the tetrahedral anionic intermediate T^–[R¹C(OH)(O^–)(OR²)] is the same. T^– is assumed to be in equilibrium or in a steady-state condition with the neutral intermediate T⁰[R¹C(OH)₂(OR²)]. It is proposed that, in the case of the A-B_AC³ mechanism, a concerted general base–general acid-catalysed decomposition of T⁰ leads to the observed acid catalysis which thus follows the sequence of steps Ester → T^–→ T⁰→ Products. Kinetic data for the acid hydrolyses of chloromethyl and 2,2-dichlorovinyl acetates together with other available data for exceptional, acid-catalysed hydrolyses were treated by a non-linear least-squares procedure.