Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase

The hydrogenation of benzaldehyde to benzyl alcohol on carbon‐supported metals in water, enabled by an external potential, is markedly promoted by polarization of the functional groups. The presence of polar co‐adsorbates, such as substituted phenols, enhances the hydrogenation rate of the aldehyde by two effects, that is, polarizing the carbonyl group and increasing the probability of forming a transition state for H addition. These two effects enable a hydrogenation route, in which phenol acts as a conduit for proton addition, with a higher rate than the direct proton transfer from hydronium ions. The fast hydrogenation enabled by the presence of phenol and applied potential overcompensates for the decrease in coverage of benzaldehyde caused by competitive adsorption. A higher acid strength of the co‐adsorbate increases the intensity of interactions and the rates of selective carbonyl reduction. Electrocatalytic hydrogenation of benzaldehyde to benzyl alcohol on carbon‐supported metals proceeds through proton‐coupled electron transfer. The presence of polar co‐adsorbates such as phenols enhances the hydrogenation rate of the aldehyde by the polarization of the carbonyl group because of the formation of a hydrogen bond. Increasing either the concentration or acidity of the co‐adsorbate increases the rates of selective carbonyl reduction.