Effect of Acoustic Field

The ultrasonic irradiation of a solution induces acoustic cavitation, a transient process that promotes chemical activity. Acoustic cavitation is generated by the growth of preexisting nuclei during the alternating expansion and compression cycles of ultrasonic waves. For example, in aqueous liquid, temperatures as high as 4300 K and pressures over 1000 atm are estimated to exist within 

It seems reasonable to note that the micro-jet stream generated by the ultrasonic cavitation promotes mass transport. Such an effect was discussed for proton transport in aqueous solutions (Atobe et al. 1999). Understandably, a proton moves in the solution as a hydrated particle. Nevertheless, we should pay attention on the similarity between proton and electron, in the sense that both are essentially quantum particles. A solvated electron, therefore, can be considered as a species that is similar to a hydrated proton. Hence, the micro-jet stream can promote electron transfer. 

Sonolysis provokes electron-transfer reactions in which hindered phenols act as donors (Aleksandrov et al. 1995). Steric hindrance does not allow a donor and an acceptor to come closer and thus, prevents or significantly hampers overlapping of the corresponding orbitals. Acoustic field effect helps in overcoming this hindrance. 

Russell and Danen (1968) studied the sonication effect on the reaction between the lithium salt of 2-nitropropane and 4-nitrobenzyl bromide. A dual mechanism, ionic and ion-radical, characterizes this reaction. The ionic mechanism leads to the 0-alkylation product and, eventually, to 4-nitrobenzaldehyde  

The ion-radical route leads to the C-alkylated product, namely, 2-(4-nitrobenzyl)-2-nitro propane  

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