Abstract
The potentiometric titration method has been used to study the equilibria of cationic heteroconjugation in systems formed by substituted pyridine N-oxides in the polar non-aqueous solvents; nitrobenzene, acetonitrile, acetone and methanol. For comparison, the systems with trimethylamine N-oxide as a representative of aliphatic amine N-oxides and pyridine representing parent heterocyclic amines were also considered. The cationic heteroconjugation constants, i.e. the constants of conjugation reactions between free and protonated N-bases leading to the formation of asymmetric BHB1 + cations determined in experimental systems without proton transfer in media studied have been compared and discussed. Summing up: investigations of the equilibria of cationic heteroconjugation in non-aqueous media revealed that only a small range can be noted in the amphiprotic methanol, whereas in the basic aprotic solvent, acetone, the range observed is slightly greater. A full scheme of the heteroconjugation equilibria can be observed in two other aprotic protophobic solvents studied — acetonitrile and nitrobenzene.
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References and Notes
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Chmurzynski, L. Cationic Heteroconjugation Equilibria in Systems with Heterocyclic N-Oxides in Non-Aqueous Media. Molecules Online 1, 99–105 (1996). https://doi.org/10.1007/s007830050017
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DOI: https://doi.org/10.1007/s007830050017