Acid–base and cationic homoconjugation equilibria of substituted pyridine N-oxides in acetone

Abstract

The pK_a and cationic homoconjugation constants in acetone have been determined for a series of eight pyridine N-oxide derivatives with a wide spectrum of acid–base properties. The acetone pK_a values were found to correlate well with the aqueous ones. The values are lower than those determined in acetonitrile and benzonitrile, owing to the higher basicity of acetone. As in other aprotic solvents, cationic homoconjugation was found to take place for sufficiently basic N-oxides (pK_a > 5 in acetone). In contrast to anionic homoconjugation the homoconjugation constants are lower in acetone than in nitriles. For comparison, investigations were carried out for pyridine, showing the homoconjugation constant to be indeterminable which conforms with the results obtained in other aprotic solvents.