19197-14-9Relevant articles and documents
Effect of Medium on the Acidity Constant of Some New Arylazopyrazolopyrimidine Derivatives
Abd-Allah, Elham M.,Rageh, Nasr M.,Salman, Hassan M.A.
, p. 652 - 656 (2007/10/03)
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 (pKa) 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 pKa 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.
