Reactivity of dpph.

A kinetic study of the reduction of pyrocatechol and catechin by dpph radical has been carried out in various ratios of CHOH/HO mixed solvent at pH 5.5-7.5, μ = 0.10 M [( n-Bu)N]ClO, and T = 25°C. The rate constants of oxidation in aqueous solvent, k, were obtained from the extrapolation of the linear plots of the specific rate constants k vs. % HO plots at each pH value. A linear relationship between k and 1/[H] was observed for both flavonoids with k = kK/[H], where K was the first acid dissociation constant on the catechol ring and k is the rate constant of the oxidation of the mononegative species HX. The values of k obtained from the slopes of the plots are (8.2 ± 0.2) × 10 and (6.1 ± 0.1) × 10 M s 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 M s for the self-exchange rate constant of dpph/dpphH couple. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 147-153, 2011 [ABSTRACT FROM AUTHOR]