Influence of microwave

E. Oks, Series on Atoms and Plasmas, Vol. 9, Plasma Spectroscopy The Influence of Microwave andEaserFields, Spdnger-Vedag, New York, 1995. 

Analysis of soils is an important task in the environmental researches. Reliability of ICP-MS results of soil analysis mainly depends on chemical sampling. Recently microwave systems are widely used for preparation of different samples. Influence of microwave radiation on sample ensures a complete decomposition of sample, greatly increases the mineralization, and allows possible losses of volatile elements to be minimized. In the given study to intensify decomposition of soils we applied the microwave sample preparation system MULTIWAVE (Anton Paar, Austria and Perkin-Elmer, USA) equipped with rotor from 6 autoclaves with TEM reaction chambers of 50 ml volume. 

A case study on the influence of microwave-assisted reactions carried out in open or closed vessel has been described by Kappe and co-workers [ 158]. One of the examples deals with the cyclocondensation of tetrahydroquinohne and malonic esters. The reaction gave tricyclic hydroxyquinolones with loss of two molecules of ethanol, similar to the reaction described in Scheme 79. The results showed clearly that this reaction carried out in an open vessel gave more reproducible results. 

Influence of microwave pretreatment of fresh orange peels on pectin extraction... 

The aim of the present investigation is to study the influence of microwave pretreatment of fresh orange peels on the extraction time of orange pectin and the influence of the intensity and duration of microwave heating on pectin yield. 

In the first series of experiments (Table 2) the influence of microwave pretreatment on extraction of pectin was followed. Microwave pretreatment had a slight positive effect on the... 

In order to study any chemoselectivity influences of microwave irradiation on the domino Knoevenagel/hetero-Diels-Alder process (the so-called Tietze reaction), Raghunathan and coworkers [31a] investigated the transformation of 4-hydroxy coumarins (10-85) with benzaldehydes 10-86 in EtOH to afford pyrano[2,3-c]cou-marin 10-87 and pyrano[2,3-b]chromone derivatives 10-88. Normal heating of 10-85a and 10-86a at reflux for 4h gave a 68 32 mixture of 10-87a and 10-88 in 57% yield, whereas under microwave irradiation a 97 7 mixture in 82% yield was obtained. Similar results were found using the benzo-annulated substrates 10-85b and 10-86b. 

Symmetrical and unsymmetrical benzoins have been rapidly oxidized to benzils in high yields using solid reagent systems, copper(II) sulfate-alumina [105] or Oxone-wet alumina [105, 106] under the influence of microwaves (Scheme 6.32). Conventionally, the oxidative transformation of a-hydroxy ketones to 1,2-diketones is accomplished by reagents such as nitric acid, Fehling s solution, thallium(III) nitrate (TTN), ytterbium(III) nitrate, ammonium chlorochromate-alumina and dayfen. In addition to the extended reaction time, most of these processes suffer from drawbacks such as the use of corrosive acids and toxic metals that generate undesirable waste products. 

The interaction of microwaves with solid materials has proven attractive for the preparation and activation of heterogeneous catalysts. It has been suggested that micro-wave irradiation modifies the catalytic properties of solid catalysts, resulting in increasing rates of chemical reactions. It is evident that microwave irradiation creates catalysts with different structures, activity, and/or selectivity. Current studies document a growing interest in the preparation of microwave-assisted catalysts and in the favorable influence of microwaves on catalytic reactions. 

Fast, solid state, microwave-assisted, synthesis. A short history of reactions carried out in the solid state under the influence of microwaves was reported by Kniep (1993) who presented this technique as characteristic of a fast solid-state chemistry and described the preparation of CuInS2 by using a method first developed by Whittaker and Mingos (1992) and extended by Landry and Barron(1993). Copper, indium and sulphur (all powders between 100 and 300 mesh) in a molar ratio 1 1 2 were thoroughly mixed and loaded into a silica ampoule. This, sealed under vacuum, was placed in a domestic microwave oven and irradiated at 400 W and 2450 MHz. 

Prasad, P.S.S., Lingaiah, N., Rao, P.K., Berry, F.J. and Smart, L.E., The influence of microwave-heating on the morphology and benzene hydrogenation activity of alumina-supported and silica-supported palladium catalysts, Catal. Lett., 1995, 35, 345-351. 

Several workers have claimed that under the influence of microwaves, some reactions proceed faster than under conventional conditions at the same temperature because of various non-thermal microwave effects 48,53-56. Other investigators have rejected the theory of specific activation at a controlled temperature in homogeneous media57-62. A study by Stadler el al,63 on the rate enhancements observed in solid-phase reactions revealed that the significant rate enhancements were a result of direct, rapid in-core heating of the solvent by microwave energy and not a specific non-thermal microwave effect . The existence or otherwise of non-thermal microwave effects continues to be a source of great debate and if proven would have serious potential consequences for scale-up, particularly if such effects were unpredictable. 

Although the first publications concerned with the possibilities of use of microwave irradiation in organic synthesis appeared in 1980 s [12,13] and in polymer chemistry even earlier at the end of 1960 s [14], the sudden growth of interest in the application of microwave irradiation in almost all fields of chemistry took place at the end of 1980 s. Nowadays, there is hardly find any reaction that has not been attempted under microwave conditions. The application of microwaves in chemistry is therefore so attractive that from the very beginning it was realized that a number of chemical processes can be carried out with a substantial reduction in the reaction time in comparison to conventional processes. Reactions that usually take many hours or days, under influence of microwave irradiation can be run in considerably shorter time of several minutes or even seconds [15]. These phenomena are not fully understood yet however, there are two groups of theories that are proposed to explain the reduction of the reaction time under microwave conditions in comparison with processes under conventional conditions. 

All the mentioned above examples prove that for the investigated reactions the influence of microwave irradiation was limited to pure thermal effects (i.e., higher pressure and/or bulk temperature of the reaction mixture, development of temperature gradient, overheating of solvents) that can be responsible for seemingly higher reaction rates under microwave conditions. However, a careful consideration of all the factors that influence the reactions under microwave conditions can eliminate most of these effects, there are several of reactions in which the influence of only thermal effects do not suffciently explain the enhancement of yield and rate of the reactions. 

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

The influence of microwave power input and inlet temperature on microwave fluid-bed drying. (From... 

A relatively unexplored extension of the Kramers theory is the escape of a Brownian particle out of a potential well in the presence of an external periodic force. Processes such as multiphoton dissociation and isomerization of molecules in high-pressure gas or in condensed phases/ laser-assisted desorption/ and transitions in current-driven Josephson junctions under the influence of microwaves " may be described with such a model, where the pieriodic force results from the radiation field. 

Bogdal, D. Influence of microwave irradiation on the rate of coumarin synthesis by the Knoevenagei condensation. ECHET98 Eedronic Conference on Heterocyclic Chemistry, June 29-July 24, 19981998, 387-390. 

Normally stable sulfonyl ylide 46 reacts with benzaldehyde derivatives under the influence of microwave irradiation to produce a,fl-unsaturated sulfones. P-Silylated olefins are obtained from the corresponding aldehydes or ketones and [Ph3PCH3]I."... 

Graft Copolymerization Under the Influence of Microwave Radiations (MW)... 

Similarly, N-arylsulfoximines and related species have been prepared by Pd-catalyzed coupling of a sulfoximine with aryl chlorides under the influence of microwave irradiation [72], Appropriately functionalized systems gave rise to benzo-thiazines directly in a one pot process. 

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