Can sonochemistry take place in the absence of cavitation? - A complementary view of how ultrasound can interact with materials

A hypothesis is advanced for a mechanism by which ultrasound could help to activate chemical reactions, even in the absence of cavitation. It is suggested that the compression phase of an acoustic wave could produce transient solid-like structures within the solution. These structures would result in an "ordering effect" of the molecules in which electrical charges could develop. Such electrical charges could facilitate electron movement from one molecule to another triggering therefore chemical reactions. Such reactions could occur even in the absence of cavitation especially if the solvent or reagents employed show piezoelectric/electrostriction properties. Similar transient ordering effects could be induced by the shockwave accompanying bubble collapse and these would help to explain some of the anomalous effects observed in sonochemistry under cavitation.

The ultrasonically induced reaction of benzoyl chloride with nitrobenzene: an unexpected sonochemical effect and a possible mechanism

A mixture of benzoyl chloride and nitrobenzene is not known to be chemically reactive, indeed the mixture is chemically inert when subjected individually to either ultrasonic irradiation or heating. However, if this system is initially subjected to ultrasound and then heated for several hours at 200 degrees C, a reaction does occur and the products are benzoic anhydride, hydrochloric acid, nitrous and nitric acid together with some minor products. To the best of our knowledge, this is the first example of a reaction where the effect of ultrasound does not appear to be the consequence of the direct action of acoustic cavitation bubbles. A possible explanation of this behaviour is advanced which involves an electron transfer reaction in which nitrobenzene is first activated by ultrasound and then acts as oxidant in the thermal stage of reaction.

Sonochemical and thermal redox reactions of triphenylmethane and triphenylmethyl carbinol in nitrobenzene

The reaction of triphenylmethane and triphenylcarbinol with nitrobenzene under thermal or ultrasonic activation was studied. It was shown beyond doubt that the thermal reaction of the aforementioned systems at 210 degrees C occurs through electron transfer. The sonochemical reactions occur at 40 degrees C, although slowly, while heating at the same temperature leaves the system unchanged. Electron transfers are also involved but an unexpected reductive process was evident.