Thermodynamics of ionization of solvent and 4-substituted phenols in t-butanol + water mixtures

Abstract

Enthalpies of neutralization of hydrogen chloride and potassium hydroxide in t-butanol + water solvent mixtures (0.02 ⩽x_BuOH⩽ 0.30) have been determined calorimetrically. Standard enthalpies of autoionization of the solvent mixtures were deduced and combined with Gibbs free energies of autoionization to obtain the corresponding standard entropies of autoionization.

Enthalpies of solution of phenol and four 4-substituted phenols (^tBu, Br, CHO and NO₂) in t-butanol + water mixtures alone and in corresponding mixtures containing potassium hydroxide have also been determined. Standard enthalpies of ionization of the phenols were evaluated. Standard free energies and entropies of ionization are also reported.

Enthalpies of transfer of phenols and their anions from water to t-butanol + water mixtures showed indothermic maxima at solvent compositions corresponding to the most structured arrangement of t-butanol and water molecules. Some specificity in behaviour showed no simple dependence on the size or polarity of solute species. Thermodynamic changes accompanying the ionization of phenols showed less evidence for specific behaviour but were influenced by the electronic character of 4-substituents in the phenol ring. Free energies, enthalpies and entropies of ionization showed linear correlations with Hammett σ-parameters for each solvent mixture. The influence of solvent on the ionization of a particular phenol, or on the effect of a substituent on that ionization, leads to entropy and enthalpy changes which usually compensate to some extent in their contributions to the corresponding free-energy changes. The enthalpy and entropy data emphasize the important influence of solvent structure on equilibrium behaviour in aqueous alcoholic media.