Classical carbonium ions. Part 11. Solvent effects on solvolysis rates: the Grunwald–Winstein equation

Abstract

Although originally proposed for ‘limiting’ solvolyses in any solvent, the Grunwald–Winstein equation accurately fits the reactions of 1-methylethyl toluene-p-sulphonate in water–ethanol mixtures, which are known to be non-limiting. Solvolyses of 1-adamantyl derivatives (picrate and toluene-p-sulphonate) cannot involve nucleophilic assistance, yet in solvent systems like methanol–tetramethylene sulphone or ethanol–chloroform, seriously non-linear Grunwald–Winstein plots are observed. In an alternative treatment of the data, the solvolysis of 1-adamantyl picrate in methanol–tetramethylene sulphone is shown, by the remarkable insensitivity to solvent composition of its rate and its entropy of activation, to be a particularly simple process mechanistically. The relationship between the activity of the alcohol component and the solvolysis rate indicates that in the other three systems studied, the alcohol molecules participate stoicheiometrically in hydrogen bonding to the incipient anion. Criteria for linear Grunwald–Winstein plots can therefore be suggested.