Reactivity of dpph • in the oxidation of catechol and catechin

A kinetic study of the reduction of pyrocatechol and catechin by dpph • radical has been carried out in various ratios of CH 3 OH/H 2 O mixed solvent at pH 5.5–7.5, μ = 0.10 M [( n ‐Bu) 4 N]ClO 4 , and T = 25°C. The rate constants of oxidation in aqueous solvent, k   H   2 O , were obtained from the extrapolation of the linear plots of the specific rate constants k vs. % H 2 O plots at each pH value. A linear relationship between k   H   2 Oand 1/[H + ] was observed for both flavonoids with k   H   2 O= k 1 K a1 /[H + ], where K a1 was the first acid dissociation constant on the catechol ring and k 1 is the rate constant of the oxidation of the mononegative species HX − . The values of k 1 obtained from the slopes of the plots are (8.2 ± 0.2) × 10 5 and (6.1 ± 0.1) × 10 5 M −1 s −1 for pyrocatechol and catechin, respectively. The analysis of the reaction on the basis of Marcus theory for an outer‐sphere electron transfer reaction yielded a value of 3.7 × 10 3 M −1 s −1 for the self‐exchange rate constant of dpph • /dpphH couple. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 147–153, 2011