A COMPARISON OF ACID-BASE PROPERTIES OF SUBSTITUTED PYRIDINES AND THEIR N-OXIDES IN PROPYLENE CARBONATE

Acid-base equilibria in systems consisting of substituted pyridines an d their N-oxides have been studied in propylene carbonate, a strong io nizing, polar, protophobic, aprotic solvent. For the sake of compariso n, aliphatic trimethylamine N-oxide was also included. Of particular i nterest in this study were acid dissociation equilibria of cations con jugated with the N-bases and the N-oxides, as well as homoconjugation equilibria of these cations with conjugated bases. Appropriate equilib rium constants were determined that enabled acid-base properties of th e two classes of compounds to be compared. The heterocyclic amines tur ned out to be stronger bases in propylene carbonate than their N-oxide s by two to three powers of 10. At the same time, the difference was t wo to three powers of 10 smaller than in aqueous solutions. The differ ence in basicities of heterocyclic and aliphatic amine N-oxides turned out to be markedly larger. For instance, the aliphatic trimethylamine N-oxide was by approximately eight powers of 10 a stronger base than pyridine N-oxide, a representative of the heterocyclic amine N-oxides. Again, the tendency towards cationic homoconjugation in propylene car bonate was much more pronounced in systems with substituted pyridine N -oxides (logarithms of the cationic standard homoconjugation constant values fell within the range 1.6 to 4.7) than in those with parent het erocyclic amines, while with the aliphatic amine N-oxides the tendency was still stronger {lg K(f)degrees(BHB+) value in the system with tri methylamine N-oxide was 5.89}. Numerical values of the standard homoco njugation constants in the majority of systems with pyridine derivativ es could not be determined from potentiometric measurements, while in those where they could be determined, they were usually low (Ig K(f)de grees (BHB+) values ranged from 0.8 to 1.7) and imprecise. Further, a comparison of the standard acidity constants and cationic standard hom oconjugation constants determined in propylene carbonate with those de termined in acetonitrile showed that the enhanced polarity of the apro tic solvent was favourable for dissociation of the cationic acids and did not appreciably affect the cationic standard homoconjugation const ants. (C) 1998 Academic Press.