Activation volumes for ester hydrolysis via elimination–addition

Abstract

Esters that have an acidic proton α to the carboxyl group (or in a vinylogous situation) and a good leaving group may undergo hydrolysis by elimination to an intermediate ketene, which rapidly hydrates. Examples are found in p-hydroxybenzoate, malonate, and acetoacetate esters of nitrophenols. The evidence for this mechanism (E1cb type) includes the independence of rate with pH in the region of dissociation of the acidic proton and, in particular, positive volumes of activation that contrast sharply with negative values typical of the more usual BAc2 mechanism of hydrolysis.