Acidity in methanol–water Original Research Article

The different factors influencing acidity of solutes in methanol–water (acidity of the solute, basicity and dielectric constant of the solvent, and specific solute–solvent interactions) are discussed. General thermodynamic models and preferential solvation models are used to establish linear relationships between the acidity pK values of a family of compounds in any methanol–water mixture and the pK values of the compounds in water. The parameters (slope and intercept) of the relationships are related with solvent composition for the most common families of compounds (phenols, carboxylic acids, amines and pyridine derivatives). These equations allow accurate calculation of the pK values of any member of these families at any methanol–water mixture from the pK value of the compound in water, or conversely estimation of the pK value in water from pK values measured in methanol–water mixtures. Some examples of prediction, including chromatographic retention of acid–base compounds, are presented.