Effect of mixtures of water and organic solvents on the acidities of 5‐membered heteroaromatic carboxylic acids

pKa values of various 5‐membered heterocyclic aromatic carboxylic acids (pKa^s) were determined in solutions of 20.3, 35.2, 50.1, 65.1, and 79.9 weight percent of organic solvents in water. The pKa^s values show good linearity when they are ploted as a function of the dielectric constants of the mixed solvents methanol, ethanol, isopropyl alcohol, and tert‐butyl alcohol. On the other hand, the pKa^s values show a poor correlation with the dielectric constants in aqueous acetonitrile, N,N‐dimethylformamide, dimethyl sulfoxide, and dioxane. The pKa values in pure water and in pure organic solvent could be calculated by extrapolation of the plot of the pKa^s versus percentage of organic solvent. The pKa^s values of 4‐ and 5‐sub‐stituted 2‐thiophenecarboxylic acids were also determined, and the ρ values are calculated in the same series of the solvents.