Substituted Aromatic Aldehyde Decomposition under Hydrothermal Conditions

The reaction kinetics and product formation from a range of hydroxy (−OH) and methoxy (−OCH3)-substituted aromatic aldehydes were determined under hydrothermal conditions between 280 and 360 °C. The presence of methoxy functionalities in the aromatic aldehyde led to hydrolytic demethylation reactions, whereas hydroxy moieties – including those generated by hydrolytic demethylation – enhanced cleavage of the formyl group. The effect was most prominent with −OH substitution at the ortho-positions. A hypothetical reaction pathway in which H2O plays a role in the aldehyde cleavage from the aromatic ring is proposed. Aldehyde rearrangement was identified experimentally at high temperature and long reaction times, consistent with a high energy barrier predicted by density functional theory. Importantly, the decomposition of aldehydes following a Cannizzaro pathway, involving the disproportionation of an aldehyde to an equimolar mixture of an alcohol and a carboxylic acid, was not observed.