Potentiometric determination of standard acidity and cationic standard homoconjugation constants of substituted pyridine N-oxide systems in (acetonitrile + methanol)

Abstract

An attempt has been made to determine potentiometrically standard acidityKao (BH+) constants of protonated substituted pyridine N-oxides and cationic standard homoconjugation constantsKfo (BHB+) of these acids conjugated with the N-oxides in a binary solvent system. The system consisted of solvents with different proton-donating properties, namely, the aprotic protophobic acetonitrile (CH3CN) and amphiprotic methanol. The measurements were carried out in solutions with mole fractions of acetonitrile ranging between 0 andx(CH3CN) = 1. The standard acidity constants expressed as p Ka(BH+) values in these systems were determined for cations resulting from protonation of a variety of substituted pyridine N-oxides, namely those of 4- N′,N′- dimethylaminopyridine, 4-methylpyridine, pyridine, and 4-nitropyridine. Whenever the respective equilibria were established, the cationic standard homoconjugation constants of the protonated N-oxides with conjugate bases were also calculated. For comparison, the equilibrium constants were also determined for quinoline N-oxide, trimethylamine N-oxide, and pyridine, representing, respectively, binuclear heterocyclic amines, aliphatic amines, and mononuclear heterocyclic amines. Inspection of the determined standard acidity dissociation constants showed them to be nonlinearly related tox(CH3CN) in the binary solvent in all the acid–base systems studied. The character of the relations was specific for a particular cationic acid. It was also found that the cationic standard homoconjugation constants were determinable over the whole composition range of the (acetonitrile + methanol) system for the strongest bases only, namely trimethylamine N-oxide and 4- N′,N′- dimethylaminopyridine N-oxide. Nonetheless, inspection of the standard homoconjugation constants determined in the majority of the remaining acid–base systems revealed that, similar to the pKa (BH+) values, logarithms of the cationic standard homoconjugation constants do vary nonlinearly as a function of composition of the solvent system.