Ion–dipole interactions in the unimolecular reactions of isolated organic ions: some isomers of CnH2n+ 1O+

Abstract

The slow, unimolecular dissociations of four isomers of CH₃CHO–C₄H₉ and two isomers of CH₃CHO–C₅H₁₁ are compared. The results are interpreted in terms of a general mechanism involving an intermediate in which the incipient carbonium ion, C₄H₉⁺ or C₅H₁₁⁺, is co-ordinated to acetaldehyde. Rearrangement of this intermediate can take place, to form a complex in which acetaldehyde and a potential olefin, C₄H₈ or C₅H₁₀, are bound to a common proton. Dissociation can then occur, the nascent neutral species having the greater proton affinity retaining the proton. In some cases, rate-determining isomerisation of the incipient carbonium ion takes place. The mechanism explains the observed competition between CH₃CHO and C₄H₈(or C₅H₁₀) elimination and also the relative magnitude of the kinetic energy released upon decomposition of isomeric ions.