Base-Assisted Nitrate Mediation as the Mechanism of Electrochemical Benzyl Alcohol Oxidation

Nitrate anion (NO3-) oxidation to nitrate radical (NO3 center dot) is chemically irreversible in acetonitrile (MeCN) solvent due to solvent-based hydrogen-atom transfer (HAT). Introducing benzyl alcohol (PhCH2OH) leads to competition with MeCN for electrochemically generated NO3 center dot and affords benzaldehyde (PhCHO) product with similar to 80% faradaic efficiency (FE) in 250 mM PhCH2OH. Stoichiometric HNO3 forms during HAT reactions (observed by UV-vis spectroscopy) and exists as an electrochemically inert and weak electrolyte; this off-cycle form of nitrate can be reintroduced to the catalytic cycle upon deprotonation by 2,6-lutidine while maintaining the base-free FE. Oxygen reduction complements nitrate oxidation during catalysis and reduced oxygen species (ROS) generated during protoncoupled oxygen reduction are identified through rotating ring-disk electrochemistry; proton-coupled oxygen reduction indicates ROS are capable of rendering NO3- catalytic when collocal. Directly observing ROS as the stoichiometric base generated during nitrate anion oxidation resolves differences in photocatalytic vs photoelectrochemical reactivity of NO3- in base-free conditions and points toward HAT as the general mode of reactivity for nitrate radical in acetonitrile solutions.

Automated summary

The oxidation of nitrate anion to nitrate radical in acetonitrile solvent is chemically irreversible due to solvent-based hydrogen-atom transfer, leading to the formation of stoichiometric HNO3 during hydrogen-atom transfer reactions. Introduction of benzyl alcohol competes with acetonitrile for electrochemically generated nitrate radical and results in the production of benzaldehyde product. Additionally, oxygen reduction complements nitrate oxidation during catalysis, with reduced oxygen species identified as being capable of rendering nitrate catalytic when collocal.