A comparison of acid – base properties of substituted pyridines and their N-oxides in propylene carbonate

Abstract

Acid–base equilibria in systems consisting of substituted pyridines and their N-oxides have been studied in propylene carbonate, a strong ionizing, polar, protophobic, aprotic solvent. For the sake of comparison, aliphatic trimethylamine N-oxide was also included. Of particular interest in this study were acid dissociation equilibria of cations conjugated with the N-bases and the N-oxides, as well as homoconjugation equilibria of these cations with conjugated bases. Appropriate equilibrium constants were determined that enabled acid–base properties of the two classes of compounds to be compared. The heterocyclic amines turned out to be stronger bases in propylene carbonate than their N-oxides by two to three powers of 10. At the same time, the difference was two to three powers of 10 smaller than in aqueous solutions. The difference 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 carbonate 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 heterocyclic amines, while with the aliphatic amine N-oxides the tendency was still stronger {lgKf°(BHB+) value in the system with trimethylamine N-oxide was 5.89}. Numerical values of the standard homoconjugation constants in the majority of systems with pyridine derivatives could not be determined from potentiometric measurements, while in those where they could be determined, they were usually low {lgKf°(BHB+) values ranged from 0.8 to 1.7} and imprecise. Further, a comparison of the standard acidity constants and cationic standard homoconjugation constants determined in propylene carbonate with those determined in acetonitrile showed that the enhanced polarity of the aprotic solvent was favourable for dissociation of the cationic acids and did not appreciably affect the cationic standard homoconjugation constants.