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Title: Reduction of substituted p-Benzoquinones by Fe II near neutral pH

Author
item Uchimiya, Sophie
item STONE, ALAN - Johns Hopkins University

Submitted to: Aquatic Geochemistry Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2009
Publication Date: 11/17/2009
Citation: Uchimiya, M., Stone, A.T. 2009. Reduction of substituted p-Benzoquinones by Fe II near neutral pH. Aquatic Geochemistry. 16(1):173-188.

Interpretive Summary: Quinones are important component of soil organic matter. Redox chemistry of quinones is known to influence a wide range of soil biogeochemical processes, such as respiratory electron shuttling by microorganisms. In this study, the impact of quinone structure and pH on the driving force for the reaction with Fe(II), and on the rate of reaction was investigated. Results showed that ring substituents that raise the potential of the p-benzoquinone/hydroquinone half reaction raise reaction rates. Electron transfer reactions between quinones and iron may be an important link in networks of electron transport taking place in suboxic and anoxic environments.

Technical Abstract: The oxidation of dihydroxyaromatics to benzoquinones by FeIII (hydr)oxides is important in respiratory electron shuttling by microorganisms and has been extensively studied. Prior publications have noted that the Gibbs Free Energy (DG) for the forward reaction is sensitive to dihydroxyaromatic structure, pH, and concentrations of reactants and products. Here, we address the back reaction, benzoquinone reduction by FeII. Rates markedly increase with increasing pH, in accord with increases in DG. Ring substituents that raise the potential of the p-benzoquinone and hydroquinone half reaction raise reaction rates:–OCH3\–CH3\–C6H5\–H\–Cl. p-Naphthoquinone, with a reduction potential lower than the five substituted p-benzoquinones just listed, yields the lowest reaction rates. The complexity of the reaction is reflected in lag periods and less-pronounced S-shaped time course curves. Benzoquinone reduction by FeII may be an important link in networks of electron transport taking place in suboxic and anoxic environments.