A Potentiometric Study on Proton-Transfer Equilibria and Cationic Conjugation in Pyridine N-Oxide Systems in Acetone and Methanol

Abstract

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.

Potentiometric studies of cationic heteroconjugation equilibria in systems involving free and protonated pyridine derivatives in dimethyl sulfoxide

“…For instance, no relationship could be established between cationic standard heteroconjugation constants and solvent basicities, which is characteristic for substituted pyridine N -oxides. (17) Furthermore, in the case of the (OHO) + bridges, the cationic standard heteroconjugation constant values increase with increasing acceptor basicity and decrease with proton donor basicity, (18)(19)(20)(21) a regularity not observed in the case of the (NHN) + bridges. Another feature influencing the tendency towards cationic heteroconjugation in the systems with the N -oxides is the capacity of the constituents to form hydrogen bonds, as defined by the arithmetic mean of their cationic standard homoconjugation constant values.…”

“…Also, the (OHO) + bridges, where different amine N -oxides are bound together with a hydrogen bond, turned out to be less stable. (18)(19)(20)(21) However, it should be emphasised that analysing cationic standard heteroconjugation constants determined in non-aqueous media in systems involving organic N -bases is not straightforward for two reasons. First, there were many methods employed to study the equilibria considered.…”

The transfer of neutral molecules, ions and ionic species from water to benzonitrile; comparison with nitrobenzene

“…Chmurzyński and Wawrzyniak [37] have determined pKa values for pyridine-N-oxide and the three methylpyridine-N-oxides in benzonitrile and in water. These can be used in Eq.…”

A potentiometric study of cationic heteroconjugation equilibria in nitromethane andN,N- dimethylformamide

“…Furthermore, cationic homoconjugation shown in equations (3) and (4) is a reaction whereby cationic acid BH + (B 1 H + ) reacts with conjugate base B (B 1 ) to yield a symmetrical complex cation BHB + (B 1 HB 1 ) + , whereas the cationic heteroconjugation phenomenon shown in equation (5) takes place when cationic acid BH + reacts with base B 1 conjugated with another cationic acid to afford an asymmetric hydrogen-bonded complex cation BHB + 1 . Cationic heteroconjugation equilibria in systems comprising amine N -oxides (mainly substituted pyridine N -oxides, as well as aliphatic trimethylamine N -oxide) have been extensively studied in polar protophobic aprotic solvents, such as nitrobenzene, (7) acetonitrile, (8) propylene carbonate, (9) and acetone, (10) as well as in methanol, (10) which is a polar amphiprotic solvent. These studies have shown that the tendency towards heteroconjugation of substituted pyridine N -oxides depends on solvent basicity and decreases with increasing basicity of solvents in the following order: nitrobenzene > propylene carbonate > acetonitrile > acetone > methanol.…”