Ionization constants of phenols in methanol + water mixtures

Abstract

The pK_a values of six 2-substituted phenols, three 3-substituted phenols and their 3,5-disubstituted analogues, 1-naphthol and 2,6-di-t-butyl-4-nitrophenol in methanol + water mixtures at 298 K have been determined either spectrophotometrically (0.05 ⩽x_MeOH⩽ 0.6) or potentiometrically (0.7 ⩽x_MeOH⩽ 0.9). The acidities of the phenols in methanol were also determined where literature values were not available. Values of the Hammett ρ-parameter have been deduced. The variation of ρ with solvent composition differs for 2- and 3-substituted phenols. 2-Substituents caused steric interference with the solvation of phenoxide anions and thereby led to an acid weakening effect which became greater as the methanol content of the solvent was increased. The Hammett equation pK_a,s=–ρσ+pK_a,u where pK_a,s and pK_a,u refer to a substituted phenol and phenol itself respectively was applicable with σ independent of solvent for 3-substituted phenols in all methanol + water mixtures. However for 2-substituted phenols the equation was not strictly valid.