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

Abstract

The proton-abstraction by O 2 − from weakly acidic compounds, like aliphatic ketones (with p K as≈21), is endothermic and consequently slow. In this work, by employing selected aromatic ketones as proton donors (with p K as≈19), it has been possible to accelerate the rate of the α-proton abstraction by O 2 − to the point that it is observable within the time scale of cyclic voltammetry. Digital simulations of the cyclic voltammograms of O 2 in the presence of those ketones allow determination of the rate constants of the proton abstraction. Those rate constants were found to increase as the degree of conjugation in ketones increased, consistent with a slow endothermic rate determining step. Furthermore, because ketones as proton donors are very weak acids, it was possible to avoid any subsequent acid–base type of reaction between HO 2 − 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 HO 2 /HO 2 − redox couple.