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 f 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 AP( p-chlorotoluene) were measured: p-chlorotoluene, 0; p-fluorotoluene, 1.2 * 0.2; o-chlorotoluene, 16 f 0.8; o-fluorotoluene, 28 f 1.5; rn-chlorotoluene, 33.9 * 1.4; mfluorotoluene, 36.9 f 1.4 kJ mol-'.

Protonation of fluorophenols and fluoroanisoles in the gas phase: experiment and theory

“…For this purpose, we have used the reported entropies of protonation of the isomeric 2-, 3-and 4-fluorotoluenes. 42,44,45 With the use of the reported entropy of protonation of 2-fluorotoluene, the TD r S o value becomes À29.5 J mol À1 K À1 for the related fluorophenol. This value is close to the result obtained by the crude assumption that the entropy of the proton determines the change for the protonation reaction in eqn.…”

Thermodynamics of gas phase proton transfer reactions involving substituted benzaldehydes

“…Thus the main contribution to ∆S is given by ∆S rot [eqn. (5)], where σ is the rotational symmetry constant.…”

“…This reaction is thought to be induced by proton migration around the ring at high temperatures and 'ring walk' of the substituent. 4, 5 The possibility of 'acid catalysed' aromatic isomerisation was further investigated here through a study of substituted benzaldehyde compounds, the difference being, that the proton is expected to attach to the substituent, the carbonyl group, rather than to the ring.…”