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

Acid-base equilibrium constants, i.e. acid dissociation, cationic homoconju gation and cationic heteroconjugation constants in 4-R-2.6-dimethylpyridine N-oxide systems (where R denotes methoxy-, methyl-, chloro- or nitro-group ) in polar nonaqueous solvents: protophobic aprotic nitromethane, acetonitr ile 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 subst ituent effect and to the sequence of acidity changes in water. This finding allowed to correlate the pK(a) values determined in water with those deter mined in the solvents studied. Further, it was found that the cationic homo - and heteroconjugation constant values increased with decreasing solvent b asicity and with increasing basicity of the N-oxide, and in the case of het eroconjugating systems, with increasing basicity of the the proton acceptor . (C) 2000 Elsevier Science B.V. All rights reserved.