The E1cb route for ester hydrolysis; volumes of activation as an additional criterion of mechanism

Abstract

Hydrolyses of esters which possess an acidic proton at the α or vinylogous position can, in principle, hydrolyse by the E1cb route via a ketenoid intermediate. To the kinetic evidence for such a mechanism in the hydrolyses of 4-hydroxybenzoates, malonates, acetoacetates and fluorenecarboxylates is now added the further criterion of volumes of activation. Values of ΔV^‡ for reactions proceeding by the E1cb route are positive and contrast with the negative values associated with hydrolyses by the more usual B_Ac2 mechanism.