Unexpectedly ambivalent O₂ role in the autocatalytic photooxidation of 2-methoxybenzyl alcohol in water

Abstract An unusual autocatalytic photooxidation of 2-methoxybenzyl alcohol has been observed under UV irradiation in aqueous medium. The homogeneous oxidation is catalyzed by the corresponding aldehyde that is also the main oxidation product. The trend of alcohol disappearance rate matches the typical shape of an autocatalytic process, where a crucial and ambivalent role is played by the presence of molecular oxygen. Low oxygen concentrations give rise to a zero-order reaction since the beginning of irradiation, while higher amounts of oxygen reduce the alcohol oxidation rate until the aldehyde reaches a concentration high enough to speed up the alcohol's conversion. Experiments performed by varying alcohol, aldehyde and oxygen concentrations in both aqueous and organic media suggest that the alcohol oxidation is initially favored by the presence of oxygen. Once the formed aldehyde competes for photon absorption, the oxidation is driven by aldehydes alpha cleavage leading to reactive oxygenated species in aqueous medium. An excess of oxygen quenches the latter processes thus inhibiting/slowing down the reaction.