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

Abstract It 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 H 2 O, CH 3 ˙ 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 H 2 O loss, but the loss of CH 3 ˙ shows additional mechanistic complexities. It was shown from the metastable peak shapes of 13 C 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 CH 3 ˙ from the isomeric ester methylcrotonate and the absence of H 2 O loss in both methylcrotonate and the methyl ester of 3‐butenoic acid.