A potentiometric study of cationic heteroconjugation equilibria in systems with substituted pyridine N-oxides in nitrobenzene

Cationic heteroconjugation equilibria have been studied potentiometrically in systems containing substituted pyridine N-oxides in a polar protophobic aprotic solvent, nitrobenzene. For comparison, systems with trimethylamine N-oxide, Me3NO, representing aliphatic amine N-oxides, and pyridine, Py, as a representative of heterocyclic amines, were also studied. Generally, systems without proton transfer were studied which satisfied the condition pKa (acceptor) < pKa (donor). The highest values of the heteroconjugation constants (logKBHB1+ ≈ 4.70) were obtained in systems containing amine N-oxides with [OHO]+ bridges in which both the proton donor and acceptor had intermediate and comparable basicities. In systems with amine N-oxides and amines forming [OHN]+ bridges, the cationic heteroconjugation constants were markedly smaller (logKBHB1+ of the order of 2.5–3.0) as compared with those measured in the N-oxide systems. In systems with pyridine as a proton acceptor, the heteroconjugation constants decreased with increasing proton donor basicity. Further, a comparison of the cationic heteroconjugation constants in nitrobenzene with those measured in other non-aqueous solvents showed that similar to the cationic homoconjugation, the tendency towards cationic heteroconjugation declined with increasing solvent basicity.