Organic Sonochemistry: Challenges and Perspectives for the 21st Century

The sound is a mechanical vibration which propagates by elasticity through matter whatever its physical state. On liquid state, an interesting and unique physical phenomenon was identified at the end of nineteenth century and designated as cavitation, which is the birth, growth and collapse of tiny gas bubbles. The intensity of bubbles occurrence and of collapse violence is very dependent on the sound frequency. The most energetic cavitation activity occurs when using ultrasound frequencies, i.e. above the upper limit of human hearing (18 kHz). The incident irradiative frequency is therefore of crucial importance leading to effects on chemical systems of physical and/or chemical nature. Several other operational parameters do also greatly influence the cavitation process and are here described as well as most frequent types of ultrasonic devices working either on direct or indirect mode. ‘Hotspot’ theory and generally admitted reacting zones establishing rules of sonochemistry are also examined. Finally, some guidelines for good experimental use of ultrasonic devices are tentatively established by authors based on their own experience.