Ortho effects—VII : Conjugative and field effects of dimethylsulfonio group on the ionization of benzoic acid

Abstract The effect of the Me 2 S + group on the ionization of benzoic acid has been re-examined in comparison with that of the Me 3 N + group. The thermodynamic p K a values of o -, m -, p -Me 2 S + and o -, m -, p -Me 3 N + substituted benzoic acids have been determined in 65 vol% aq. dimethyl sulfoxide, 50 vol% aq. ethanol, and in water potentiometrically. The result of this work, σ p -Me 2 S + ⪢ σ m -Me 2 S + is contrary to that previously found by others, σ p -Me 2 S + m -Me 2 S + . The following relations were found to exist irrespective of the solvent used: σ m -Me 2 S + = σ m -Me 3 N + + 0 ·06, and σ p -Me 2 S + = σ p -Me 3 N + + 0 The conjugative effect of Me 2 S + group has been found to be acid-strengthening from the meta or para positions, while it becomes acid-weakening from the “ortho” position.