Oxidation of Small Phenolic Compounds by Mn(IV)

Plant secondary metabolites, including phenolics, represent a large quantity of organic material that enters soil and contributes to the formation of soil organic matter (SOM). The process of phenolics forming SOM remains poorly understood. One possible mechanism is oxidation of the phenolic compound catalyzed by redox active metals such as manganese (Mn) and iron (Fe) in soils. In this work, we report how three phenolic compounds react with a redox active environmentally relevant metal, Mn(IV). The reactions were monitored via nuclear magnetic resonance (NMR), high-performance liquid chromatography (HPLC), and direct CO₂ measurements. Using these techniques, we demonstrate that gallic acid reacts with Mn(IV) less efficiently than pyrogallol. The products of the gallic acid:Mn(IV) reaction are more oxidized than the products of the pyrogallol reaction. Gallic acid produces small molecules and releases CO₂, while pyrogallol produces a less oxidized product, likely a quinone, and releases less CO₂. Benzoic acid did not react with Mn(IV). This work provides a framework for how different classes of plant secondary metabolites may be degraded abiotically by redox active metals in soil.