Sonochemical reactions homogeneous

In 1981, the first report on the sonochemistry of discrete organometallic complexes demonstrated the effect of ultrasound on iron carbonyls in alkane solutions (174). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(CO)5 provides an excellent example of the unique chemistry which homogeneous cavitation can... 

The synthesis of colloidal paramagnetic nanoparticles is a complex process. The requirements of the produced magnetic nanoparticles are homogeneous composition, narrow size distribution, and repeatable and facile chemical conditions. Magnetic nanoparticles can be synthesized by numerous chemical methods such as microemulsions [113], sol-gel synthesis [114], sonochemical reactions [115], hydrothermal reactions [116], hydrolysis and thermolysis of precursors [117], flow injection syntheses [118], electrospray syntheses [119], and the most commonly used technique chemical coprecipitation of iron salts [120,121],... 

Rule 1 applies to homogeneous processes and states that those reactions which are sensitive to the sonochemical effect are those which proceed via radical or radical-ion intermediates. This statement means that sonication is able to effect reactions proceeding through radicals and that ionic reactions are not likely to be modified by such irradiation. 

There have been examples of sonochemical switching in homogeneous reactions. The decomposition of lead tetraacetate in acetic acid in the presence of styrene at 50 °C generates a small quantity of diacetate via an ionic mechanism. Under otherwise identical conditions sonication of the mixture gives 1-phenylpropyl acetate predominantly through an intermediate methyl radical which adds to the double bond (Scheme 3.8) [55,56]. These results are in accord with the proposition that radical processes are favoured by sonication. 

Ultrasound is known for its capacity to promote heterogeneous reactions (Ley and Low, 1989) mainly through greatly increased mass transport, interfacial cleaning and thermal effects. In addition, homogeneous chemical reactions have been reported to be modified (Suslick et ai, 1983 Luche, 1990 Colarusso and Serpone, 1996) for example the sonochemical generation of radical species in aqueous media is important in environmental detoxification (Kotronarou et al., 1991 Serpone et al., 1994). 

In homogeneous liquid systems, sonochemical effects generally occur either inside the collapsing bubble, — where extreme conditions are produced — at the interface between the cavity and the bulk liquid —where the conditions are far less extreme — or in the bulk liquid immediately surrounding the bubble — where mechanical effects prevail. The inverse relationship proven between ultrasonically induced acceleration rate and the temperature in hydrolysis reactions under specific conditions has been ascribed to an increase in frequency of collisions between molecules caused by the rise in cavitation pressure gradient and temperature [92-94], and to a decrease in solvent vapour pressure with a fall in temperature in the system. This relationship entails a multivariate optimization of the target system, with special emphasis on the solvent when a mixed one is used [95-97]. Such a commonplace hydrolysis reaction as that of polysaccharides for the subsequent determination of their sugar composition, whether both catalysed or uncatalysed, has never been implemented under US assistance despite its wide industrial use [98]. 

Balazsik, K., Torok, B., Felfoldi, K., Bartok, M. (1999) Homogeneous and Heterogeneous As5mimetric Reactions. Part 11. Sonochemical Enantioselective Hydrogenation of Trifluoromethyl Ketones over Platinum Catalysts, Ultrasonics Sonochem. 5,149-155. 

The behavior in the same reaction of several phosphorus compounds with a labile P-H bond was investigated under a variety of conditions (homogeneous, heterogeneous, heating, photochemical, sonochemical, "dry medium").Qf these, sonication appears to be especially efficient and selective, but the system is made complex by the important reactivity differences between the various substrates used (phosphonate, phosphine, phosphine oxide, or sulfide) and no general rule can be formulated. 

These reactions were used in the asymmetric syntheses of several members of this family of compounds. All the experiments were conducted in a bath sonicator, i.e., under irradiation conditions which are not necessarily optimal. Most of the time, they were run under heterogeneous conditions, since the quinones are insoluble in the diene taken in excess. The presence of a solvent, toluene for example, is undesirable, and most of the time it makes the sonochemical effect disappear. In a few cases, the presence of methanol in amounts just sufficient to make the mixture homogeneous is compatible with good results, but larger quantities must be avoided. 

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