(700g) The Mechanism of Ultrasound-Driven OH-Mediated Aqueous Benzyl Alcohol Oxidation with Analogies to Atmospheric Chemistry.

Ultrasonic irradiation of aqueous solutions forms OH radicals by driving the inertial cavitation of suspended gas cavities with Ar, O₂, and H₂O vapors. These OH radical are transported to the surrounding solution where they drive reactions with organic substrates, thus presenting a promising strategy for radical-mediated chemical synthesis using only water, electricity, and a piezoelectric material. Products from gas-phase reactions of OH radicals with benzyl alcohol in the presence of O₂ (e.g. glyoxal, butenedial, hydroxymethylphenol isomers) were shown to form during ultrasonic irradiation of aqueous benzyl alcohol. A network of reaction steps based on established radical reaction steps (Fig. 1a and 1c) is proposed to account for distributions of products formed from OH-benzyl alcohol reactions. The reaction energies and activation barriers for these reaction steps were calculated using density-functional theory (DFT) methods and standard statistical mechanism formalisms. Kinetic parameters based on these calculations successfully captured directional trends for dependences of product selectivity on O₂ pressure (Fig. 1b). Increased O₂ activity drives O₂ addition to cyclic intermediates forming bicyclic compounds that ultimately fragment to glyoxal and 3-hydroxy-2-oxo-propanal; these dicarbonyl products further oxidize to oxalic acid (Fig. 1c). The computational and experimental results reveal that sonication drives reactions of aqueous aromatic compounds via mechanisms analogous to those established in atmospheric chemistry. In doing so, we show that atmospheric chemistry can be harnessed in ultrasound reactors thus inspiring future applications of sono-chemical synthesis.

Figure 1: (a) Steps for benzyl alcohol-OH reactions with O₂. (b) Selectivity to phenol (x), benzaldehyde (★), phenol (●), and hydroxymethylphenol isomers (+) as a function of O₂ pressure during sonication of aqueous benzyl alcohol (5 mM; 20 kHz; 310 K) with bubbling Ar/O₂. Trends from kinetic model with DFT-derived rate constants and one adjustable parameter. (c) Scheme for benzyl alcohol fragmentation and oxalic acid formation.