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The use of ultrasound in chemistry (sonochemistry) offers the chemist a technique of chemical activation which has wide applications and uses equipment which is relatively economical. When laboratory research in sonochemistry began it seemed to be mostly a technique of initiating intransigent reactions especially those which rest on the activation of metallic or solid reagents. Its expansion in the past 25 years however has shown that it has far broader applicability than this and also that it presents an important scientific challenge to understanding its underlying physical phenomenon—acoustic cavitation. The ever increasing number of applications of sonochemistry in synthesis has made the subject striking to many scientists and interest has spread outside academic laboratories into industry and chemical engineering. There are variety of applications for the uses of ultrasound in chemistry which include synthesis, environmental safety (the obliteration of both biological and chemical contaminants) and process engineering (upgraded extraction, crystallization, electroplating and new approaches in polymer technology). True sonochemistry is the advancement of single electron transfers induced by ultrasonic waves, while mechanical effects of ultrasonic waves yield false sonochemistry. Mechanochemistry and sonochemistry are closely related during cavitation collapse, because both phenomena occur under indistinguishable local circumstances. The primary result of sonochemistry in their view is cavitation, which delivers the mechanical energy for all following chemical reactions, including bond scission induced by viscous frictional forces. Mechanochemistry is concerned with the physical and chemical variations in the properties of materials due to the mechanical forces applied to organic or inorganic solids, powders and liquids. Organic reactions under solvent-free conditions are beneficial because of improved selectivity and efficiency; comfort of manipulation, and more prominently, because toxic and often volatile solvents are avoided. Solvent-free methods involve mechanochemical mixing (grinding), microwave (MW)/Ultrasound irradiation of neat reactants (undiluted), or catalysis by the surfaces of cheap and recyclable mineral supports, such as alumina, silica, clay, or ‘doped’ surfaces. These approaches are applicable to a wide range of cleavage, condensation, cyclization, oxidation, and reduction reactions, including convergent one-pot synthesis of heterocyclic compounds from in situ produced reactive intermediates. The approach is adaptable to multi-component reactions for high-speed parallel synthesis of a library of biologically active molecules. |
