Behaviour of isomeric methyl ethyl and ethyl methyl halosuccinates under electron impact. Elimination of a halogen atom by a multi-step mechanism
✍ Scribed by D. Bornstein; A. Mandelbaum; I. Vidavsky; B. Domon; D. Mueller; W. J. Richter
- Publisher
- John Wiley and Sons
- Year
- 1991
- Tongue
- English
- Weight
- 545 KB
- Volume
- 26
- Category
- Article
- ISSN
- 1076-5174
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✦ Synopsis
Abstract
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 abstraction 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 C~2~H~4~ and CO~2~ is observed.