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Part-IIImpact of Microwave…Figure-2 (a) Dipolar polarization mechanism (b) Dipolar molecular try to align with anoscillating electric field. Ionic conduction mechanism: Ions in solution willmove in the electric field.2.2.1 Dielectric PropertiesThe heating characteristics of a particular material (for example, a solvent) undermicrowave irradiation conditions are dependent on the dielectric properties of thematerial. The ability of a specific substance to convert electromagnetic energy into heat ata given frequency and temperature is determined by the so-called loss tangent, (tan δ).The loss factor is expressed as the quotient tan δ= ε˝/ε′, where ε˝ is the dielectric loss,indicative of the efficiency with which electromagnetic radiation is converted into heat,and ε′ is the dielectric constant describing the polarizability of the molecules in theelectric field. A reaction medium with a high tan δ is required for efficient absorption and,consequently, for rapid heating. Materials with a high dielectric constant such as water (ε′at 25 o C = 80.4) may not necessarily also have a high tan δ value. In fact, ethanol has asignificantly lower dielectric constant (ε′ at 25 o C = 24.3), but heats much more rapidlythan water in a microwave field due to its higher loss tangent (tan δ: ethanol = 0.941,water = 0.123). In general, solvents can be classified as high (tan δ > 0.5), medium (tan δ0.1-0.5), or low microwave-absorbing (tan δ < 0.1).2.2.2 Microwave versus Conventional Thermal HeatingMicrowave irradiation produces efficient internal heating (in core volumetric heating) bydirect coupling of microwave energy with the molecules (solvents, reagents, catalysts)that are present in the reaction mixture. Since the reaction vessels employed are typicallymade out of (nearly) microwave-transparent materials such as borosilicate glass, quartz orteflon, the radiation passes through the walls of the vessel and an inverted temperaturegradient as compared to conventional thermal heating results 7 (Figure-3).274