Ortho effects—VIII : Thermodynamics of the ionization equilibria of o- and p-substituted benzoic acids in dimethyl sulfoxide-water mixed solvents

Abstract Thermodynamic quantities, Δ H ° and Δ S °, in the ionization of various o - and p -substituted benzoic acids have been determined m 65 and 95 vol % aqueous dimethyl sulfoxide (DMSO) and in water. A linear correlation between Δ G ° and Δ H ° for both o - and p -substituted acids exists in 65 and 95 vol % DMSO, whereas in water the same plot shows wide scattering. For p -substituted acids, a trend can be seen that the ionization is controlled by Δ S ° in water while in 65 and 95 vol % DMSO the ionization is evidently controlled by Δ H °. These facts can be interpreted as a result of strong solvation of the acid anions in water and of weak solvation in aqueous DMSO. The characteristic features of aqueous DMSO as a solvent for ionization of organic acids are clarified in terms of Δ H ° and - T Δ S °.