The pKa values of mono-substituted phenols and benzenethiols and the conjugation of substituents having a strong +K effect

Abstract

The acidity constants of meta- and para-substituted phenols and benzenethiols in 20% water–ethanol were measured at 20°. The pK_a values range for meta-substituted phenols from 8·58 to 10·42, and for para-substituted phenols from 9·76 to 10·90. For meta-substituted benzenethiols these values range from 6·07 to 6·99 and for para-substituted benzenethiols from 6·53 to 7·47. The Hammett ρ parameter is found to be 2·463 for phenols and 1·81 for benzenethiols. The σ and σ_R values derived from the Hammett empirical relation indicate significant differences in conjugative effect between the corresponding phenols and benzenethiols having strongly mesomeric para-substituents such as NMe₂, NH₂, and OMe.