Temperature dependence of the acid–base equilibrium constants of substituted pyridine N-oxides in acetonitrile

Abstract

Temperature dependence of acid–base equilibrium constants in systems consisting of substituted pyridine N-oxides was studied in acetonitrile, a strongly ionizing polar protophobic aprotic solvent. N-oxides, which have an enhanced tendency towards formation of cationic homo- and heteroconjugates, (OHO +) as a result of their relatively strong basicity, were investigated. For the sake of comparison, also aliphatic trimethylamine N-oxide and pyridine, the parent compound of its N-oxide, were included. Of particular interest were acid dissociation constants of cations of the protonated bases, homoconjugation equilibria of the cationic acids with conjugate bases, as well as cationic heteroconjugation equilibria of the protonated bases with non-conjugated bases. The constants were determined at three temperatures, 292.1, 298.1 and 304.1 K. On the basis of the temperature dependence of the constants, enthalpy changes were calculated for the acid dissociation of the cationic acids, as well as for cationic homo- and heteroconjugation. These values were derived from the known thermodynamic relationships, Δ G = − RTln K and Δ G = Δ H − TΔ S. It has been found that enthalpy changes for the dissociation reactions of the cationic acids and for cationic homoconjugation reactions are consistent with the variations of the protonation and homoconjugation energies, respectively calculated by both the semi-empirical and the ab initio methods on the 4-31G level.