A potentiometric study of (acid + base) equilibria in substituted 4-nitropyridine N-oxide systems in methanol and dimethyl sulfoxide

The acid dissociation constants for cationic acids conjugated with 4-nitropyridine N-oxides have been determined using potentiometric titration method. The measurements in the systems of thirteen 4-nitropyridine N-oxide derivatives were carried out in the polar amphiprotic methanol (MeOH) and in the aprotic protophilic dimethyl sulfoxide (DMSO). Likewise as in the polar aprotic protophobic solvents (acetonitrile, acetone) it was found that in MeOH for all N-oxides studied the pKa values were readily determinable, whereas in DMSO the pKa values were hardly determinable or indeterminable by using the potentiometric method. In addition, just like in our previous investigations it was revealed that the sequence of the pKa values of the cationic acids in methanol is the same as in the water and the values are lower than those determined in acetonitrile and acetone. Also, it was found that the phenomenon of cationic homoconjugation equilibria was not present in the systems involving 4-nitropyridine N-oxide derivatives in both solvents used. Furthermore, protonation energies, ΔEprot, and Gibbs free enthalpies, ΔGprot, in vacuo   have been compared with acid dissociation constants (expressed as pKaMeOH values) of the protonated N-oxides determined by potentiometric titration in methanol to establish a correlation between these magnitudes.