Sonolysis solvents

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Another example is the influence of ultrasonic sound treatment. In chlorinated or bromi-nated solvents it leads to extreme rate accelerations and higher selectivities (Table 6)84. This observation was explained by the formation of hydrogen halide from the sonolysis of the solvent molecules, followed by protonation of the dienophiles and ordinary acid catalysis. Nevertheless, although there are quite a few aspects of the Diels-Alder reaction which are not totally understood, the general mechanisms leading to selectivities and catalysis are clear. 

A synthetic application of the sonolysis of iron carbonyls is the preparation of useful ferrilactones. The alkenyl epoxides (2, R = H, Ph, 1-hexanyl) are smoothly converted to the corresponding ferrilactone complexes (3) on reaction with Fe2(CO)9 suspended in THF and sonicated at room temperature [53]. Such complexes undergo several synthetically useful transformations (Scheme 3.7) including oxidation with Ce(IV) as a route to P-lactone natural products or P-lactam antibiotics and reaction with CO to afford 5-lactones [54]. Somewhat surprisingly this reaction is efficient even in diethyl ether, a volatile solvent which delivers low cavitation energy. 

Palladium metallic clusters have been prepared at room temperature by sonochemical reduction of Pd(OAc)2 and a surfactant, myristyltrimethylammonium bromide, in THE or MeOH [160[. It is noteworthy that nanosized amorphous Pd is obtained in THE, but in a crystalline form in MeOH. In this solvent, and in higher homologous alcohols, sonolysis of tetrachloropalladate(II) leads to Pd nanoclusters in which carbon atoms, formed by complete decomposition of the solvent, can diffuse. This results in an interstitial solid having the formula PdQ (0 < x < 0.15) [161]. Noble metal nanoparticles of Au, Pd, and Ag are obtained by sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilise the naked col-... 

It is thought that both active species react with each other and produce H2 and H202. It is apparent that H2 is pioduced by dimerization of H radicals and that H202 is produced by dimerization of OH radicals. The recombination of H and OH radicals may also occur. However, there is no distinction between K20 formed by the radical reaction and the one from solvent and/or the reactant. As a result, watei reacted to H2 and H202 by sonolysis In order to accomplish overall water splitting, H202 must be decomposed tc 02 and II20, as shown in Eq. (12.6). 

Decomposition reactions are another reaction class often employed in nonaque-ous solvents. In these reactions, the starting materials are decomposed to create the final product. The preferred starting materials have ligands that are very good leaving groups, such as carbonates, carbonyls, and acetates. The decomposition is facilitated by several different techniques, such as heat in thermolysis, light in photolysis, and sound in sonolysis. The reaction is the same in almost every case ... 

Ultrasonic strong digestion is based on mechanical and, especially chemical effects. The chemical effects result from the reactivity of the chemical agents (oxidants or reduc-tants) promoted by the radicals generated by sonolysis of the solvents in the liquid phase (particularly in aqueous solutions). In fact, the radicals act as promoters of the chemical reactions involved in matrix decomposition. 

Another factor that inoreases the efficienoy of USAL is the presenoe of free radioals formed through cavitation. In fact, the oxidative energy of radioals created by sonolysis of the solvent dramatioally improves the effioienoy of leaching, at the expense of potential alteration in the stability of the target analytes. 

The fact that US influences the mechanism of chemical reactions via the action of highly reactive radicals such as OH- and H- formed during solvent sonolysis is well known (see Chapter 7). Solvents sensitive to thermolysis or sonolysis (e.g. dimethylformamide [158], dimethylsulphoxide [159]) decompose slowly in the presence of intense US. Thus, radical species formed by cavitation are highly reactive and may participate as activators or enhancers in the electrode process [160]. In fast, qt/asr-reversible or irreversible systems, however, the only effect of US is to enhance mass transport without any direct effect on the rate of simple electron transfer processes. 

Reactions in the interfacial region correspond to an indirect mechanism in which sonolysis of the solvent in the bubble or a volatile solute constitutes a first step. The sonolysis of amphiphilic compounds in water occurs with preferential hydroxylation and subsequent oxidations induced by the hydroxyl radical. 

Among the recently developed synthetic methods, many reactions necessitate a preliminary sonolysis. A direct mechanism can be involved if one of the reactants penetrates into the bubble. The relative volatility with respect to the solvent should then be an important factor if this mechanism is accepted. 3 The ease of the subsequent bond cleavage should depend on the bond energies, but systematic studies have not yet been undertaken, and the bond cleaved is not necessarily the least stable one (p. 71). Examples are known in which substrates much less volatile than the solvent undergo homolysis. In these cases, an indirect process can exist, with sonolysis of the solvent acting as a relay. 

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