THE MECHANISM OF ETHYLENE ELIMINATION FROM THE OXONIUM IONS CH3CH2CH=O+CH2CH3 AND (CH3)2C=O+CH2CH3

The reactions of the metastable oxonium ions CH3CH2CH=O+CH2CH3 and (CH3)2C=O+CH2CH3 are reported and discussed. Various mechanisms for ethylene elimination, which is the principal dissociation route for these ions, are considered. It is shown by means of H-2-labelling experiments and analysis of collision-induced dissociation spectra that routes involving ion-neutral complexes pre-empt 'conventional' mechanisms for these processes. In contrast, the behaviour of the lower homologues CH3CH2CH=OR+ and (CH3)2C=OR+ (R = H, CH3) is consistent with the operation of 'conventional' mechanisms for ethylene expulsion. This contrast is interpreted in energetic terms. The significance of these results for the chemistry of homologous and analogous 'onium' ions containing a Z+-R function (Z = O, S, NH, NCH3; R = CnH2n+2m n greater-than-or-equal-to 2) is explained.