A POTENTIOMETRIC STUDY ON PROTON-TRANSFER EQUILIBRIA AND CATIONIC CONJUGATION IN PYRIDINE N-OXIDE SYSTEMS IN ACETONE AND METHANOL

The potentiometric titration method has been used to study the equilib ria of cationic heteroconjugation in systems formed by substituted pyr idine N-oxides in the polar, non-aqueous solvents acetone and methanol . For comparison, the systems With trimethylamine N-oxide as a represe ntative of aliphatic amine N-oxides and pyridine representing parent h eterocyclic amines were also studied. The cationic heteroconjugation c onstants, i.e. the equilibrium constants for conjugation reactions bet ween free and protonated N-bases leading to the formation of unsymmetr ic BHB'(+) cations, were determined in experimental systems with and w ithout proton transfer. It was found that there were significant diffe rences in the values of the cationic heteroconjugation constants deter mined in these two acid-base systems. The proton-transfer reactions li mit and even preclude the determination of the cationic heteroconjugat ion constants. On this basis it was concluded that the heteroconjugati on constants should be determined in systems without proton transfer. In such systems, in the amphiprotic solvent methanol, cationic heteroc onjugation was ascertained in all substituted pyridine N-oxide systems , the values of heteroconjugation constants being relatively low (loga rithms of their values of the order of 2-2.5), and only negligible in systems involving trimethylamine N-oxide. A more pronounced tendency t owards cationic heteroconjugation of the [OHO](+) type was observed in the aprotic protophobic acetone, where heteroconjugation constants we re determined for all amine N-oxide systems studied including those co ntaining 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 K-BHB'+ 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 sys tems containing. pyridine, i.e. [NHO](+) type bridges formed by amine N-oxides and heterocyclic amines. In methanol the heteroconjugation co nstants turned out to be determinable for all such systems studied (lo garithms of the equilibrium constants being of the same order as for N -oxide systems), whereas in acetone the hetero constants were indeterm inable for all systems.