Acid ionization of solvent, acetic acid and 4-substituted phenols in t-butanol + water mixtures

Abstract

Apparent ionic products are reported for t-butanol + water mixtures at six mole fractions of t-butanol in the range 0.02 [graphic omitted] xBuOH [graphic omitted] 0.3. Acid ionization constants of acetic acid, phenol and four 4-substituted phenols (tBu, Br, CHO and NO2) in the six solvent mixtures have also been determined. The variations in pKa with solvent composition are discussed in terms of the free energies of transfer of phenols, phenoxide anions and the proton from water to t-butanol + water mixtures. Comparison of the data with results for other alcohol + water systems emphasizes the importance of non-electrostatic contributions to the medium effects on pKa. The applicability of the Hammett ρσ relationship to the ionization of 4-substituted phenols in mixtures of water with t-butanol, methanol or ethanol has revealed the existence of an empirical relationship between ρ values and the excess free energies of mixing of the alcohol + water solvents. The effect of 4-substituents on the influence of solvent composition on the free energy of ionization of phenol is predominantly governed by non-electrostatic effects. Electrostatic theories completely fail to account for the variations of ρ with solvent composition.