Sonochemical theory

It is observed in Chapters 2, 4, and 5 that the link between sonochemical theory and experiment is still tenuous. Many reactions remain difficult to predict when the a priori (theory based) approach is followed. An a posteriori (experiment based) method should then be used as an alternative route. To make this empirical approach fruitful, the choice of model reactions is crucial. 

Theory Cavitational properties of ultrasound accelerate those organic reactions which involving free radical mechanism, hydrolysis, protonation, etc. However the sonochemical effects are negative for those reactions in which one of the reactants is volatile. 

However, this commonly accepted theory is incomplete and applies with much difficulty to systems involving nonvolatile substances. The most relevant example is metals. For a heterogeneous system, only the mechanical effects of sonic waves govern the sonochemical processes. Such an effect as agitation, or cleaning of a solid surface, has a mechanical nature. Thus, ultrasound transforms potassium into its dispersed form. This transformation accelerates electron transfer from the metal to the organic acceptor see Chapter 2. Of course, ultrasonic waves interact with the metal by their cavitational effects. 

Margulis MA, Margulis IM. Theory of local electrification of cavitation bubbles new approaches. Ultrason Sonochem 1999 6 15-20. 

The supercritical-water theory describes the elfects of high temperatures and pressures in aqueous systems when conditions are reached under which supercritical water is likely. Supercritical water is known to have a strong solvent action towards organic compounds and extreme chemical activity. Sonochemical effects are possible inside the supercritical water layer surrounding a transient bubble. At the present time, no direct evidence for the generation of supercritical water in ultrasound fields has been found experimentally. 

Russian workers questioned the existence of true cavitation and postulated a somewhat different sonochemical elfect called charge theory . Oscillating bubbles undergo very rapid size changes. Friction forces at the gas/liquid bubble boundary can create charged species that can lead to secondary chemical reactions in the bulk liquid. 

It is surprising to see the apparent parsimonious interest of sonochemists in the Diels-Alder addition. This reaction with a large negative activation volume is frequently carried out in heated sealed tubes. Then, from the sonochemist s viewpoint, the high pressures and temperatures produced by the cavitational collapses should activate the reaction, provided the reactants are able to penetrate into the bubble. Non-volatile reagents should not be activated since the pressures and temperatures decrease sharply outside the bubble. Thus, the hot-spot theory predicts that the sonochemical Diels-Alder reactivity should more or less be related to the volatility of the partners. 

If the electrical theories are considered, the high electric fields should ionize one of the reagents, with results comparable to those of radical ion reactions. In this case, the sensitivity of a given system should correspond to the ability of one of the reactants to become ionized. Then, in both situations, whether thermal or electrical effects predominate, the Diels-Alder reaction should respond to sonochemical activation. 

Another important aspect is the question of the reaction mechanisms. Since the sonochemical activation of Diels-Alder systems seems to have no direct link with the volatility of the substrates, the reactions should not take place in the "microreactor" bubble, and a purely thermal interpretation based on the hot-spot theory then seems inadequate. Alternative explanations could be based on a redox reaction between the two partners, or an ionization of one reactant. The existence of intermediate charge transfer complexes in Diels-Alder chemistry has been postulated for a long time, but most authors consider that no complete electron exchange is actually involved. l With the strong electron-accepting tetracyano-ethylene, a partial transfer occurs, but not at all with acrylic compounds. 

For example, with regard to the hot-spot theory outlined above, it would clearly be useful to understand the effects of changing the solvent, or the ambient temperature and at which the reaction was carried out. Furthermore, the design of ultrasonic probe systems allows for ready variation of the power input and occasionally variation of the frequency of the output. Hence, there are a number of factors that must be bom in mind when setting up a viable system. For this reason, the following section is devoted to a discussion of the effects of extrinsic variables on the sonochemical process. 

The credibility of the hot spot theory is reinforced by its ability to account for the effects of extrinsic variables on the sonochemical process. Nevertheless, the frequency of ultrasound applied is surprisingly irrelevant to the course of the reaction. Cleaning baths produce a range of frequencies which often vary from day to day, or even during the course of a reaction, and yet this has no discemable effect on the sonochemistry observed. 

The possibility of using sound energy in chemistry was established more than 70 years ago. By definition, sonochemistry is the application of powerful ultrasound radiation (10 kHz to 20 kHz) to cause chemical changes to molecules. The physical phenomenon behind this process is acoustic cavitation. Typical processes that occur in sonochemistry are the creation, growth and collapse of a bubble. A typical laboratory setup for sonochemical reactions is shown in Fig. 8.17. More details of sonochemistry and the theory behind it can be found elsewhere. - ... 

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