Studies on correlations of acid-base properties of substituted pyridine N-oxides in solutions. Part 2: Correlations of the pKa values in non-aqueous media

The method of comparison of the capacity of polar non-aqueous solvents to differentiate between the strengths of cationic acids, based on the magnitude of slope in correlations of the non-aqueous and aqueous pKa values, has been found inapplicable to acids obtained by protonation of substituted pyridine N-oxides. Therefore an attempt has been made to find correlations between the pKa values in various polar non-aqueous solvents. Correlations of this type have been found to exist in protophobic and protophilic aprotic solvents, and their combinations as well as in cases when the pKa values in aprotic (both protophobic and protophilic) solvent were correlated with the corresponding values in amphiprotic methanol. The magnitude of slope a in these correlations has been suggested as a criterion for comparison of differentiation properties of non-aqueous solvents with respect to the strength of cationic acids. On this basis, the differentiating capacity has been determined for a variety of polar non-aqueous solvents in relation to the strength of protonated substituted pyridine N-oxides. The sequence of solvents obtained according to their decreasing differentiation capacity is as follows: acetonitrile > nitromethane > acetone > propylene carbonate ≥ N,N-dimethylformamide > methanol > dimethyl sulfoxide.