(708e) Selective Oxidation of Aldehyde Functions of Aqueous Organic Molecules Under Nominally Ambient Temperature Using Ultrasound Irradiation

Ultrasound irradiation forms •OH in aqueous solution by driving the homolysis of H₂O within cavitating gas bubbles. These •OH species are powerful oxidants capable of oxidizing aldehyde functions into carboxylic acids. Ultrasound therefore holds potential as a driver for oxidation chemistry via •OH using water as a solvent and electricity as a power source.

Here, the oxidation of glyoxal, a model aldehyde, was performed under ultrasound irradiation at frequencies of 20 kHz and 580 kHz. Kinetic measurements showed a monotonic decrease in glyoxal concentration with a concomitant increase in formic, oxalic, and glyoxylic acid products with increasing ultrasound exposure. A reaction network initiated by •OH was proposed to explain the formation of these oxidation products. A kinetic model of these reactions was constructed with rate constants parameterized by experimentally measured values (where available) and supplemented with values from density functional theory calculations. The model accurately predicted yields to predominant products from experiments with small mean percentage errors of 9.8% and 13% at 20 kHz and 580 kHz, respectively.

A kinetic analysis of steps included in the model uncovered strategies to favor selective formation of C₂ acid products. Indeed, lowering the pH avoided the deprotonation of acid products into their carboxylate counterparts, thereby inhibiting the total oxidation to CO₂. Increasing the rate that •OH formed by adjusting the ultrasound frequency, moreover, increased rates of glyoxal initiation by •OH while minimizing the rates of secondary C-C cleavage by oxidation products (specifically H₂O₂ and •O₂^–). These mechanistic insights thus showcase the promise of sonochemistry for the selective and sustainable oxidation of aqueous organic species.

References:

¹Fischer et al., ChemRxiv (2024) DOI: 10.26434/chemrxiv-2024-01sv9

Figure 1: Maximum yields (normalized the initial glyoxal concentration) to C₂ acid products, and corresponding concentrations of C₁ products and unreacted glyoxal at different pH, calculated the kinetic model.¹