Hammett-Taft and Drago models in the prediction of acidity constant values of neutral and cationic acids in methanol

The Hammett and Taft one-parameter model and Drago dual-parameter model have been applied to a very wide series of dissociation constants in methanol of carboxylic aliphatic acids, benzoic acid derivatives, phenols, protonated amines, anilinium and pyridinium derivatives. Both models can be successfully used to predict pK values in methanol of any member of the neutral and cationic acid families studied. Similarities and differences in the solvation behavior between the studied series of compounds have been pointed out. The relationship between the solvent dependent parameters in both models has been established and it shows that the Hammett-Taft rho value can be explained by means of the electrostatic and covalent components without any other contribution.