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.