Gas-phase proton-transfer reactions in xylene–dimethyl ether mixtures. Further evidence for mobile protons

Abstract

The temperature dependences of the proton-transfer equilibria between the three xylenes and dimethyl ether and between benzaldehyde and diethyl ether have been studied and the standard enthalpy and entropy changes for these reactions have been evaluated from van't Hoff plots. In the two equilibria involving o- and p-xylene, the values of ΔS° were greater than could be accounted for on the basis of changes in rotational entropy or mixing of isomeric protonated species and are rationalised in terms of partial mobility of the proton in the aromatic ring. Trends in the values of ΔH° and ΔS° for the reactions involving O-, m- and p- xylenes are similar to those found for the corresponding isomeric halogenotoluenes, although the differences are less marked. In the benzaldehyde/diethyl ether equilibrium, the value of ΔS° was low, consistent with protonation on the side-chain.