Determination of the overlapping pKa values of resorcinol using UV-visible spectroscopy and DFT methods

In this paper we determine the overlapping pKa values of resorcinol in water, applying a UV-Vis spectroscopic method that uses absorbance diagrams. On the other hand, in order to explain the pKa values obtained, we also investigate the molecular conformations and solute-solvent interactions of the resorcinate anions, using ab initio and density functional theory methods. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-donor capability, are proposed. The mentioned reactions and equilibria constituted the indispensable theoretical basis to calculate the acidity constants of resorcinol. Basis sets at the HF/6-31 + G(d) and B3LYP/6-31 + G(d) levels of theory were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the resorcinate anions and water molecules. In this way, it was determined that in alkaline aqueous solutions the monoanion and dianion of resorcinol are solvated with two and four molecules of water, respectively. The agreement between the experimentally determined pKa values and those reported in the literature demonstrates the applicability and accurateness of the spectroscopic method here used. On the other hand, the agreement between the experimental and theoretically calculated pKa values provides solid support for the acid-base reactions proposed in this work.