Microwave heating methods

Taylor CR, Shi S-R, Chen C, et al. Comparative study of antigen retrieval heating methods microwave, microwave and pressure cooker, autoclave, and steamer. Biotech. Histochem. 1996 71 263-270. 

In the case of protein, heat treatment of pork does not greatly affect retention as long as the critical temperature or time is not greatly exceeded according to Sebranek (1988). This has been suggested to be about 100°C and less than 1 hr, respectively. Heating methods (microwave, steam, infra-... 

Reactions conducted through microwaves are cleaner and more environmental friendly than conventional heating methods. Microwaves heat the compounds directly and therefore, use of solvents in the chemical reaction can be reduced or in some cases even eliminated. A method was developed to carry out a solvent-free chemical reaction on sponge like material (alumina) with the help of microwave heating. The reactants are absorbed on alumina and exposed to microwaves. It was observed that they react at a faster rate than conventional heating. The use of microwaves has also reduced the extent of purification required for the end products of chemical reactions. 

The one-pot, three-component synthesis of a 20-membered dihydrotri-azine hbrary was also dramatically accelerated through the use of microwave irradiation [79]. Heating a subset of substituted anilines, cyanoguanidine and acetone in the presence of concentrated hydrochloric acid for 35 min at 90 °C in a single-mode microwave reactor gave the corresponding 2,2-dimethyl-1,2-dihydro-s-triazine hydrochloride 51 in comparable yield to conventional conductive heating methods but in a much shorter reaction time and increased purity (Scheme 21). 

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Freitag and John [96] studied rapid separation of stabilisers from plastics. Fairly quantitative extraction (>90% of the expected content) of stabilisers from a powdered polymer was achieved by MAE within 3 to 6 min, as compared to 16 h of Soxhlet extraction for the same recovery. MAE and Soxhlet extraction have also been compared in the analysis of cyclic trimer in PET [113]. On the other hand, Ganzler et al. [128] compared the extraction yields for various types of compounds from nonpolymeric matrices for microwave irradiation with those obtained by the traditional Soxhlet or shake-flask extraction methods. Microwave extraction was more effective than the conventional methods, in particular in the case of polar compounds. As expected, the efficiency of the former is high especially when the extraction solvents contain water. With the high dipole moment of water, microwave heating is more... 

Most importantly, microwave processing frequently leads to dramatically reduced reaction times, higher yields, and cleaner reaction profiles. In many cases, the observed rate enhancements may be simply a consequence of the high reaction temperatures that can rapidly be obtained using this non-classical heating method, or may result from the involvement of so-called specific or non-thermal microwave effects (see Section 2.5). 

The corresponding bridgehead heterocydes are difficult to obtain by conventional heating methods since the reactions of a-tosyloxyketones with ethylenethioureas remain incomplete in an oil bath. The microwave-accelerated process, on the other hand, gets completed in a short time (Scheme 6.44) [147,148],... 

The purity of ionic salts obtained by use of microwave heating was found to be superior to that of those prepared by use of conventional heating methods. This eco-... 

Microwave heating is achieved with a pulse of microwave energy generated within a magnetron. The only requirement for the use of this heating method is that the solvent has a permanent dipole moment.24 This technique can be employed to study reactions in non-aqueous solvents since the presence of electrolytes is not necessary.30 However, the temperature changes attained were much lower than in the case of Joule or laser heating.24... 

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