Enthalpies and entropies of ionization of 4-substituted phenols in methanol + water mixtures

Abstract

Enthalpies and entropies of ionization of phenol and five 4-substituted phenols in methanol + water mixtures are reported for the complete solvent composition range. The results confirm the inability of simple electrostatic theories to give a satisfactory account of the solvent dependence of the thermodynamics of ionization of weak neutral acids. The effect of solvent on the influence of 4-substituents on the thermodynamics of ionization of phenol is analysed in detail. Specific solvation effects appear to be less important than general effects, the magnitude of which depends primarily on the electron withdrawing or donating properties of the substituent groups. Solvation phenomena involving the phenoxide anions are dominant over the corresponding phenomena involving the neutral phenols in their contributions to the overall influence of solvent on substituent effects. The implications of the results with respect to Hepler's β and γ parameters are considered.