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 alpha-cleavage of ionise d amines of general structure, CH3CH2CH2(CH3CH2)CHN(CE3)R+. or CH3CH2CH2CH2 (CH3CH2CH2)CHN(CH3)R+., where R = n-CnH2n + 1 (n = 1-10, 12 or 14), iso-C5H 11, CH2CH(CH3)C2H5, neo-C-5,H-11 or CH2CH2C(CH3)(3), are reported. The size of the spectator alkyl group, R, affects the ratio of ethyl and propyl rad ical loss from metastable ionised amines containing a 3-hexyl group; the sl ight preference for expelling the smaller ethyl radical increases initially before gradually falling as n increases from 1 to 14. In contrast, the deg ree of branching in isomeric pentyl or hexyl spectator alkyl groups has ver y little effect on the ratio of ethyl and propyl radical loss. For faster r eactions 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, t he kinetic energy release which accompanies loss of either radical increase s slowly with n, but remains small, as would be expected if the reaction oc curred without appreciable reverse critical energy. Similar trends are foun d for the relative rates of propyl and butyl radical elimination from ionis ed amines containing a 3-hexyl group.