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

Abstract

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, Me 3NO, 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 p K a (acceptor) < p K a (donor). The highest values of the heteroconjugation constants ( logK BHB1 + ≈ 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 ( logK BHB1 + 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.