Mechanism of propene and water elimination from the oxonium ion CH3CHO+CH2CH2CH3

Abstract

The site-selectivity in the hydrogen transfer step(s) which result in propene and water loss from metastable oxonium ions generated as CH₃CHO⁺CH₂CH₂CH₃ have been investigated by deuteriumlabelling experiments. Propene elimination proceeds predominantly by transfer of a hydrogen atom from the initial propyl substituent to oxygen. However, the site-selectivity for this process is inconsistent with β-hydrogen transfer involving a four-centre transition state. The preference for apparent α- or γ-hydrogen transfer is interpreted by a mechanism in which the initial propyl cation accessible by stretching the appropriate bond in CH₃CHO⁺CH₂CH₂CH₃ isomerizes unidirectionally to an isopropyl cation, which then undergoes proton abstraction from either methyl group {CH₃CHO⁺CH₂CH₂CH₃→ CH₃CHO⁺CH₂CH₂CH₃→[CH₃CHO ⁺CH(CH₃)₂]→[CH₃CHOH⁺ CH₃CHCH₂]}. This mechanism involving ion-neutral complexes can be elaborated to accommodate the minor contribution of expulsion of propene containing hydrogen atoms originally located on the two-carbon chain. Water elimination resembles propene loss insofar as there is a strong preference for selecting the hydrogen atoms from the α- and γ-positions of the initial propyl group. The bulk of water loss is explicable by an extension of the mechanism for propene loss, with the result that one hydrogen atom is eventually transferred to oxygen from each of the two methyl groups in the complex [CH₃CHO ⁺CH (CH₃)₂]. This site-selectivity is strikingly different from that (almost random participation of the seven hydrogen atoms of the propyl substituent) encountered in the corresponding fragmentation of the lower homologue CH₂O⁺CH₂CH₂CH₃. This contrast is explained in terms of the differences in the relative energetics and associated rates of the cation rearrangement and hydrogen transfer steps.