Acid–base and cationic homoconjugation equilibria in nitromethane solutions of substituted pyridine N-oxide systems

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Abstract
The acid-dissociation and cationic-homoconjugation constant values for nine substituted pyridine N-oxide systems in nitromethane have been determined by the potentiometric titration method. A series of N-oxides having a wide spectrum of acid–base properties (aqueous pKa values in the range 3.88 to –1.7) has been studied. A linear relationship, between the nitromethane and the aqueous pKa values, pKNM a= 1.64 pKW a+ 7.41, has been found. As in other polar aprotic solvents, cationic homoconjugation has been demonstrated in nitromethane. The homoconjugation constants are higher than those in acetonitrile and increase with increasing basicity of the N-oxides. The relationship between the logarithms of the homoconjugation constants and the nitromethane pKa values can be expressed by the equation: log KBHB+= 0.39 pKa+ 0.33. The tendency of substituted pyridine N-oxides towards cationic homoconjugation in nitromethane has been found to be comparable with that of bicyclic amine N-oxides and is ca. two orders of magnitude stronger than that of the parent amines. The applicability of the direct method of pKa determination of substituted pyridine N-oxides in nitromethane solutions has been proven. Using this method, the pKa values of an extended series of eleven pyridine N-oxide derivatives have been estimated and compared with those calculated from a complete titration curve.