Quinone and iron porphyrin mediated reduction of nitroaromatic compounds in homogeneous aqueous solution

Abstract

The kinetics of reductive transformations of a series of monosubstituted nitrobenzenes and nitrophenols have been investigated in aqueous solution containing reduced sulfur species and small concentrations of either a naphthoquinone or an iron porphyrin. Boith the two naphthoquinones and the iron porphyrin used in this study mediated the reduction of the nitro group. In all three cases, the rate of reduction of the nitrobenzenes and of the nitrophenols was strongly pH dependent. Dissociated nitrophenols were reduced ~ 3-4 times more slowly as compared to the nondissociated species. For the substituted nitrobenzenes, the effect of substitution on the reaction rate could be described by a linear free energy relationship (LFER) of the general form log k = aEh1' + b, where k is the second-order rate constant for reaction with the hydroquinone monophenolate or the iron porphyrin, respectively, and Eh1' is the one-electron reduction potential of the nitroaromatic compound. Competition between different nitrobenzenes was observed in the case of the iron porphyrin, while no effects were found for the reaction with the hydroquinones. The results of this study form an important base for the evaluation and interpretation of reductive transformation processes of nitroaromatic compounds in the environment.