Thermodynamic study of the acid dissociation of substituted pyridinium ions in methanol

Abstract

Standard Gibbs' free energies, enthalpies and entropies of dissociation of the pyridinium ion and thirteen substituted pyridinium ions in methanol have been measured. The electronic effects of substituents on the thermodynamic data are discussed in terms of specific solvation around the substituent groups and of changes in solute–solvent interactions at the pyridine N atom or the pyridinium ion ⁺NH group charge centre. Reference to gas-phase data for the dissociation of pyridinium ions and to enthalpies of solvation of neutral pyridines in methanol has allowed separation of substituent effects involving the pyridines and the pyridinium ions. The steric effects of 2-t-butyl and 2,6-di-t-butyl substitution on solute–solvent interactions are assessed and compared with corresponding effects involving the addition of t-butyl substituents on the acid ionization of phenol. An acidity function for hydrogen chloride in methanol is defined.