A potentiometric study of acid-base equilibria of substituted pyridines in acetonitrile

Acid dissociation, as well as cationic homo- and heteroconjugation equilibria have been studied by potentiometric titration for pyridine and a variety of its mono- and di-substituted derivatives in the polar protophobic aprotic solvent, acetonitrile. The heteroconjugation equilibria were investigated in several BH+/B-1 systems without proton transfer in which the pK(a) values of the protonated form of the proton acceptor were lower than those of the proton donor. The pK(a) values of acids conjugated with the heterocyclic N-bases determined in acetonitrile fall within the pK(a) range of 4.5-18.5 and are on an average 8-9 orders of magnitude higher than corresponding pK(a)'s of these species in water. The ranking order of the pK(a) values of the protonated amines is the same in both solvents, thus providing the basis for establishing linear correlations of these values. Moreover, the results of potentiometric measurements have also shown that cationic homoconjugation equilibria are present in the BH+/B systems studied. The homoconjugation constants are relatively small falling in the log KBHB+ range between 1 and 2. Further, the acidity and cationic homoconjugation constant values in acetonitrile were compared with those determined in another polar protophobic aprotic solvent, propylene carbonate. It has been concluded that acetonitrile better differentiates between the strengths of cationic acids and is more favourable for attaining cationic homoconjugation equilibria than propylene carbonate. Furthermore, it has been found that cationic heteroconjugation equilibria are not attained in the majority of the amine systems studied. The heteroconjugation constant could be determined in one system studied only and was of the order of two on the logarithmic scale. (C) 1999 Published by Elsevier Science B.V. All rights reserved.