Mass spectrometric decomposition processes of 2‐methyl‐1‐hexene

Abstract

Abstract Unimolecular decompositions of 2‐methyl‐1‐hexene and several labelled analogues were studied following 70 eV electron impact (normal and metastable spectra) and field ionization (field ionization kinetic measurements). Molecules labelled with 13 C in the 1‐position and the methyl position were found to behave essentially identically. This is attributed to rapid transfer of a hydrogen atom mainly from C‐5 to C‐1 (γ‐hydrogen shift). Loss of ethene, propene or propenyl do not involve loss of the methyl carbon or C‐1. All three reactions are better than 90% specific in this respect under all conditions studied. At shorter times, C 3 H 6 loss is the dominant reaction, while at longer times C 2 H 6 loss accounts for >90% of the ion current. It is proposed that at least two distinct pathways for C 2 H 4 loss operate in linear 1‐alkenes, one of which (loss of carbons 1 and 2) is blocked by a 2‐methyl substituent. The [C 6 H 11 ] +˙ and [C 5 H 10 ] +˙ ions formed from 13 C labelled 2‐methyl‐1‐hexenes fragment in an essentially statistical fashion.