A potentiometric study of acid-base equilibria of substituted pyridine N-oxides in nitrobenzene

Acidic dissociation and homoconjugation constants were determined for a number of substituted pyridine N-oxides in nitrobenzene. The acidic dissociation constants of acids conjugated with the N-oxides studied turned out to be in the range of 4 < pKa < 11, which is about 3 and more times higher than in other aprotic solvents with comparable polarity and donor properties (e.g. acetonitrile), whereas the determined homoconjugation constants were higher by about a factor of 0.5–1.5. For comparison, measurements were carried out for parent heterocyclic amine-pyridine, as well as trimethylamine N-oxide representing aliphatic N-oxides. The determined acidity constant of the pyridinium cation was more than a factor of 1 lower than that of pyridine N-oxide, and the homoconjugation constant was about 2 orders of magnitude lower, which is in conformity with the results in other aprotic solvents. The obtained pKa and log kBHB+ for trimethylamine N-oxide are, approximately, 7 and 4 orders of magnitude lower respectively, than those characteristic of pyridine N-oxide, which conforms with the results in other polar aprotic solvents. A preliminary multiparametric analysis of the acidity and cationic homoconjugation constants in a series of non-aqueous polar solvents was carried out. The relevant parameters of solvents were studied which described their polarity and donor-related and acceptance properties. It has been found that the pKa values are a linear function of β values (characterizing solvent basicity), whereas the variation in logarithms of cationic homoconjugation constants with solvent properties is explained by two parameters: β and ETN (characterizing solvent polarity).