The Sonochemical Reactivity

From the empirical systematization of sonochemistry, it is sufficient to remember that, when the possibility exists, radical pathways are privileged at the expense of polar pathways, and sonochemical switching can result in a number of cases. In heterogeneous systems, reactions which follow ionic mechanisms are still sensitive to the mechanical effects of sonication. This false sonochemistry can, in principle, also be observed when efficient mixing techniques are applied, but the so-called simple mechanical effects are strongly dependent on geometrical factors and prove to be much more complicated than expected. 

9 Luche, J.L. Einhorn, C. Einhom, J. Sinisterra-Gago, J.V. Tetrahedron Lett. 1990, 31, 4125-4128. 

From a purely geometrical viewpoint, the state of a surface can be described by a mathematical parameter, the fractal dimension D.li For the sake of simplicity, let us consider a one-dimensional space (a straight line) with a size L (Fig. 1). 

It is established that for a higher D value of a surface, the rates of heterogeneous reactions increase because of the higher number of active sites available. In diffusion-controlled reactions, the initial rate is directly dependent on D and the steady-state rate on a parameter derived from it. It was fotmd recently for silica, commonly used in solid-supported reactions, that the value of D, originally 1.8-2.3, increases up to 2.8 by sonication.12 If this observation can be generalized, the higher reactivity of sonicated solids should be explained by the transformation of a reaction in a two-dimensional space to a process occurring in a volume. 

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After an irradiation time of 300 min, (500 kHz, 30 W, 20 °C) the hydroxylated intermediates disappear and the chlorine atoms are completely mineralised to chloride ions. The concentrations of the final products (CO and CO2) rise slowly and after 400 min they represent 21 % when starting with cMorophenol and 18 % when starting with phenol. These results are in agreement with previous studies in which it was demonstrated that the sonochemical reactivity of an organic compound is related to its vapor pressure and hydrophobicity. [26,27]. These studies have been extended to the destruction of a number of chloroaromatics [25] (Tab. 4.3). 

Most likely, the sonochemical reactivity of solid reagents consists of the addition of several convergent factors not yet fully listed or explored. An in-depth interpretation of the reactions then seems very difficult given the present state of knowledge, leading to a paradoxical situation for the synthetic chemist, sonochemical reactions are simple and easy to use most of the time, but their fundamental understanding remains very fragmented and difficult. 

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). 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

As stated earlier that a paradoxical reactivity could often be encountered in sono-chemical studies. The decomposition of [Fe(SCN)6]3 complex had been witnessed, if the sonication of the solution was continued long. Contrary to the sonochemically assisted formation of [Fe(SCN>6]3 in the preceding experiment, used for the detection of the conversion of Fe2+ to Fe3+, here is another opposite result of the sonicated... 

For the sonochemical mineralization of reactive dye Cl Reactive Black 5 with 20, 279 and 817 kHz irradiation, the discoloration and radical formation both are directly dependent upon ultrasonic frequency, acoustic power and irradiation time and indirectly on the number of free radicals thus generated, as their suppression decreased the discoloration rate due to radical scavenging effect. Although ultrasound alone is capable of decolorizing Reactive Black 5 but inefficient in mineralization as only 50% degradation was observed after 6 h of ultrasonic irradiation [121]. The sonochemical... 

Vajnhandl S, Marechal AML (2007) Case study of the sonochemical decolouration of textile azo dye Reactive Black 5. J Hazard Mater 141 329-335... 

The early studies of the chemical effects of ultrasound have been thoroughly reviewed (5-7). Only the most important and most recent research is mentioned here as needed to provide a perspective on sonochemical reactivity patterns. The sonolysis of water is the earliest and most exhaustively studied (3,93,96,98-105). The first observations on the experimental parameters which influence sonochemistry come from these reports. The primary products are H202 and H2, and various data supported their formation from the intermediacy of hydroxyl radicals and hydrogen radicals ... 

There are two types of reaction involving metals (1) in which the metal is a reagent and is consumed in the process and (2) in which the metal functions as a catalyst. While it is certainly true that any cleansing of metallic surfaces will enhance their chemical reactivity, in many cases it would seem that this effect alone is not sufficient to explain the extent of the sonochemically enhanced reactivity. In such cases it is thought that sonication serves to sweep reactive intermediates, or products, clear of the metal surface and thus present renewed clean surfaces for reaction. Other ideas include the possibility of enhanced single electron transfer (SET) reactions at the surface. 

A very reactive form of a finely divided metal is a so-called Rieke powder [79]. These materials are produced as fine powders by chemical precipitation during the reduction of various metal halides ivith potassium metal in refluxing tetrahydrofuran. Obviously this is a potentially hazardous laboratory procedure and ultrasound has provided an alternative method of preparation of these extremely valuable reagents [80]. The sonochemical technique involves the reduction of metal halides with lithium in TH F at room temperature in a cleaning bath and gives rise to metal powders that have reactivities comparable to those of Rieke powders. Thus powders of Zn, Mg, Cr, Cu, Ni, Pd, Co and Pb were obtained in less than 40 min by this ultrasonic method compared with reaction times of 8 h using the experimentally more difScult Rieke method (Tab. 3.1). 

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