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 pK_a 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 pK_a. 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.