Cationic heteroconjugation equilibria
have been studied potentiometrically in systems containing substituted pyridine Noxides in a polar protophobic aprotic solvent, nitrobenzene. For comparison, systems with trimethylamine N-oxide, MesNO, 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 pK, (acceptor) < pK, (donor). The highest values of the heteroconjugation constants (logKanai+ M 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 (IogKBHBI I of the order of 253.0) 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.