Unimolecular reactions of isolated organic ions: The chemistry of the oxonium ions CH₃CH₂CH₂CH₂⁺O = CH₂ and CH₃CH₂CH₂CH = O⁺CH₃

Abstract

Abstract The reactions of the metastable oxonium ions CH 3 CH 2 CH 2 CH 2 + O = CH 2 and CH 3 CH 2 CH 2 = O + CH 3 are reported and discussed. Both these isomers of C 5 H 11 O + expel predominantly CH 2 O (75–90% of the metastable ion current), a moderate amount of C 3 H 6 (5–15%), a minor amount of CH 3 OH (2–8%) and a very small proportion of H 2 O (0.5–3%). All these processes give rise to Gaussian metastable peaks. The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH 3 CH 2 CH 2 CH = O + CH 3 . The behaviour of labelled analogues confirm that the reactions of CH 3 CH 2 CH 2 CH = O + CH 3 are closely related, but subtly different. Elimination of CH 2 O and C 3 H 6 is intelligible by means of mechanisms involving CH 3 CH + CH 2 CH 2 OCH 3 . This open‐chain cation is accessible to CH 3 CH 2 CH 2 + O = CH 2 by a 1,5‐H shift and to CH 3 CH 2 CH 2 ‐CH = O + CH 3 by two consecutive 1,2‐H shifts (or, possibly, a direct 1,3‐H shift). The rates of these 1,2‐, 1,3‐ and 1,5‐H shifts are compared with one another and also with the rates of CH 2 O and C 3 H 6 loss from each of the two oxonium ions. The 1,5‐H shift that converts CH 3 CH + CH 2 CH 2 OCH 3 formed from CH 3 CH 2 CH 2 CH = O + CH 3 into CH 3 CH 2 CH 2 + O = CH 2 prior to CH 2 O elimination is essentially unidirectional. In contrast, the corresponding step converting C 5 H 11 O + ions generated as CH 3 CH 2 CH 2 CH 2 + O = CH 2 into CH 3 CH + CH 2 CH 2 OCH 3 competes effectively with expulsion of CH 2 O and C 3 H 6 . The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon‐carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro ‘ene’ pericyclic process are emphasized.