Behaviour of isomeric methyl ethyl and ethyl methyl halosuccinates under electron impact. Elimination of a halogen atom by a multi-step mechanism

The electron impact mass spectra of isomeric methyl ethyl and ethyl methyl halosuccinates (X = Cl and Br) are surprisingly different. Only the isomers with the ethyl group remote from the halogen give rise to [M ‐ X]+ ions. A low‐energy collision‐induced dissociation study of deuterium‐labelled analogues of the former isomers indicates that the [M ‐ X]+ ions are mixtures of protonated methyl ethyl maleate (major component, > 85%) and fumarate, and the loss of the halogen atom is a multi‐step process including at least two specific hydrogen transfers. Migration of a β‐hydrogen atom to the carbonyl oxygen within the ethoxycarbouyl group produces a primary radical site in a distonic intermediate which, by subsequent ion of a hydrogen atom from C(3), triggers the ejection of X from C(2) with concomitant double bond formation. Whereas in the other isomer an [M ‐ X]+ ion is absent or negligible, a characteristic double loss of C2H4 and CO2 is observed.