The potentiometric titration method has been used to study the equilibria of cationic in sytems formed by substituted pyridine N-oxides in the polar, non-aqueous solvents acetone and methanol. For comparison, the systems with trimethylamine N-oxide as a representative of aliphatic amine N-oxides and pyridine representing parent heterocyclic amines were also studied. The cationic heteroconjugation constants, i.e. the equilibrium constants for conjugation reactions between free and protonated N-bases leading to the formation of unsymmetric BHB'+ cations, were determined in experimental systems with and without proton transfer. It was found that there were significant differences in the values of the cationic heteroconjugation constants determined in these two acid-base systems. The proton-transfer reactions limit and even preclude the determination of the cationic heteroconjugation constants. On this basis it was concluded that the heteroconjugation constants should be determined in systems without proton transfer. In such systems, in the amphiprotic solvent methanol, cationic heteroconjugation was ascertained in all substituted pyridine N-oxide systems, the values of heteroconjugation constants being relatively low (logarithms of their values of the order of 2-2.5), and only negligible in systems involving trimethylamine N-oxide. A more pronounced tendency towards cationic heteroconjugation of the [OHO]+ type was observed in the aprotic protophobic acetone, where heteroconjugation constants were determined for all amine N-oxide systems studied including those containing protonated trimethylamine N-oxide as a proton donor. However, the values of the cationic heteroconjugation constants were found to be, in methanol likewise, relatively low (log KBHB'+ of the order of 2-3). On the contrary, a greater extent of cationic heteroconjugation equilibria was observed in methanol than in acetone in the case of systems containing pyridine, i.e. [NHO]+ type bridges formed by amine N-oxides and heterocyclic amines. In methanol the heteroconjugation constants turned out to be determinable for all such systems studied (logarithms of the equilibrium constants being of the same order as for N-oxide systems), whereas in acetone the hetero constants were indeterminable for all systems.

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An attempt has been made to determine potentiometrically (1) acid dissociation constants of cations obtained by protonation of nine trisubstituted pyridine N-oxides, namely 2-halo(Cl, Br and I)-4-nitropicoline N-oxides with the methyl group at positions 3, 5, and 6, as well as (2) the cationic homoconjugation constants of these cationic acids with conjugated N-oxides in acetonitrile. On the basis of the substitution effect, variations of the acid dissociation constants of the trisubstituted pyridine N-oxide cations are discussed. The determined pK a values of the protonated 2-halo-4-nitropicoline N-oxides are compared with the previously determined equilibrium constants of the cationic acids conjugated with the mono-and disubstituted pyridine N-oxides in acetonitrile. Further, based on the pK a values of the protonated 2-halo-4-nitropicoline N-oxides in acetonitrile, supplemented with correlations between pK a 's of the protonated mono-and disubstituted pyridine N-oxides in acetonitrile and water, the pK a 's of the acids conjugated with the trisubstituted N-oxides studied in aqueous solutions have been estimated. Moreover, it has been concluded that the cationic homoconjugation constants cannot be determined by potentiometric titration in acetonitrile solutions of the 2-halo-4-nitropicoline N-oxide systems.

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