Studies on acid–base equilibria of 4-R-2,6-dimethylpyridine N-oxide systems in non-aqueous solvents

Add time:07/13/2019    Source:sciencedirect.com

Acid–base equilibrium constants, i.e. acid dissociation, cationic homoconjugation and cationic heteroconjugation constants in 4-R-2,6-dimethylpyridine N-oxide systems (where R denotes methoxy-, methyl-, chloro- or nitro-group) in polar non-aqueous solvents: protophobic aprotic nitromethane, acetonitrile and acetone, protophilic aprotic N,N-dimethylformamide and amphiprotic methanol have been determined. The acidity constant values of protonated N-oxides in solvents studied have been found to change according to the substituent effect and to the sequence of acidity changes in water. This finding allowed to correlate the pKa values determined in water with those determined in the solvents studied. Further, it was found that the cationic homo- and heteroconjugation constant values increased with decreasing solvent basicity and with increasing basicity of the N-oxide, and in the case of heteroconjugating systems, with increasing basicity of the the proton acceptor.

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