Electron-Transfer Reaction of Cinnamic Acids and Their Methyl Esters with the DPPH Radical in Alcoholic Solutions

The kinetic behavior of cinnamic acids, their methyl esters, and two catechols 1-10 (ArOH) in the reaction with DPPH in methanol and ethanol is not compatible with a reaction mechanism that involves hydrogen atom abstraction from the hydroxyl group of 1-10 by DPPH. The rate of this reaction at 25°C is, in fact, comparatively fast despite that the phenolic OH group of ArOH is hydrogen bonded to solvent molecules. The observed rate constants (k1) relative to DPPH + ArOH are 3-5 times larger for the methyl esters than for the corresponding free acids and, for the latter, decrease as their concentration is increased according to the relation k1 ) B/[ArOH]0 m, where k1 is given in units of M-1 s-1, m is ca. 0.5, and B ranges from 0.02 (p-coumaric acid) to ca. 3.48 (caffeic acid) in methanol and from 0.04 (p-coumaric acid) to ca. 13 (sinapic acid) in ethanol. Apparently, the reaction mechanism of DPPH¥ + ArOH involves a fast electron-transfer process from the phenoxide anion of 1-10 to DPPH¥. Kinetic analysis of the reaction sequence for the free acids leads to an expression for the observed rate constant, k1, proportional to [ArOH]0 -1/2 in excellent agreement with the experimental behavior of these phenols. The experimental results are also interpreted in terms of the influence that adventitious acids or bases present in the solvent may have. These impurities dramatically influence the ionization equilibrium of phenols and cause a reduction or an enhancement, respectively, of the measured rate constants.