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

Abstract

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 pK a 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 pK a 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 pK a 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.