The mechanism of propene elimination from the immonium ions CH 2 N + (CH 3 )CH(CH 3 ) 2 and CH 2 N + (CH 3 )CH 2 CH 2 CH 3

It is shown by 2H-labelling experiments that β-hydrogen transfer from the propyl group to the nitrogen atom is not an adequate general explanation for propene loss from CH2N+(CH3)C3H7 ions: β-hydrogen transfer occurs for CH2N+(CH3)CH(CH3)2, in which the N-alkyl substituent corresponds to a stable isopropyl cation; but α- and γ-hydrogen transfer dominate for CH2N+(CH3)CH2CH2CH3, in which isomerisation of the unstable incipient n-propyl cation precedes propene expulsion.