Abstract
Ultrasound can be used in chemistry to increase both reaction rates and yields of products. Most effects of ultrasound on chemical reactions are due to cavitation: the formation and collapse of small bubbles in the solvent. In this review, we first outline the physical background of cavitation, and discuss its dependence on factors such as sound intensity and frequency, solvent and temperature. The impact of ultrasound on chemical reactions is considered for homogeneous reactions and for heterogeneous liquid‐solid systems. The first area is mainly illustrated by a discussion of the effect of ultrasound on polymerization and depolymerization reactions, the second by selected examples in organic synthesis. The tendency of ultrasound to change reaction mechanisms in favour of homolytic (instead of heterolytic) pathways, is also briefly discussed. The specific preference for a particular pathway under sonochemical conditions, different from that under mechanical stirring has been termed “sonochemical switching”. Ultrasonic equipment for lab‐scale experiments are compared, and some practical “tricks and traps” are given.