Advantage of microwaves

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

In recent years, parallel to the emergence of SPOS, microwave-mediated organic synthesis has come to hght and has developed into a popular field [24-31]. The main advantage of microwave dielectric heating compared to other conventional methods, such as hot plate, oil bath or isomantle, is the tremendous rate enhancement generally observed under microwave irradiation conditions. Various theories have been proposed to explain the source of the rapidity of microwave chemistry [32,33]. However, the gener-... 

In 1996, the first examples of intermolecular microwave-assisted Heck reactions were published [85]. Among these, the successful coupling of iodoben-zene with 2,3-dihydrofuran in only 6 min was reported (Scheme 75). Interestingly, thermal heating procedures (125-150 °C) resulted in the formation of complex product mixtures affording less than 20% of the expected 2-phenyl-2,3-dihydrofuran. The authors hypothesize that this difference is the result of well-known advantages of microwave irradiation, e.g., elimination of wall effects and low thermal gradients in the reaction mixture. 

Polycondensations of 3,3-bis(chloromethyl)oxetane and a variety of bisphenols were studied using the microwave-PTC technique (Eq. 23) [35]. The results obtained showed the advantages of microwaves in terms of the molecular weights for crystalline polymer, as reflected in higher values of the transition temperature (Tg) and melting point (Tm) but also in reduction of reaction times for all types of structure. 

The reactions of substituted furo[3,2- ]pyrrole-5-carbohydrazides with 5-arylfuran-2-carbaldehydes, 4,5-disubsti-tuted furan-2-carbaldehydes, and thiophene-2-carbaldehyde have been studied <2005CEC622>. The advantage of microwave (MW) irradiation on some of these reactions was reflected in the reduced reaction time and increased yields (Table 8). The series of substituted hydrazones 241-246 was obtained from these... 

Abstract Recent developments in the microwave-assisted synthesis of heterocycles are surveyed with the focus on diversity-oriented multi-component and multi-step one-pot procedures. Both solution- and solid-phase as well as polymer-supported methodologies for the preparation of libraries of heterocycles are reviewed. Advantages of microwave dielectric heating are highlighted by comparison with conventional thermal conditions. 

General advantages of microwave heating for chemistry with either open or closed vessels, with or without solvents, now are well recognized [17,18]. In the main ... 

Microwaves heat the liquid phase, whereas vapors do not absorb microwave energy. The temperature of the vapor phase is therefore lower than the temperature of the liquid phase and vapor condenses on cool vessel walls. As a result, the actual vapor pressure is lower than the predicted vapor pressure. This sort of sustained dynamic, thermal non-equilibrium is a key advantage of microwave technology, because very high temperatures (and, in turn, short digestion times) can be reached at relatively low pressures. 

The advantage of microwave pyrolysis over conventional pyrolysis methods do not rely on changes in chemical pathways, but in the advantages that have been mentioned previously. 

Mixing carbon with microwave-transparent materials, particularly plastics, and subjecting the mix to microwave radiation, is a very efficient way to heat up such materials, increasing their bulk temperature to a point where pyrolysis occurs. In this chapter the main characteristics of a number of microwave pyrolysis processes, for plastics and other materials, have been introduced, showing that these processes combine the advantages of microwave heating with the commercial and environmental opportunities intrinsic to the pyrolysis of wastes. 

---