The kinetics of the base-catalyzed oxygenation of flavonol have been investigated in 50% DMSO-H2O solution in the pH range 6.4-10.8 and an ionic strength of 0.1 mol L-1 using spectrophotometric techniques at temperatures between 70 and 90 °C. The rate law -d[flaH]/dt = kobs [OH-][flaH][O2] (kobs = kK1/[H2O]) describes the kinetic data. The rate constant, activation enthalpy, and entropy at 353.16 K are as follows: k/mol-1 L s-1 = (4.53 ± 0.07) × 10-2, ΔH‡/kJ mol-1= 59 ± 4, ΔS‡/J mol-1 K-1 = -110 ± 11. The reaction showed specific base catalysis. It fits a Hammett linear free energy relationship for 4′-substituted flavonols and electron-releasing substituents enhanced the reaction rate. The linear correlation between the oxidation potential of the flavonols and the rate constants supports that a higher electron density on the flavonolate ion makes them more nucleophilic and the electrophilic attack of O2 easier.
ASJC Scopus subject areas
- Organic Chemistry