On the mechanism of isomerization reactions in the esters of some simple α,β‐unsaturated carboxylic acids

AbstractIt is shown from a detailed examination of first field free region metastable peak shapes that the molecular ion of the methyl ester of acrylic acid rearranges into 2‐ and 3‐butenoic acid ions prior to metastable fragmentations involving the losses of H2O, CH3˙ and CO. The key intermediate ion in this ester‐acid isomerization is shown to be the enol form of ionized γ‐butyrolactone. The C‐5 homologues methylmethacrylate and ethylacrylate display a similar mechanism for H2O loss, but the loss of CH3˙ shows additional mechanistic complexities. It was shown from the metastable peak shapes of 13C and deuterium labelled compounds that the larger part of the methyl loss does not occur from acid type ions, but directly from methyl substituted enol ions of γ‐butyrolactone. The mechanistic proposals also account for the presence of a pronounced loss of CH3˙ from the isomeric ester methylcrotonate and the absence of H2O loss in both methylcrotonate and the methyl ester of 3‐butenoic acid.

On the mechanism of isomerization reactions in the esters of some simple α,β‐unsaturated carboxylic acids Journal Articles