A potentiometric study of acid-base equilibria of substituted pyridines inacetonitrile

Acid dissociation, as well as cationic homo- and heteroconjugation equilibr ia have been studied by potentiometric titration for pyridine and a variety of its mono- and di-substituted derivatives in the polar protophobic aprot ic solvent, acetonitrile. The heteroconjugation equilibria were investigate d in several BH+/B-1 systems without proton transfer in which the pK(a) val ues 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 heterocycli c 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 p K(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 basi s for establishing linear correlations of these values. Moreover, the resul ts of potentiometric measurements have also shown that cationic homoconjuga tion equilibria are present in the BH+/B systems studied. The homoconjugati on 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 protopho bic aprotic solvent, propylene carbonate. It has been concluded that aceton itrile better differentiates between the strengths of cationic acids and is more favourable for attaining cationic homoconjugation equilibria than pro pylene carbonate. Furthermore, it has been found that cationic heteroconjug ation equilibria are not attained in the majority of the amine systems stud ied. The heteroconjugation constant could be determined in one system studi ed only and was of the order of two on the logarithmic scale. (C) 1999 Publ ished by Elsevier Science B.V. All rights reserved.