Reactions of isolated organic ions. Alkene loss from the immonium ions CH3CHN+HC2H5 and CH3CHN+HC3H7
Abstract
The slow unimolecular reactions of metastable CH3CHN+HR [R = C2H5, (CH3)2CH, and CH3CH2CH2] ions are reported and discussed in energetic and mechanistic terms. These immonium ions eliminate an alkene (R–H) derived by hydride abstraction from the incipient carbonium ion R+. This reaction is interpreted as occurring via an ion–dipole complex comprising the incipient carbonium ion co-ordinated to CH3CHNH, followed by rearrangement to another complex in which (R–H) and CH3CHNH are bound to a common proton. 2H-Labelling experiments show a different specificity in the hydrogenabstraction step for R = ethyl and R =n-propyl: transfer of a deuterium atom from the β-carbon predominates when R = CH2CD3, but CH3CHN+HCD2CH2CH3 expels a substantial proportion of C3H5D. The behaviour of the CH3CHN+HR ions is compared with that of their CH2N+HR homologues. An analysis is presented of the influence of the structure of R on the properties of the associated ion-dipole complexes.