Effect of Medium on the Acidity Constant of Some New Arylazopyrazolopyrimidine Derivatives

Article abstract of DOI:10.1021/je020194t

The acid dissociation constants of some new 8-[(aryl)azo]-2,4-dimethylpyrazolo[1,5-a]pyrimidine-7(6H)-one derivatives were determined in aqueous-organic solvent mixtures (methanol, ethanol, acetone, and dimethylformamide (DMF)). Methanol and DMF had approximately similar relative permittivity constants (32.6 and 36.7, respectively, at 25°C), and all the compounds were more acidic in water + DMF than in water-methanol although the same mole fraction of each was used. The increase in the acid dissociation constants (pK_a) of the compounds as the proportion of the organic cosolvent in the medium was increased could be ascribed, in addition to the electrostatic effect, to the hydrogen-bonding interaction between the conjugate base (A-) and solvent molecules. The pK_a values in the presence of the poorer hydrogen bond donor DMF were less than those obtained in the presence of corresponding amounts of the other solvents.