Microwave oven components

Electrical components, coffeemakers, hair dryers, microwave oven components... 

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Hgure 4 (A) Microwave oven components. (B) Microwave guide. (Adapted with permission from Kingston HM and Jassie LB (1998) Introduction to Microwave Sample Preparation, Theory and Practice, ACS Professional Reference Book Series, pp. 18-19, Washington DC American Chemical Society.)... 

Microwave ovens emit microwave radiation that is absorbed by water. The absorbed radiation is converted to heat that is transferred to other components of the food. Suppose the microwave radiation has wavelength 12.5 cm. How many photons are required to increase the temperature of 100 mL of water (d = 1 g/mL) from 20°C to 100°C if all the energy of the photons is converted to heat ... 

The common microwave oven has been brought into the laboratory. Using special Teflon reaction vessels, components are mixed together, the vessel sealed and put into the microwave oven. Reaction times are greatly accelerated in many reactions, and reactions that took hours to be complete in refluxing solvents are done in minutes. Benzyl alcohol was converted to benzyl bromide, for example, using microwaves (650 W) in only 9 min on a doped Montmorillonite K-10 clay. This is a growing and very useful technique. 

Using a similar format, dihydropyrimidines were obtained in a microwave-expedited version of the classical Biginelli three-component condensation (Scheme 12.24) [73]. Neat mixtures of /i-kctocstcrs, aryl aldehydes and (thio)ureas with polyphosphate ester (PPE) as reaction mediator were irradiated in a domestic microwave oven for 1.5 min. The desired dihydropyrimidines were obtained in 61-95% yield after aqu-... 

Quiroga and co-workers55 have described a facile three-component microwave assisted, one-pot synthesis of 5-aryl-6-cyano-7-phenyl-5,8-dihydropyrido[2,3-d]pyrimidin-4 (3/f)-ones suitable also in a combinatorial set-up, Scheme 5.36. Equimolar quantities of the starting compounds were placed in open vessels and irradiated in a domestic microwave oven for 15-20 min at 600 W. When irradiation was complete, the resulting solid was treated with ethanol and filtered to give the products in 70-75% yield. Under reflux in ethanol, a much longer reaction time (40-48 h) was required to provide the product in very modest yields (21-25%). 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems. 

Temperatures were measured at each power and time setting. Eleven combinations of power vs. time settings were conducted for each chemical and their average temperature value was computed. Data was collected and a pattern emerged which reflected the tested material s unique heat absorption in the microwave oven. Each component was given a representative value, Delta T (AT ), which corresponds to the ratio of the temperature increase of the sample to the temperature increase of the standard. 

AT values are thus divided into two major groups flavor components which were found to possess a high AT (greater than 1.0, the value for water), and those with low AT (less than 1.0). High AT components tend to get hotter in the microwave oven and therefore can be used most effectively in "reaction"-type flavors where browning and caramelization is desirable. Conversely, low AT values reflect the reduced heat absorbance of flavor components within the microwave oven. They are less prone to microwave-related "modifications" or "flashing-off" and are therefore likely to have superior flavor retention. Experimental data for chemical combinations, essential oils, and flavor systems will appear in a future publication. 

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