Safety with microwaves

Vessels designed for microwave-assisted SPOS must fulfill several require-menfs because of fhe harsh conditions (i.e., high temperatures and pressures) usually associated with microwave heating. Open vessels are often impractical because of the possible loss of solvent and/or volatile reagents during the heating process. However, in cases where a volatile byproduct inhibits a reaction, their use may be superior over closed systems. A sealed vessel retains the solvents and reagents, but must be sturdily constructed to avoid the obvious safety implications due to the buildup of pressure. 

Ionic liquids interact very efficiently with microwaves through the ionic conduction mechanism (see Section 2.2) and are rapidly heated at rates easily exceeding 10 °C s"1 without any significant pressure build-up [52]. Therefore, safety problems arising from over-pressurization of heated sealed reaction vessels are minimized. 

The last, but not least, benefit is the possibility of using solvent-free techniques coupled with microwave irradiation. This new type of activation is now more frequently employed but often the presence of solvents prevents its use for safety reasons. This difficulty can be overcame by solvent-free processes. 

Precautions should be taken, especially in a scale-up approach, when dealing with exothermic reactions in the microwave field. Due to the rapid energy transfer of microwaves, any uncontrolled exothermic reaction is potentially hazardous (thermal runaway). Temperature increase and pressure rise may occur too rapidly for the instrument s safety measures and cause vessel rupture. 

Operating with chemicals and pressurized containers always carries a certain risk, but the safety features and the precise reaction control of the commercially available microwave reactors protect the users from accidents, perhaps more so than with any classical heating source. The use of domestic microwave ovens in conjunction with flammable organic solvents is hazardous and must be strictly avoided as these instruments are not designed to withstand the resulting conditions when performing chemical transformations. 

The MARS-S is constituted of a multimode cavity very close to domestic oven with safety precautions (15 mL vessels up to 0.5 L round-bottomed flasks, magnetic stirring, temperature control). The magnitude of microwave power available is 300 W. The optical temperature sensor is immersed in the reaction vessel for quick response up to 250 °C. A ceiling mounted is available in order to make connection with a conventional reflux system located outside the cavity or to ensure addition of reactants. These ports are provided with a ground choke to prevent microwave leakage. It is also possible to use a turntable for small vessels with volumes close to 0.1 mL to 15 mL vessels (120 positions for 15 mL vessels). Pressure vessels are available (33 bar monitored, 20 controlled). 

Significant improvements in yields or reaction conditions can be achieved, together with considerable simplification of operating procedures. The powerful synergistic combination of PTC and microwave techniques has certainly enabled an ever increasing number of reactions to be conducted under clean and mild conditions. The inherent simplicity of the method can, furthermore, be allied with all the advantages of solvent-free procedures in terms of reactivity, selectivity, economy, safety, and ease in manipulation. 

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