Green Chemistry in Teaching Laboratory
Microwave Induced Reactions
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Microwave Induced Reactions
This teaching manual comprises of chemistry experiments based on reactions that are induced by microwave. The reactions are carried out in a microwave similar to one used for cooking. Microwaves are electromagnetic radiation in the 0.3-300 GHz frequency range (corresponding to 0.1-100 cm wavelength). To avoid interference with communication networks, all microwave heaters (domestic or scientific) are designed to work at either 2.45 GHz or 0.9 GHz, of which, the former is more prevalent. The energy of microwave radiation is insufficient for molecular excitation, thus most of the energy is used in substrate heat-up. The mechanism of microwave heating is different from that of conventional heating, where heat is transferred by conduction, convection or radiation. In microwave heating, electromagnetic energy is transformed into heat through ionic conduction and the friction due to rapid reorientation of the dipoles under microwave radiation. The larger the dipole moment of a molecule, the more vigorous is the oscillation in the microwave field, consequently more heating. This type of heating is fast, has no inertia, and is in-situ without heating the surroundings. Chemistry under microwave radiation is known to be quite different, fast and efficient . It also reduces the need for solvents, thus it is eco-friendly. It has been exploited in a variety of organics synthesis including hetero cyclic, organ metallic, and combinatorial chemistry. Some of the reported advantages are rapid reactions under controlled temperature and pressure (especially in a closed system), higher purity products achieved due to short residence times at higher temperatures, and better yields at even very short residence times. Solvents for Microwave Experiments Typically all polar solvents which have OH bonds can absorb microwave radiation. Usually polar solvents includes, water, acids, alcohols, and amides. The polarities of these compounds are in the following order: Water > Acids > Amides > Alcohols
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