Problems associated with microwave heating

Cycloaddition reactions often require the use of harsh conditions such as high temperatures and long reaction times. These conditions are not compatible with sensitive reagents or products such as natural products. The applicability of Diels-Alder cycloadditions is, moreover, limited by the reversibility of the reaction when a long reaction time is required. The short reaction times associated with microwave activation avoid the decomposition of reagents and products and this prevents polymerization of the diene or dienophile. All these problems have been conveniently solved by the rapid heating induced by microwave irradiation, a situation not accessible in most classical methods. With the aid of microwave irradiation, cydoaddition reactions have been performed with great success [9, 10]. 

Cycloaddition reactions have been performed with great success with the aid of microwave irradiation (Chapter 11). All the problems associated with these reactions have been conveniently solved by the rapid heating achieved with microwave irradiation, a situation not accessible by classical methods [4a, 6]. In some examples the selectivity of the reaction has also been modified. Langa described the cydoad-dition of N-methylazomethine ylides to C70 to give three regioisomers (83a-c) by attack at the 1-2, 5-6, and 7-21 bonds (Scheme 5.24) [68]. Under the action of conventional heating the 7-21 83c isomer was formed in only a low proportion... 

The use of various pretreatments of the plastic wastes such as chemical soaking, heat treatments, microwave, and plasma treatments, etc. in conjunction with the pressurized method might be attractive areas for future research. Co-pyrolysis with other wastes such as food wastes is also plausible. Much work has been carried out on other pressurized carbonization methods such as biomass hydrothermal carbonization [111, 112]. If an industrial process is to emerge from the research, the combined use of various carbon sources would be attractive for economy-of-scale purposes. Producing porous carbons for further applications from plastic wastes would not only yield useful products from cheap precursors, but it would also help reduce the problems associated with the ever-growing plastic waste stream. 

However, several articles in the area of microwave-assisted parallel synthesis have described irradiation of 96-well filter-bottom polypropylene plates in conventional household microwave ovens for high-throughput synthesis [16-19]. While some authors have not reported any difficulties associated with the use of such equipment [19], others have experienced problems in connection with the thermal instability of the polypropylene material itself [17] and with respect to the creation of temperature gradients between individual wells upon microwave heating [17, 18]. While Teflon (or similar materials such as PFA) can eliminate the problem of thermal stability, the issue of bottom-filtration reaction vessels has not yet been adequately addressed. 

Microwave ovens are now being used in chemistry labs as a source of heat. These ovens produce radiation at 2.54 GHz. Federal standards limit the amount of radiation that can leak from an oven to levels far below those known to harm people. The only hazards associated with these ovens are the problems of arcing if a metal object is placed inside or the oven door interlock switch is damaged or overridden, which would allow the oven to generate radiation while the door is open. 

Infra-red radiation is generated by fires and hot substances and can cause eye and skin damage similar to that produced by ultraviolet radiation. It is a particular problem to fire fighters and those who work in foundries or near furnaces. Eye and skin protection are essential. Microwaves are used extensively in cookers and mobile telephones and there are ongoing concerns about associated health hazards (and several inquiries are currently underway). The severity of any hazard is proportional to the power of the microwaves. The principal hazard is the heating of body cells, particularly those with little or no blood supply to dissipate the heat. This means that tissues such as the eye lens are most at risk from injury. However, it must be stressed that any risks are higher for items, such as cookers, than for low-powered devices, such as mobile phones. 

---