Effect of variations of the size and structure of the principal alkyl group on alkene elimination from the immonium ions CH3CH2CHN+(CH3)R, CH3CH2CH2CHN+(CH3)R and CH3CH2CH2CH2CHN+(CH3)R11Dedicated to Professor Henri Edouard Audier on the occasion of his 60th birthday.

The reactions of three homologous series of metastable immonium ions of general structure, CH3(CH2)mCHN+(CH3)R, [m = 1–3; R = n-CnH2n+1, n = 1–10, 12, and 14, iso-C5H11, CH2CH(CH3)C2H5, neo-C5H11, and CH2CH2C(CH3)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, CnH2n or Cn−1H2n−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 CnH2n 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, Cn−1H2n−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 Cn−1H2n−2 is favoured by branching at the γ-carbon atom, provided there is a γ-hydrogen atom, but it does not occur when R = CH2CH2C(CH3)3 because γ-hydrogen transfer is impossible. Keywords: Metastable ions; Degree of freedom effects; Immonium ions; Alkene elimination; kinetic energy release