Acidity in water (pKa values) of carboxylic acids derived from simple heterocycles (azoles and azines)

The pKa of eighteen heterocyclic carboxylic acids derived from thiophene, furan, pyrrole, benzothiophene, benzofuran, indole, pyridine and quinoline have been determined by spectrophotometry and potentiometry. The values thus obtained are discussed using additive models of the Free-Wilson type. Some theoretical calculations within the AM1 approximation have been carried out in order to understand why annelation effects are dependent on the position, α or β, of the carboxylic group. In the case of pyridine and quinoline carboxylic acids, the problem of tautomerism between neutral and zwitterionic forms has been considered.