Gas-phase proton transfer in halogenotoluene mixtures. Evidence for a ‘dynamic’ ion structure

Abstract

Gas-phase proton transfer equilibria between fluoro- and chloro-toluene mixtures have been studied as a function of temperature to determine relative proton affinities and entropy changes. Some of the entropy changes are much greater than any values previously reported for proton-transfer reactions. It is suggested that this can only be explained if the ortho- and para-halogenotoluene isomers have a ‘dynamic’ protonated ion structure in which a mobile hydrogen atom is shared by two or more unsubstituted ring carbon atoms. There is also an apparent linear correlation between the proton affinities measured and the corresponding entropy of proton bonding of these species. This ‘isokinetic’ relationship has a slope of 589 ± 50 K and is consistent with the suggestion in these compounds that the internal translational movement of the proton within the plane of the aromatic ring decreases the more tightly bound it becomes. This appears to be the first time such a relationship has been observed for gas-phase proton transfer. The following proton affinities relative to A_p(p-chlorotoluene) were measured: p-chlorotoluene, 0; p-fluorotoluene, 1.2 ± 0.2; o-chlorotoluene, 16 ± 0.8; o-fluorotoluene, 28 ± 1.5; m-chlorotoluene, 33.9 ± 1.4; m-fluorotoluene, 36.9 ± 1.4 kJ mol^–1.