An LFER study of the protolytic equilibria of 4-aryl-2,4-dioxobutanoic acids in aqueous solutions

Abstract

The protolytic equilibria of 13 4-aryl-2,4-dioxobutanoic acids (ADKs) were spectrophotometrically studied in aqueous solutions in the pH range 1–9 at 25±1 °C and an ionic strength of 0.1 mol l-1 (NaCl), with the exception of the 4-OH- derivative which was also potentiometrically studied in the pH range 7–10 at 25±1 °C and an ionic strength of 0.1 mol l-1 (NaCl). In solution, the compounds simultaneously exist in one diketo and two enolic forms; therefore, the determined acidity constants (pKa1 1.87–2.29, pKa2 6.63–8.13 and pKa3(4-OH-) 9.52) represent system macro constants. The 1H-NMR spectrum of the basic compound (4-phenyl-2,4-dioxobutanoic acid) (25 °C, pD 5.0) proved the existence of all tautomeric forms. Using the extended Hammett relation, the determined pKa values were correlated with literature σ values. The predicted pKa values were in fair accordance with the experimentally observed ones. Molecular, monoanionic and dianionic forms of the basic compound were optimized by the semi-empirical molecular orbital PM6 method using the implicit water solvation model (COSMO). The obtained geometries were used to explain the quality of the LFER models.

Keywords

• acidity constants
• 4-aryl-2
• 4-dioxobutanoic acids (ADK)
• linear free energy relationship (LFER)