A POTENTIOMETRIC STUDY OF CATIONIC HETEROCONJUGATION EQUILIBRIA IN SYSTEMS WITH SUBSTITUTED PYRIDINE N-OXIDES IN NITROBENZENE

Cationic heteroconjugation equilibria have been studied potentiometric ally in systems Containing substituted pyridine N-oxides in a polar pr otophobic aprotic solvent, nitrobenzene. For comparison, systems with trimethylamine N-oxide, Me(3)NO, representing aliphatic amine N-oxides , and pyridine, Py, as a representative of heterocyclic amines, Were a lso studied: Generally, systems without proton transfer were studied w hich satisfied the condition pK(a) (acceptor) < pK(a) (donor). Tke hig hest values of the heteroconjugation constants (IogK(BHB1+) approximat e to 4.70) were obtained in systems containing amine N-oxides with [OH O](+) bridges in which both the proton donor and acceptor had intermed iate and comparable basicities. In systems with amine N-oxides and ami nes forming [OHN](+) bridges, the cationic heteroconjugation constants were markedly smaller (logK(BHB1+) of the order of 2.5-3.0) as compar ed with those measured in the N-oxide systems. In systems with pyridin e as a proton acceptor, the heteroconjugation constants decreased with increasing proton donor basicity. Further, a comparison of the cation ic heteroconjugation constants in nitrobenzene with those measured in other non-aqueous solvents showed that similar to the cationic homocon jugation, the tendency towards cationic heteroconjugation declined wit h increasing solvent basicity.