Mechanistic Insights into the Aerobic Oxidation of Aldehydes: Evidence of Multiple Reaction Pathways during the Liquid Phase Oxidation of 2‑Ethylhexanal

The liquid-phase aldehyde oxidation by molecular oxygen (autoxidation) has been known for about 2 centuries and is a critical organic transformation in both industrial applications and academic research. However, the general reaction pathway proposed for the aerobic oxidation of aldehydes into the corresponding carboxylic acid exhibits some inconstancies, in particular, for β-substituted aliphatic aldehydes. Thus, the liquid-phase aerobic oxidation of 2-ethylhexanal was further studied in acetonitrile at 20 °C with O₂ at atmospheric pressure. By precisely monitoring the primary intermediate (peracid), product (carboxylic acid), and byproducts as a function of time and catalysts used, we demonstrated the pivotal role of the acylperoxy radical. The direct formation of peracid and carboxylic acid from the latter was highlighted by analyzing the composition of the reaction mixture at low conversion. Peracid could be converted into carboxylic acid by metal catalysts or through reaction workup. Consequently, the commonly accepted pathway of aerobic oxidation of aldehyde via a Criegee intermediate can be overlooked under these conditions.