Kinetic energy release as a probe for rate-determining unimolecular isomerisations. Energetics of ring expansion in ionised halogeno-benzenes and alkylbenzenes

Abstract

The mechanism whereby radical loss occurs from the molecular ions of numerous halogeno-aromatic and alkyl-aromatic compounds is investigated using the kinetic energy release which accompanies dissociation, the non-occurrence of plausible competing reactions, and appearance potential measurements. Slow elimination of a bromine radical from ionised bromobenzene is formulated as a simple cleavage and is evidenced by little kinetic energy release. In contrast, the major slow dissociations (metastable transitions) of ionised halogenotoluenes (halogen radical loss), xylenes and polymethylbenzenes (methyl radical loss), and toluene (hydrogen radical loss) all occur after ring expansion to ionised cycloheptatriene isomers. In these cases (apart from ionised toluene and ethylbenzene) the ring expansion process is rate-determining and the subsequent fast decompositions give rise to relatively large releases of kinetic energy. Further support for the analysis may be found from earlier work on these systems involving ²H- and ¹³C-labelling experiments; in particular, the observation of an isotope effect associated with the ring expansion of ionised p-chloroethylbenzene is strong evidence that this is the rate-determining step for Cl˙ loss.