Ultrasound: A boon in the synthesis of organic compounds

Abstract

The use of ultrasound for promoting chemical reactions is described. Ultrasound is the name given to sound waves having frequencies higher than those to which human ears can respond, i.e. greater than 16kHz and with wavelength between 7.0 and 0.015 cm. It is transmitted through any substance – solid, liquid or gas, which possesses elastic properties. The first commercial application of ultrasonics appeared in 1917 with Langevin’s echo-sound technique for the estimation of depths of water resulting in the system known as SONAR (sound navigation and ranging). Some of the broader applications of ultrasound in various fields are for homogenisation and cell disruption in biology and biochemistry; to assist in drilling, grinding, cutting, welding of hard materials and testing of materials in engineering; for cleaning and drilling teeth in dentistry; for dispersal of pigments and solids in paints, inks, resins; for acoustic filtration and ultrasound drying in industry; for ultrasound imaging in obstetrics and treatment of muscle strains in medicine (frequency range 1 – 10 MHz); for welding of thermoplastics, polymer degradation, curing of resins and initiation of polymerisation in plastics and polymers ; for breakdown of aromatic pollutants and cell disruption of bacteria in waste treatment, to name a few. Ultrasound has also been employed for specific chemical applications. The chemical effects of ultrasound were first reported by A L Loomis in 1927. The hydrolysis of dimethyl sulphate and the iodine clock reaction were found to be accelerated by ultrasound. Initially, the use of ultrasound was restricted. However, it has increased manifold in a variety of chemical reactions, resulting in a sub-discipline called ‘Sonochemistry’.