Thermodynamic analysis of substituent effects on proton dissociation equilibria of substituted pyridines

Abstract

Equilibrium constants for the dissociation of RC₅H₄NH⁺(R = 3- and 4-Br, 3- and 4-CN, and 2- and 3-NH₂) have been measured as a function of temperature in the range 5–65 °C. For these and other literature data (R = H, 2-, 3-, and 4-NO₂, 2-, 3-, and 4-Me, 2-, 3-, and 4-CHO, 2-, 3-, and 4-Ac, 2-CH₂NHMEe, and 4-NH₂) thermodynamic functions (ΔG⁰, ΔH⁰, and ΔS⁰) have been calculated by a thermodynamic optimization method and by the Clarke and Glew procedure. Substituent effects, the environmental parameter for the reaction series examined, and solvent–solute interactions are discussed on the basis of Hepler's dichotomized enthalpy–entropy model, and the approach developed by Bolton et al.