Text: Sonochemistry began (officially) in 1927 when Richards and Loomis described two types of chemical reactions brought about by ultrasonic waves: (1) the acceleration of conventional reactions, such as the hydrolysis of dimethyl sulphate; and (2) redox processes in aqueous solution, such as the oxidation of sulphite. Reactions of type 1 are of interest in preparative chemistry. Ultrasound has an accelerating effect in many organic syntheses, sometimes less by-products are formed and the yield of the main product is increased, and reactions may be carried out at a lower temperature or lower excess pressure. Reactions of type 2 may play a role in the irradiation of biological important molecules in aqueous media. These reactions often resemble the ones that are initiated by ionizing radiations. The third type of sonochemical reaction is the degradation of macromolecules in solution. Brohult first described the degradation of biological polymer in 1937. Schmid carried out the first experiments with synthetic polymers. He found that the rate of depolymerization was proportional to the difference, P-Pf, between the actual degree of polymerization, P, and the final degree, Pf, which is reached after long-term irradiation. The molecular weight distribution of a polymer is greatly narrowed down during depolymerization. When a macromolecule is broken in the main chain, free macromolecules are often formed which can be used to initiate the polymerization of a vinyl monomer yielding a block copolymer. The macroradicals can also be detected by their reactions with scavengers, such as diphenylpicryl hydracyl and iodine. Their mutual deactivation reactions have also been investigated. The fourth type of sonochemical reaction is the decomposition of organic liquids. In 1953 it was found that methanol solutions of diphenylpicryl hydracyl were decolored, indicating the formation of free radicals. In 1958, the decomposition of chloroform, in which HCl was produced, was reported. Other products that indicated that free radicals and carbenes are present as intermediates were found much later. The next examples were described by Weissler, Pecht, and Anabar, who found that elemental chlorine was formed in the sonolysis of carbon tetrachloride and that various gases, such as N2, CH4 and H2, were produced from acetonitrile. Examples of the studies in more recent times are the sonolysis of alkanes, metal carbonyls, nitrobenzene, the sonolytic polymerization of vinyl monomers and the sonolytic decomposition of dipenylpicryl hydracyl in various solvents. Finally, Webster gives a number of chemical processes such as oxidation, hydrolysis and depolymerization that are initiated or accelerated by ultrasound. Recent investigations of sonochemistry usually use commercial devices producing ultrasound in the 20-40kHz range. The emitting titanium horn is dipped into the liquid to be irradiated. A field of standing waves is not generally formed and the chemical effects occur mainly close to the horn. Intensities of the order of 10-100 W cm-2 are often used.

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