The Influence of the Size and Structure of a Spectator Alkyl Group on the Relative Rates of Alkyl Radical Elimination from Ionised Tertiary Amines

The relative rates of alkyl radical elimination by α-cleavage of ionised amines of general structure, CH3CH2CH2(CH3CH2)CHN(CH3)R+• or CH3CH2CH2CH2(CH3CH2CH2)CHN(CH3)R+•, where R = n-CnH2n + 1 (n = 1–10, 12 or 14), iso-C5H11, CH2CH(CH3)C2H5, neo-C5H11 or CH2CH2C(CH3)3, are reported. The size of the spectator alkyl group, R, affects the ratio of ethyl and propyl radical loss from metastable ionised amines containing a 3-hexyl group; the slight preference for expelling the smaller ethyl radical increases initially before gradually falling as n increases from 1 to 14. In contrast, the degree of branching in isomeric pentyl or hexyl spectator alkyl groups has very little effect on the ratio of ethyl and propyl radical loss. For faster reactions occurring in the ion source, there is at most a marginal variation in the ratio of ethyl to propyl radical loss as n increases. In general, the kinetic energy release which accompanies loss of either radical increases slowly with n, but remains small, as would be expected if the reaction occurred without appreciable reverse critical energy. Similar trends are found for the relative rates of propyl and butyl radical elimination from ionised amines containing a 4-octyl group.