Abstract
The acid dissociation constants (pKa) of 7-hydroxyflavone, 3-hydroxyflavone and 5-hydroxyflavone, which are practically insoluble compounds in water but of great biological and physicochemical interest, were determined by UV–visible spectroscopy in ethanol–water and acetonitrile–water solutions, varying the solvent relative permittivity in the interval of 61 to 75, at constant ionic strength (0.050 mol·kg–1) and temperature (298.15 K). The pKa values of the compounds increase as the permittivity of the reaction medium decreases. Correlations were established between the pKa values and empirical parameters of the solvents, such as the relative permittivity and hydrogen-bond donor capacity. These equations allow the interpretation of the solvent effect on the acid–base equilibria and the determination of pKa values at 298.15 K in pure water. The ordering of the pKa values was 7-hydroxyflavone (7.28) < 3-hydroxyflavone (8.68) < 5-hydroxyflavone (11.75). In addition, the dissociation constants were also calculated by means of DFT methods (B3LYP/6-311+G(2d,p) level of theory), employing several thermodynamic cycles. The solvent effect on the optimized structures in the gas-phase was evaluated using the polarizable continuum model. A good agreement was observed between the theoretical and experimental pKa values. Finally, the experimental ordering in the acidity of the hydroxyflavones was explained using natural bond orbital analysis.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.