Ion–dipole complexes in the unimolecular reactions of isolated organic ions. Effect of N-methylation on olefin and amine loss from protonated aliphatic amines

Abstract

The slow unimolecular fragmentation reactions of 18 gaseous protonated aliphatic amines of general formula R¹NH⁺R²R³(R¹= Prⁿ, Prⁱ, Buⁿ, Buⁱ, Bu^s, or Bu^t; R²,R³= H,CH₃) are reported and discussed. Two decomposition routes are observed for metastable ions R¹NH⁺R²R³. The first involves elimination of a neutral amine, R²R³NH, and formation of a carbocation, R¹⁺, via a mechanism involving an incipient cation bound to the developing amine by an ion–dipole attraction. Rearrangement of the cation, to give thermodynamically more stable isomers, is feasible in these ion–dipole complexes. Further reorganization of the complexes leads to a species in which an incipient olefin [R¹– H] and an amine [R²R³NH] are co-ordinated to a common proton. Dissociation of these proton-bound complexes, with retention of the proton by the developing amine, results in olefin loss, which is the second reaction undergone by metastable ions R¹NH⁺R²R³. The relative abundance of amine expulsion is greater for protonated amines containing a primary alkyl group, R¹, than is the case for isomeric ions containing secondary or tertiary alkyl groups. Progressive methylation of the nitrogen atom decreases the relative abundance of amine loss from R¹NH⁺R²R³, regardless of the nature of the principal alkyl group. These two trends are explained in terms of the energetics of the intermediates and products involved in the decomposition of the protonated amines.