Sonochemistry: Theory, Reactions, Syntheses, and Applications PDF by Filip M. Nowak

CONTENTS

PREFACE
The study of sonochemistry is concerned with understanding the effect of sonic waves and wave properties on chemical systems. This book reviews research data in the study of sonochemistry including the application of sonochemistry for the synthesis of various nanostructured materials, ultrasound irradiation in pinacol coupling of carbonyl compounds, ultrasound and hydrophobic interactions in solutions, as well as the use of ultrasound to enhance anticancer agents in sonochemotherapy and the ultrasound-enhanced synthesis and chemical modification of fullerenes.

Chapter 1 – Recently, sonochemistry has been employed extensively in the synthesis of nano-structured materials. Rapid reaction rate, controllable reaction conditions, simplicity and safety of the technique as well as the uniform shape, narrow size distribution, and high purity of prepared nano-sized materials are some of the main advantage of sonochemistry. Sonochemistry uses the ultrasonic irradiation to induce the formation of particles with smaller size and high surface area.

Because of its importance, sonochemistry has experienced a large promotion in various fields concerned with production of new nano-structured materials and improvement of their properties during the recent years. However, it has encountered limitations in the case of production of some nano-materials with specific morphology, size and properties, but the growth of the number of researches and published articles in the field of sonochemistry during the recent years shows a large interest and attempt to apply sonochemistry in nanotechnology. The improvement of shape, size, purity and some other chemical and physical properties of such produced materials has been the scope of the researchers recently.

Sonochemistry uses the powerful ultrasound irradiation (20 kHz to 10 MHz) to induce chemical reaction of molecules. During the ultrasonic irradiation, the acoustic cavitations will occur which consist of the formation, growth and implosive collapse of bubbles in a liquid. The implosive collapse of the bubbles generates a localized hotspot or shock wave formation within the gas phase of the collapsing bubbles (The hot-spot theory). This chapter is planned to deal with the application of sonochemistry for the synthesis of various nano-structured materials such as metals, metal carbides, metal oxides, chalcogenides and nanocomposites with unique properties. The effect of different ultrasonic parameters on the prepared structures including their size, morphology and properties are investigated. Also, some applications of prepared nano-materials are introduced, e.g. electrochemical energy storage, catalysis, biosensor and electrooxidation.

Chapter 2 – A multitude of useful physical and chemical processes promoted by ultrasonic cavitation have been described in laboratory studies. Industrial-scale implementation of high-intensity ultrasound has, however, been hindered by several technological limitations, making it difficult to directly scale up ultrasonic systems in order to transfer the results of the laboratory studies to the plant floor. High-capacity flow-through ultrasonic reactor systems required for commercial-scale processing of liquids can only be properly designed if all energy parameters of the cavitation region are correctly evaluated. Conditions which must be fulfilled to ensure effective and continuous operation of an ultrasonic reactor system are provided in this chapter, followed by a detailed description of “shockwave model of acoustic cavitation”, which shows how ultrasonic energy is absorbed in the cavitation region, owing to the formation of a spherical micro-shock wave inside each vapor-gas bubble, and makes it possible to explain some newly discovered properties of acoustic cavitation that occur at extremely high intensities of ultrasound. After the theoretical background is laid out, fundamental practical aspects of industrial-scale ultrasonic equipment design are provided, specifically focusing on:

· electromechanical transducer selection principles;

· operation principles and calculation methodology of high-amplitude acoustic horns used for the generation of high-intensity acoustic cavitation in liquids;

· detailed theory of matching acoustic impedances of transducers and cavitating liquids in order to maximize the ultrasonic power transfer efficiency;

· calculation methodology of ―barbell horns‖, which provide the impedance matching and can help achieving the transference of all available acoustic energy from transducers into the liquids. These horns are key to industrial implementation of high-power ultrasound because they permit producing extremely high ultrasonic amplitudes, while the output horn diameters and the resulting liquid processing capacity remain very large;

· optimization of the reactor chamber geometry.

Chapter 3 – Carbon-carbon bond formation is one of the most important topics in organic synthesis. One of the most powerful methods for constructing a carbon-carbon bond is the reductive coupling of carbonyl compounds giving 1,2-diols. Of these methods, the pinacol coupling, which was described in 1859, is still a useful tool for the synthesis of vicinal diols. 1, 2-Diols obtained in the reaction were very useful synthons for a variety of organic synthesis, and were also used as intermediates for the construction of biologically important natural product skeletons and asymmetric ligands for catalytic asymmetric reaction. In particular, pinacol coupling has been employed as a key step in the construction of HIVprotease inhibitors.

Generally, the reaction is effected by treatment of carbonyl compounds with an appropriate metal reagent and/or metal complex to give rise to the corresponding alcohols and coupled products, The coupling products can have two newly chiral centers formed. Threo, erythro mixtures of diols are usually obtained from reactions. As a consequence, efficient reaction conditions have been required to control the stereochemistry of the 1,2-diols. Recent efforts have focused on the development of new reagents and reaction systems to improve the reactivity of the reagents and diastereoselectivity of the products.

In some of the described methods, anhydrous conditions and long reaction time are required to get satisfactory yields of the reaction products, some of the used reductants are expensive or toxic; excess amounts of metal are needed. Sonication can cause metal in the form of a powder particle rupture, with a consequent decrease in particle size, expose new surface and increase the effective area available for reaction. It was effective in enhancing the reactivity of metal and favorable for single electron transfer reaction of the aldehydes or ketones with metal to form diols. Some recent applications of ultrasound in pinacol coupling reactions are reviewed. The results are mostly from the author research group.

Chapter 4 – Sonochemistry and solution chemistry have been explicitly brought together by analyzing the effect of ultrasound on kinetics of ester hydrolysis and benzoin condensation, measured by the authors, and similar kinetic data for the solvolysis of tert-butyl chloride, compiled from literature. For the first time the power ultrasound, reaction kinetics and linear free-energy relationships were simultaneously exploited to study ionic reactions in water and aqueous-organic binary solvents and the importance of hydrophobic ground-state stabilization of reagents in aqueous solutions was discussed. This approach has opened novel perspectives for wider understanding of the effect of sonication on chemical reactions in solution, as well as on solvation phenomena in general.

Chapter 5 – Ultrasound generates cavitation, which is “the formation, growth, and implosive collapse of bubbles in a liquid. Cavitation collapse produces intense local heating (~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (>109 K/sec)” and liquid jet streams (~400 km/h), which can be used as a source of energy for a wide range of chemical processes. This review will concentrate on theory, reactions and synthetic applications of ultrasound in both homogeneous liquids and in liquid-solid systems.

Some recent applications of ultrasound in organic synthesis, such as, Suzuki reaction,
Sonogashira reaction, Biginelli reaction, Ullmann coupling reaction, Knoevenagel condensation, Claisen-Schmidt condensation, Reformatsky reaction, Bouveault reaction, Baylis-Hillman reaction, Michael addition, Curtius rearrangement, Diels-Alder reaction, Friedal-Craft acylation, Heck reaction, Mannich type reaction, Pechmann condensation and effect of ultrasound on phase transfer catalysis, oxidation-reduction reactions, ionic liquids and photochemistry are reviewed. Ultrasound found to provide an alternative to traditional techniques by means of enhancing the rate, yield and selectivity to the reactions.

Chapter 6 – Sonochemotherpy is the use of ultrasound to enhance anticancer agents. Preclinical trials have manifested this modality is effective against cancers including chemoresistant lesions. Sonochemotherapy is a target therapy, in which cavitation plays the leading role. Making the occurrence and level of cavitation under control improves the safety and therapeutic efficacy. Sonosensitizers and microbubbles enhance cavitation, being a measure to adjust the level of cavitation. Free radicals due to cavitation have the potentials of restructuring a molecule and changing the conformation; thus the molecular structure and anticancer potency of a cytotoxic agent must be investigated, especially when sonosensitizer and microbubble are employed. A potential clinical model for investigating sonochemotherapy is the residual cancer tissues when performing palliative high intensity focused ultrasound treatment.

Chapter 7 – Ultrasound (US) is a sound wave of a frequency greater than the superior audibility threshold of the human hearing. Sonochemistry is the application of ultrasound in chemistry. It became an exciting new field of research over the past decade. Some applications date back to the 1920s. The 1950s and 1960s subsequently represented the first extensive sonochemical research years and significant progresses were made throughout them. Then it was realized that ultrasound power has a great potential for uses in a wide variety of processes in the chemical and allied industries. In these early years, experiments were often performed without any real knowledge of the fundamental physical background about the US action. The situation changed in the 1980s when a new surge of activity started and the use of US as a real tool in chemistry began. It was in 1986 that the first ever international symposium on Sonochemistry was held at Warwick University U.K.

Chapter 8 – In this chapter, the use of ultrasounds on fullerenes (C60 and C70) and fullerene derivatives is described. The focus is on the articles reporting the ultrasoundpromoted treatment of these nanoparticles written in English. The ultrasound-enhanced synthesis and chemical modification of fullerenes are detailed. The improvement obtained by sonicating the reaction mixtures while carrying out traditional organic reactions is discussed.

This includes many types of reactions, such as oxidation, cycloaddition, reduction and amination. Also the ultrasound-enhanced crystallization of fullerenes, producing fullerites, and the formation of colloids when the fullerenes are sonicated in various solvent mixtures are detailed, providing the role of ultrasound in these processes.

Chapter 9 – In this chapter, the use of ultrasounds on carbon based nanotubes is reviewed with a focus on the English written articles. The synthesis of carbon nanotubes and their surface modification such as oxidation and covalent functionalization under ultrasounds are reported. The synthesis of hybrid nanocomposite materials where carbon nanotubes are added as a reinforcement agent via ultrasound-induced assembly is not described in this chapter. A detailed survey of the literature concerning the purification and separation of carbon nanotubes under ultrasounds is provided. The effect of sonication on carbon nanotubes suspensions which covers aqueous and organic solutions in the presence of surfactants is discussed with an emphasis being placed on the effect that ultrasounds have on non-covalent interactions between the carbon nanotubes and the components of the suspensions. The effect of ultrasounds on the physical properties of the carbon nanotubes, especially the introduction of wall defects is analyzed. Finally the advantages and shortcomings of sonochemistry described in this chapter are summarized, showing a possible trend in the direction of future research in this field.