Non-concrted unimolecular reactions of ions in the gas-phase: The importance of ion-dipole interactions in carbonium ion isomerizations

Abstract

The behaviour of six members of a homologous series of oxonium ions, C 3H 7O +CR 1R 2 (R 1, R 2 = H or CH 3) are discussed. For the two lowest members (R 1 = R 2 = H) isomerization of the propyl side-chain from n- to iso-structure is possible at energies lower than those required to promote decomposition of either ion. For higher members, rearrangement of the propyl side-chain from n- to iso-structure constitutes the rate-determining step in the dissociation of the ion in question. Extension of the analysis, including a consideration of plausible reactions which are found not to occur, permits estimates to be made for the heats of formation of the intermediates required for rearrangement of the propyl side-chain of C 3H 7O +CR 1R 2. It is suggested that these intermediates are stabilized by an ion—dipole interaction between the incipient propyl cation and the potential carbonyl compound, OCR 1R 2. The magnitude of this stabilization appears to lie in the range 15–25 kcal mol −1.