Sonochemical rules

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. 

Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed false sonochemistry although many industrialists would argue that the term false may not be correct because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus sonochemical . In fact the true test for false sonochemistry is that similar results should, in principle, be obtained using an efficient mixing system in place of sonication. Such a comparison is not always possible. 

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. 

The addition of anthracene to maleic anhydride (Fig. 11) was reported to be accelerated by sonication. From a mechanistic study in the presence of electron carriers, an electron transfer process was ruled out. These results could not be reproduced, and no difference between the sonochemical and thermal rates and yields was observed (adduct formation in 30% after 1 h, 50% after 3 h, with or without sonication). In the presence of monoelectronic oxidizers such as ferric chloride,or tris(4-bromophenyl)aminyl hexachloroantimonate (TBPA),28a,c change was noted in these figures, although the radical cation of anthracene was formed.43 This radical cation is not involved in the reaction pathway. 

In many cases, sonochemical methods avoid a distinct preparation step of the organometallic, and one-step Barbier procedures find an improved efficiency. A complete analysis of many reactions is not easy, since the mechanical and chemical effects of sonication are superposed. In many cases, however, some confirmation of the sonochemistry rules can be found, and the reactions correspond to the third type. The discussions given in the descriptive part below are not limited to the sonochemical results, but relate, whenever possible, to the conventional parent reactions, to permit an easy comparison of the respective merits of each approach. 

High-power, low-frequency waves are often associated with better mechanical treatment and less importantly with chemical effects. This rule, however, suffers exceptions, and sonochemical switchings can be observed, even in solutions, under low-frequency irradiation. A heterogeneous reaction with a metal was also reported to be improved by the use of a high-frequency irradiation. ... 

The intermediacy of Fe(CO)4 was ruled out on the grounds that it would be trapped by excess Fe(CO)5 as soon as it was formed, producing Fe2(CO)9. Diiron nonacarbonyl was not observed as a product of this reaction and despite his observation that sonochemical cleavage of this species occurred as fast as that of Fe3(CO)i2 production, Suslick emphatically concluded that... 

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