Acidity constants have been determined potentiometrically for a variety of conjugate acids of substituted pyridine N-oxides in N,N-dimethylformamide (dmf) and dimethyl sulfoxide (dmso). The pK(a) values in these solvents varied in the same direction and correlated with the pK(a) values of these species in water and in the protophobic aprotic solvent acetonitrile. Further, a linear relationship has been established between the pK(a) values in the two protophilic aprotic solvents under study. The most basic substituted pyridine N-oxides exhibited a weak tendency towards cationic homoconjugation in dmf, whereas in the more basic dmso the homoconjugation equilibrium was not established for any of the heterocyclic N-oxides. The phenomenon of cationic homoconjugation was also not observed with pyridine as a representative of heterocyclic amines, both in dmf and dmso. This finding complies with the results obtained in other polar aprotic solvents.
