On the mechanism of isomerization in the esters of some α,β‐unsaturated carboxylic acids—II

Abstract The effect of a phenyl group on the mechanisms of isomerization in the ionized methyl esters of simple α,β‐unsaturated acids (methyl acrylate and related compounds) has been investigated with the aid of deuterium labelling as well as information from mass analysed ion kinetic energy spectra and first field free region metastable peak shapes. Substitution of a hydrogen atom of the OCH 3 group by a phenyl group (benzyl acrylate and homologues) greatly enhances the rate of [ester] → [acid] isomerizations (loss of H 2 O and COOH˙). It is inferred that this is due to an accelerating effect of the phenyl group on the first and the third steps (ring opening of the key intermediate ion which has the structure of the ionized enol form of γ‐butyrolactone) of the reaction whose mechanism is basically the same as that in methyl acrylate. A phenyl group present at the α‐ or β‐position of the vinylic double bond appears to suppress the [ester] → [acid] isomerization in some compounds by promoting other reactions or opening up a new reaction pathway, i.e. the loss of CH 2 O from ionized methyl atropate for which mechanisms are proposed.