Studies on correlations of acid-base properties of substituted pyridine N-oxides in solutions. Correlations of cationic homoconjugation constants and pKa values in non-aqueous media

Based on potentiometrically determined cationic homoconjugation constants it has been found that in all the non-aqueous solvents studied the extent of cationic homoconjugation increases with the increasing basicity of pyridine N-oxide. The relationships between logarithms of cationic homoconjugation constants and the pKa values are linear and correlation coefficients are relatively high. Moreover it is concluded that the tendency of solutions containing substituted pyridine N-oxides towards cationic homoconjugation declines in polar non-aqueous media in the following sequence of solvents: aprotic protophobic > amphiprotic > aprotic protophilic. In the class of aprotic protophobic solvents this tendency is much stronger than in the remaining two classes. In the class of aprotic protophilic ones this tendency is negligible, cationic homoconjugation equilibria are set up in a relatively weakly basic solvent (N,N-dimethylformamide) only in solutions containing strongly basic N-oxides. The sequence of the non-aqueous solvents by the declining tendency towards cationic homoconjugation is as follows: nitromethane > benzonitrile > acetonitrile > propylene carbonate > acetone > methanol > N,N-dimethylformamide > dimethyl sulfoxide. The sequence parallels that of increasing basicity of the solvents. Therefore, the cationic homoconjugation constant values are proposed as a measure of the basicity of solvents.