Enthalpy of Solvation of Carbonium Ions

Abstract

Literature data on the rate of hydrolysis of t-butyl chloride and of the acid-catalyzed hydration of isobutene, both in dilute aqueous solution, have been combined with thermochemical data to yield standard heats of formation of the respective t-butyl cation-like transition states. Measurements of acid-catalyzed aromatic hydrogen isotope exchange have similarly been used to calculate the standard heat of formation of a cyclohexadienyl cation-like transition state. All these reactions are highly endothermic so that the transition states are expected to lie only very slightly higher in enthalpy than the corresponding carbonium ions. The transition state enthalpy values are taken to be good approximations (though maximum values) for the standard enthalpies of formation of the carbonium ions.

Combination of these results with mass-spectrometrically determined standard heats of formation of the gaseous carbonium ions allows conclusions concerning the heats of solvation of the ions. Accordingly, the t-butyl cation is stabilized by solvation by at least 50 kcal/mol (209 kJ/mol), the cyclohexadienyl cation by at least 73.5 ± 8.5 kcal/mol (308 ± 36 kJ/mol). The large size of this stabilising effect implies that the charge distribution in a solvated ion differs from that of the gaseous ion to which quantum-mechanical calculations refer.