Acid ionization constants of 4-substituted phenols in methanol + water mixtures

Abstract

The pK_a values of 4-t-butylphenol, 4-methylphenol, phenol, 4-bromophenol, 4-formylphenol and 4-nitrophenol in methanol + water mixtures in the 10–90 wt. % methanol composition range have been determined potentiometrically using a combined glass and silver/silver chloride electrode system. The _pK_a values are in accord with the Hammett ρσ relationship for all solvent compositions with a maximum value of ρ at ca. 60–70 wt. % methanol. The variation of ρ with solvent composition is indirectly attributed to the influence of bulk solvent structure on the free energies of solvation of phenoxide anions. The total medium effects on _pK_a are considered to be the sum of both electrostatic and nonelectrostatic contributions. Some factors influencing the latter are briefly discussed.