A cyclic voltammetric study of the proton abstraction from selected aromatic ketones by superoxide

The proton-abstraction by O-2(.-) from weakly acidic compounds, like alipha tic ketones (with pK(a)s approximate to 21), is endothermic and consequentl y slow. In this work, by employing selected aromatic ketones as proton dono rs (with pK(a)s approximate to 19), it has been possible to accelerate the rate of the alpha-proton abstraction by O-2(.-) to the point that it is obs ervable within the time scale of cyclic voltammetry. Digital simulations of the cyclic voltammograms of O-2 in the presence of those ketones allow det ermination of the rate constants of the proton abstraction. Those rate cons tants were found to increase as the degree of conjugation in ketones increa sed, consistent with a slow endothermic rate determining step. Furthermore, because ketones as proton donors are very weak acids, it was possible to a void any subsequent acid-base type of reaction between HO2- and the ketones , and to observe a new, kinetically reversible redox wave at E-1/2 = 0.34 V versus O-2/O-2(.-) in DMSO, assigned to the HO2./HO2- redox couple. (C) 20 00 Elsevier Science Ltd. All rights reserved.