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

An attempt has been made to determine potentiometrically standard acidity K -a(o)(BH+) constants of protonated substituted pyridine N-oxides and cation ic standard homoconjugation constants K-f(o)(BHB+) of these acids conjugate d with the N-oxides in a binary solvent system. The system consisted of sol vents with different proton-donating properties, namely, the aprotic protop hobic acetonitrile (CH3CN) and amphiprotic methanol. The measurements were carried out in solutions with mole fractions of acetonitrile ranging betwee n 0 and x(CH3CN) = 1. The standard acidity constants expressed as pK(a)(BH) values in these systems were determined for cations resulting from proton ation of a variety of substituted pyridine N-oxides, namely those of 4-N',N '-dimethylaminopyridine, 4-methylpyridine, pyridine, and 4-nitropyridine. W henever the respective equilibria were established, the cationic standard h omoconjugation constants of the protonated N-oxides with conjugate bases we re also calculated. For comparison, the equilibrium constants were also det ermined for quinoline N-oxide, trimethylamine M-oxide, and pyridine, repres enting, respectively, binuclear heterocyclic amines, aliphatic amines, and mononuclear heterocyclic amines. Inspection of the determined standard acid ity dissociation constants showed them to be nonlinearly related to x(CH3CN ) in the binary solvent in all the acid-base systems studied. The character of the relations was specific fora particular cationic acid. It was also f ound that the cationic standard homoconjugation constants were determinable over the whole composition range of the (acetonitrile + methanol) system f or the strongest bases only, namely trimethylamine N-oxide and 4-N',N'-dime thylaminopyridine N-oxide. Nonetheless, inspection of the standard homoconj ugation constants determined in the majority of the remaining acid-base sys tems revealed that, similar to the pK(a)(BH+) values, logarithms of the cat ionic standard homoconjugation constants do vary nonlinearly as a function of composition of the solvent system. (C) 1999 Academic Press.