Isotope effects on acid–base equilibria of pyridine and [2H5]pyridine in protium and in deuterium oxide

Abstract

Equilibrium constants for the dissociation of C₅H₅NH⁺ and C₅D₅NH⁺ in H₂O, as well as of C₅H₅ND⁺ and C₅D₅ND⁺ in D₂O have been measured in the range 5–65°, by spectrophotometric determination of acid–base concentration ratios. Thermodynamic functions (ΔG°, ΔH°, ΔS°) have been calculated for these equilibria, and for exchange reactions between deuteriated and non-deuteriated acid and base, in H₂O and D₂O. Primary, solvent, and secondary isotope effects were evaluated as a function of temperature.