Quantifying sonochemistry: Casting some light on a ‘black art’

Despite the ever increasing usage of power ultrasound to enhance reactivity in synthetic chemistry, many experimentalists experience difficulty in reproducing the work of other groups. Although such problems are most commonly encountered with chemical reactions which have been performed in ultrasonic baths, even reactions involving probe systems have sometimes proved difficult to reproduce. The origin of many of tehse problems can be traced to a failure of the original report to specify the exact sonication conditions, e.g. the frequency of ultrasonic irradiation, the precise power entering the reaction system, the geometry of the reaction vessel, the presence of a bubbled gas or even the temperature of the reaction∗. In this article we report the results of some model reactions performed using the newly developed Undatim Sonoreactor probe system. This particular instrument is equipped with an automatic transducer resonance frequency search device, enabling the power input to a system to be maintained accurately throughout a reaction, thereby offering considerable advantages over the normal commercially available equipment in terms of both the monitoring and control of irradiation parameters. Using this facility we are able to report how ultrasonic energy input to a chemical reaction is affected by the following important reaction parameters: ultrasonic power used, the presence of bubbled gas, temperature, solvent composition and reaction volume. In addition, a comparison is made of the ultrasonic power available from three different laboratory sonicators operating at a nominal frequency of 20 kHz.