Potential energy profiles for unimolecular reactions of isolated organic ions: some isomers of C4H10N+ and C5H12N+

Abstract

The unimolecular reactions of two isomers of C₄H₁₀N⁺ and four isomers of C₅H₁₂N⁺, of general formula RNH⁺CH₂(R = C₃H₇ or C₄H₉), are discussed in terms of a potential energy profile approach. Two main decomposition channels are observed, both of which involve loss of an olefin from the original alkyl side chain. Hydrogen transfer from the alkyl chain to nitrogen can occur in an intermediate in which the incipient carbonium ion R⁺ is weakly co-ordinated to NHCH₂. This process results in the formation of ⁺NH₂CH₂ and expulsion of an olefin containing the same number of carbon atoms as R. Alternatively, a 1,5-hydride shift can take place, from a γ-carbon atom in RNH⁺CH₂, to the isolated methylene group. This rearrangement may lead to loss of an olefin containing one less carbon atom than R, thus giving rise to CH₃NH⁺CH₂ as the daughter ion. The γ-hydrogen transfer process is analogous to the McLafferty rearrangement which is well known for ionised carbonyl compounds of sufficient chain length. Evidence is presented which shows that the 1,5-hydride shift is not synchronously concerted with olefin loss; instead, the γ-carbon atom acquires at least a partial positive charge before the olefin is expelled, with kinetic energy release.