Neighbor Group Hydration Effects on Carboxylate Basicities in Partly Aqueous Solutions

摘要

Protonation constants of carboxylate groups in a variety of compounds were determined in 1,4-dioxane + water solvent mixtures and have been found to depend on the local solvent composition, and their values are modified by adjacent moieties with diferent polarity occurring on the compounds. A relationship has been found between log10 K values and solvent composition, and by the neighboring group. Using acetic acid as a reference compound, the extent of the local hydration effect was estimated, and it has been found to be strong for α-ammonium sites, and moderate for nearby peptide and thiol groups. On the other hand, an extra methylene moiety has been found to bring about a moderate dehydration effect. The observed hydration/dehydration effects were observed in the 12–40 mole% range of the actual bulk solvent composition.