Sonochemical reactions sonochemistry

Abstract Acoustic cavitation is the formation and collapse of bubbles in liquid irradiated by intense ultrasound. The speed of the bubble collapse sometimes reaches the sound velocity in the liquid. Accordingly, the bubble collapse becomes a quasi-adiabatic process. The temperature and pressure inside a bubble increase to thousands of Kelvin and thousands of bars, respectively. As a result, water vapor and oxygen, if present, are dissociated inside a bubble and oxidants such as OH, O, and H2O2 are produced, which is called sonochemical reactions. The pulsation of active bubbles is intrinsically nonlinear. In the present review, fundamentals of acoustic cavitation, sonochemistry, and acoustic fields in sonochemical reactors have been discussed. 

By the proper choice of solvent and experimental conditions (i.e., low volatility, highly stable liquids at low temperature e.g., decane, -10° C), the rates of degradation of nonaqueous liquids can be made quite slow, well below those of water. This is of considerable advantage, since one may then observe the primary sonochemistry of dissolved substrates rather than secondary reactions with solvent fragments. In general, the examination of sonochemical reactions in aqueous solutions has produced results difficult to interpret due to the complexity of the secondary reactions which so readily occur. One may hope to see the increased use of low-volatility organic liquids in future sonochemical studies. 

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... 

In organometallic chemistry, the use of ultrasound in liquid-liquid heterogeneous systems has been limited to Hg. The emulsification of Hg with various liquids dates to the very first reports on sonochemistry (3,203,204). The use of such emulsions for chemical purposes, however, was delineated by the extensive investigations of Fry and co-workers (205-212), who have reported the sonochemical reaction of various nucleophiles with a,a -dibromoketones and mercury. The versatility of this reagent is summarized in Eqs. (30)-(36). 

A primary limitation of sonochemistry remains its energy inefficiency. This may be dramatically improved, however, if a more efficient means of coupling the sound field with preformed cavities can be found. The question of selectivity in and control of sonochemical reactions, as with any thermal process, remains a legitimate concern. There are, however, clearly defined means of controlling the conditions generated during cavitational collapse, which permit the variation of product distributions in a rational fashion. 

CONTENTS Introduction to Series An Editor s Foreword, Albert Padwa. Introduction, Timothy J. Mason. Historical Introduction to Sonochemistry, D. Bremner. The Nature of Sonochemical Reactions and Sonoluminescence, M.A. Mar-guli. Influence of Ultrasound on Reactions with Metals, 6. Pugin and A.T. Turner. Ultrasonically Promoted Carbonyl Addition Reactions, J.L. Luche. Effect of Ultrasonically Induced Cavitation on Corrosion, W.J. Tomlinson. The Effects ... 

The effect of the bulk solution temperature lies primarily m its influence on the bubble content before collapse. With increasing temperature, in general, sonochemical reaction rates are slower. This reflects the dramatic influence which solvent vapor pressure has on the cavitation event the greater the solvent vapor pressure found within a bubble prior to collapse, the less effective the collapse. Increases in the applied static pressure increase the acoustic intensity necessary for cavitation, but if equal numbers of cavitation events occui. the collapse should be nioie intense. In contiast, as die ambient pressure is reduced, eventually the gas-filled crevices of paniculate matter which serve as nucleation sites for the formation of cavitation in even pure liquids, will be deactivated, and therefore the observed sonochemistry will be diminished. 

It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry). The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction, the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices. ... 

Sonochemistry is the research area in which molecules undergo chemical reaction due to the application of powerful ultrasound radiation (20 KHz-10 MHz) [4]. The physical phenomenon responsible for the sonochemical process is acoustic cavitation. Let us first address the question of how 20 kHz radiation can rupture chemical bonds (the question is also related to 1 MHz radiation), and try to explain the role of a few parameters in determining the yield of a sonochemical reaction, and then describe the unique products obtained when ultrasound radiation is used in materials science. 

Thermal probe systems are inexpensive, easy to handle in almost all ultrasonic devices and particularly those used in sonochemistry. Field distributions and optimization of the geometry of the system can be rapidly obtained and the accuracy of the method is high enough to ensure reproducibility. Chemists who make use of ultrasonic equipment should, as a very minimum, consider this method to calibrate and optimize sonication conditions prior to carrying out sonochemical reactions. 

The stimulation of chemical reactions has been known for a many years [1-15] and it has been suggested that some of them might be used as standards for the measurement of the efficiency of ultrasonic systems. Unfortunately, as is the case in the use of sonoluminescence as a probe, there seems to be no theoretical correlation between chemical effects and ultrasonic power. Nevertheless it is an undeniable fact that when sonochemistry is reported in the literature it would be extremely useful if the response of the system used to a standard sonochemical reaction could be included. 

The sonochemistry group in Shanghai have also reported that the fragmentation of aryl /ert-phosphines in the presence of lithium can be promoted effectively using ultrasonic irradiation [45]. Triphenylphosphines could almost quantitatively be changed into lithium diphenylphosphides and phenyl lithiums in the presence of lithium metal under ultrasonic irradiation [Eq. (5)]. Sonochemical reaction was accomplished in 25 min at ambient temperature with no induction period. The reaction rate was increased by a factor of 6, compared with silent conditions. The method has been used for the preparation of some organophosphine ligands. 

Fundamental work by Luche resulted in the hypothesis that ultrasound can influence and change reaction pathways in reaction types with single electron transfer [186, 187]. Ultrasound is also believed to influence reaction systems by mechanical effects [187]. An empirical classification of sonochemical reactions is divided into three types of effects purely chemical effects induced by sonochemical cavitation, hydrodynamic effects (mechanically induced cavitation), and by-passing mass-transport limitation. The latter effects are based on physical rather than chemical phenomena and judged to be false sonochemistry [188]. Nevertheless, these false effects (e.g. emulsification) are often important. The three types of effect are ... 

Since sonochemistry takes its origin in cavitation, the reactivity depends on the characteristics of the bubbles. Their size and lifetime, and the content of the gaseous phase, depend on the physical properties of the medium and the parameters (amplitude and frequency) of the wave. Conducting a sonochemical reaction implies that a multiparameter problem is examined. 

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. - ... 

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). 

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