SOLVENT EFFECT ON PREFERRED PROTONATION SITES IN NICOTINATE AND ISONICOTINATE ANIONS

Potentiometric determinations of the two successive acidities of nicot inic and isonicotinic acids (AH(2)(+)) have been carried out in aqueou s dimethyl sulfoxide mixtures containing up to 95% Me(2)SO by volume. In both systems, the results reveal that the addition of Me(2)SO induc es a proton transfer from the pyridinium ring to the carboxylate group , the tautomeric equilibrium between the neutral forms of the two acid s being displaced toward the zwitterionic form (AH(+/-)) in aqueous so lution, but toward the molecular form (AH degrees) in Me(2)SO. An anal ysis of the data by means of Hammett relationships previously establis hed for benzoic acids over the whole range of H2O/Me(2)SO mixtures all owed the four microscopic acidity constants as well as the tautomeric equilibrium constant K-T pertaining to the complete ionization scheme of the two acids to be determined. At 20 degrees C, there are equal po pulations of the tautomeric AH degrees and AH(+/-) species in the mixt ures containing 38 and 47% Me(2)SO for the nicotinic and isonicotinic systems, respectively. Hammett relationships describing the ionization behaviour of a number of substituted pyridinium cations in H2O/Me(2)S O mixtures are also discussed. Possible reasons accounting for the pec uliar effects exerted by the NH2, CONH2, and COOH substituents on the process are suggested.