Microwave ultrasound

Chemat et al. have reported several microwave reactors, including systems that can be used in tandem with other techniques such as sonication [68], and ultraviolet radiation [69]. With the microwave-ultrasound reactor, the esterification of acetic acid with n-propanol was studied along with the pyrolysis of urea. Improved results were claimed compared with those from conventional and microwave heating [68]. The efficacy of the microwave-UV reactor was demonstrated through the rearrangement of 2-benzoyloxyacetophenone to l-(2-hydroxyphenyl)-3-phenylpropan-l,3-dione [69]. 

Chemat, F., Poux, M., Di Martino, J.-L. and Berlan, J., An original microwave-ultrasound combined reactor suitable for organic synthesis application to pyrolysis and esterification,. Microwave Power Electromag. 

Lagha, A., Chemat, S., Bartels, P.V. and Chemat, F., Microwave-ultrasound combined reactor suitable for atmospheric sample preparation procedure of biological and chemical products, Analusis, 1999, 27, 452. 

Contrasting with the microwaves, ultrasound is applied much more in synthetic coordination and organometallic chemistry, and has now become a classic conventional synthetic tool. This approach is used, in particular, for the activation of elemental metals in organic synthesis. A recent monograph [708] and previously published books [709-711] contain a complete description of the possibilities of ultrasonic treatment for obtaining metal complexes, so, in the present monograph, we give only selected applications. 

A combination of different techniques can frequently improve yields of final compounds or synthetic conditions, for example a reunion of direct electrochemical synthesis and simultaneous ultrasonic treatment of the reaction system [715]. Reunion of microwave and ultrasonic treatment was an aim to construct an original microwave-ultrasound reactor suitable for organic synthesis (pyrolysis and esterification) (Fig. 3.7) [716], The US system is a cup horn type the emission of ultrasound waves occurs at the bottom of the reactor. The US probe is not in direct contact with the reactive mixture. It is placed a distance from the electromagnetic field in order to avoid interactions and short circuits. The propagation of the US waves into the reactor occurs by means of decalin introduced into the double jacket. This liquid was chosen by the authors of Ref. 716 because of its low viscosity that induces good propagation of ultrasonic waves and inertia towards microwaves. 

Energy Considerations Microwaves, Ultrasound, Electricity and Light... 

Ionizing radiation, microwaves, ultrasound, or hypothermia are the major physical agents that can affect the fetus via direct transmission through maternal tissues. In general, the dose required for a physical agent to cause detriment to the fetus surpasses that required to induce maternal toxicity. Mechanical impact or changes in temperature, unless extreme, are likely minimized by the hydrostatic pressure of the womb and maternal homeostatic capabilities. 

Diels-Alder reaction of different 5-aminoisothiazole dioxides 69 (R = H) and 70 (R = R = H) and dienes in different reaction conditions (neat, microwave, ultrasound) afforded cycloadducts 171 and 172 (X = CH2, O R = o-MeOC6H4, H). The aco/OT /o-selectivity is dependent on the reaction conditions and the type of cyclic diene <2006EJO4285>. 

As noted earlier, not all open-vessel systems (viz. those that operate at atmospheric pressure) are of the focused type. A number of reported applications use a domestic multi-mode oven to process samples for analytical purposes, usually with a view to coupling the microwave treatment to some other step of the analytical process (generally the determination step). Below are described the most common on-line systems used so far, including domestic ovens (multi-mode systems) and open-vessel focused systems, which operate at atmospheric pressure and are thus much more flexible for coupling to subsequent steps of the analytical process. On the other hand, the increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Examples of these new units include the microwave-ultrasound combined extractor, the focused microwave-assisted Soxhlet extractor, the microwave-assisted drying system and the microwave-assisted distillation extractor, which are also dealt with in this section. Finally, the usefulness of the microwave-assisted sample treatment modules incorporated in robot stations is also commented on, albeit briefly as such devices are discussed in greater detail in Chapter 10. 

Chemat, F., M. Poux, J. L. Martino, and J. Berlan, An Original Microwave-Ultrasound Combined Reactor Suitable for Organic Synthesis Apphcation to Pyrolysis and Esterification, J. Microwave Pofwer and Electromag. Energy, 31, 2P(1996). 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

In this book, the state-of-the-art of the synthesis of materials, composites, and chemical compounds is presented. The methods that are described are based on the use of ionizing irradiation (a-particles, electron beam, y-radiation, etc., studied and applied in radiation chemistry and radiochemistry). Other modern techniques, which are frequently referred to as radiation in the literature (e.g., microwaves, ultrasound waves, laser, and UV radiation), are not discussed here. 

The increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Regarding MAE, examples of these new units include the microwave-ultrasound combined extractor and the focused microwave-assisted Soxhlet extractor. 

The microwave-ultrasound combined system has been constructed from a Prolabo Maxidigest 350 single-mode microwave oven, in which a sonotrode is placed at the base for indirect ultrasonic agitation of the sample. The ultrasonic probe is placed at a sufficient distance from the electromagnetic field to avoid interactions and short-circuits. The use of this system has allowed the digestion of C03O4 and olive oil in shorter times than those required by the conventional method (from 3 to 1 h and from 45 to 30 min, respectively). 

Palmisano, G., Bonrath, W., Boffa, L. et al. (2007) Heck reactions with very low hgandless catalyst load accelerated by microwaves or simultaneous microwaves/ultrasound irradiation. Adv. Synth. Catal., 349, 2338-44. 

Heat is one of the most efficient weapons to destroy tumor cells. But to induce a moderate heat of 41-47°C for a short duration (5-10 min) in the body without affecting normal tissue is quite difficult. Some of the commonly used heating sources to induce localized hyperthermia in the body are radiofrequency (RF), microwaves, ultrasound, magnetic field, and light [24]. 

Chemistry (Anastas and Warner 1998). In recent years, a renewed sensitivity to the environmental problems connected with organic syntheses was the driving force that motivated chemists to introduce novel techniques and particularly eco-friendly procedures. Atom economy, minimization of side product formation, solventless conditions, and use of unconventional techniques to run reactions (microwave, ultrasound, ball-milling, ionic liquids, etc.) have been introduced and play relevant roles in today s chemistry laboratories. These techniques will probably be used more intensively and in larger amounts in synthetic laboratories in the future. 

The other concern relating to the use of supported systems is that they are often, but not always, slower to react than their solution counterparts. This can be problematic, but can often be overcome by a number of methods. An obvious way to enhance reactivity is by improving the surface topology of the supported material. The use of focused microwaves, ultrasound, or novel solvents such as ionic liquids, or indeed combinations of these can also be useful in increasing reactivity and reaction rates. ... 

C. Domini, L. Vidal, G. Cravotto, A. Canals, A simultaneous direct microwave/ultrasound-assisted digestion procedure for the determination of total Kjeldahl nitrogen, Ultrason. Sonochem. 16 (2009) 564r-569. 

Local hyperthermia uses very high heat. Radio waves, microwaves, ultrasound waves, and other forms of energy can be used to heat the local area. When ultrasound is used, the technique is called high intensity focused ultrasound, or HIFU. The heat may be applied using different methods ... 

Peris and co-workers used glycerol instead of i-PrOH as the hydrogen source and the reaction medium in a TH process. In this context, they described the use of a series of NHC-Ir " complexes 99 and 100 for the TH of double bonds using glycerol as hydrogen donor under microwave, ultrasound and oil bath conditions (Figure 13.9). These complexes were found to be active for TH of carbonyl compounds, moderately active in the reduction of olefins and alkynes and, more remarkably, showed excellent chemoselec-tivity in the reduction of the alkenic double bond of o,p-unsaturated ketones. 

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