ABTS as an Electron Shuttle to Enhance the Oxidation Kinetics of Substituted Phenols by Aqueous Permanganate

In this study, it was, interestingly, found that 2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), a widely used electron shuttle, could greatly accelerate the oxidation of substituted phenols by potassium permanganate (Mn(VII)) in aqueous solutions at pH 5-9. This was attributed to the fact that these substituted phenols could be readily oxidized by the stable radical cation (ABTS(center dot+)), which was quickly produced from the oxidation of ABTS by Mn(VII). The reaction of Mn(VII) with ABTS exhibited second-order kinetics, with stoichiometries of similar to 5:1 at pH 5-6 and similar to 3:1 at pH 7-9, and the rate constants varied negligibly from pH 5 to 9 (k = (9.44 +/- 0.21) x 10(4) M-1 s(-1)). Comparatively, the reaction of ABTS(center dot+) with phenol showed biphasic kinetics. The second-order rate constants for the reactions of ABTS(center dot+) with substituted phenols obtained in the initial phase were strongly affected by pH, and they were several orders of magnitude higher than those for the reactions of Mn(VII) with substituted phenols at each pH. Good Hammett-type correlations were found for the reactions of ABTS(center dot+) with undissociated (log(k) = 2.82-4.31 sigma) and dissociated phenols (log(k) = 7.29-5.90 sigma). The stoichiometries of (2.2 +/- 0.06):1 (ABTS(center dot+) in excess) and (1.38 +/- 0.18):1 (phenol in excess) were achieved in the reaction of AETS(center dot+) with phenol, but they exhibited no pH dependency.