Acid–base equilibria of nitropyridines in protium oxide and in deuterium oxide

Abstract

Equilibrium constants for the dissociation of 2-, 3-, and 4-NO₂·C₅H₄NH⁺ in H₂O have been measured as a function of temperature in the range 3–70 °C by spectrophotometric determination of acid–base concentration ratios in aqueous solution of constant ionic strength (0·02M) or in aqueous sulphuric acid. Thermodynamic functions (ΔG°, ΔH°, and ΔS°) have been calculated for these equilibria as well-behaved functions of temperature and compared with those relative to acid–base equilibrium of unsubstituted pyridine. Entropy values for the above reactions are nearly equal, so that the free-energy differences, due to the different acidities, are mainly found as differences between the corresponding enthalpies. Equilibrium constants for the dissociation of 2-, 3-, and 4-NO₂·C₅H₄ND⁺ in D₂O have also been measured at 298·2 K. Isotope effects seem to be mainly determined by solvation characteristics of the system.