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

p Ka values of various 5‐membered heterocyclic aromatic carboxylic acids (p Ka 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 p Ka 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 p Ka s values show a poor correlation with the dielectric constants in aqueous acetonitrile, N,N ‐dimethylformamide, dimethyl sulfoxide, and dioxane. The p Ka values in pure water and in pure organic solvent could be calculated by extrapolation of the plot of the p Ka s versus percentage of organic solvent. The p Ka 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.