Effect of variations of the size and structure of the principal alkyl group on alkene elimination from the immonium ions CH 3CH 2CHN +(CH 3)R, CH 3CH 2CH 2CHN +(CH 3)R and CH 3CH 2CH 2CH 2CHN +(CH 3)R

Abstract

The reactions of three homologous series of metastable immonium ions of general structure, CH 3(CH 2) m CHN +(CH 3)R, [ m = 1–3; R = n-C n H 2 n+1 , n = 1–10, 12, and 14, iso-C 5H 11, CH 2CH(CH 3)C 2H 5, neo-C 5H 11, and CH 2CH 2C(CH 3) 3] are reported and discussed. The first member of each homologous series is anomalous in undergoing several reactions, including elimination of molecular hydrogen and a methyl radical. In contrast, higher members of all three series expel either or both alkenes, C n H 2 n or C n−1 H 2 n−2 derived from the principal alkyl group, R. The influence of the size and structure of R on these alkene eliminations is analysed. Loss of C n H 2 n produces an approximately Gaussian metastable peak; the associated kinetic energy release is relatively small and varies only slightly as the homologous series are ascended. In contrast, C n−1 H 2 n−2 elimination releases a much larger amount of kinetic energy, particularly for n = 3, when the associated metastable peak is dished; but the kinetic energy release declines dramatically on ascending the homologous series. Loss of C n−1 H 2 n−2 is favoured by branching at the γ-carbon atom, provided there is a γ-hydrogen atom, but it does not occur when R = CH 2CH 2C(CH 3) 3 because γ-hydrogen transfer is impossible. Keywords: Metastable ions; Degree of freedom effects; Immonium ions; Alkene elimination; kinetic energy release