Microwave and conventional heating

Keywords microwave and conventional heating in Diels-Alder reactions... 

Fig. 8 A comparison of the results from microwave and conventional heating for amino-dehalogenation on 2-pyridones...

Different approaches towards pyrazoles have been described using microwave-accelerated cyclizations. A classical approach to pyrazoles is the cyclization of a j6-diketone with hydrazines. A series of 5-trichloromethyl-pyrazoles 58 and pyrazolium chlorides 59 were synthesized by reaction of a 4-methoxy-trihalo-3-alken-one 57 with differently substituted hydrazines (Scheme 19). The use of microwave and conventional heating for making pyrazoles gave... 

The reaction was studied under the action of focused microwaves and conventional heating and it is noteworthy that elimination is more efficient than Michael addition (leading to aziridines) under irradiation, the conditions being the same. 

Because the reaction is driven by protonation of the carbonyl functionality, reacting species were expected to be localized on the bed of the acid catalyst subjected to microwave irradiation. Hexane was used as a nonpolar solvent to minimize solvent absorption and superheating. Elimination of catalyst superheating in a continuous-flow reactor was most probably the reason why no significant differences were observed between the reaction rates under the action of microwave and conventional heating. 

Leadbeater, N.E. and Marco, M., Rapid and amenable Suzuki coupling reaction in water using microwave and conventional heating, J. Org. Chem., 2003, 68, 888-892. 

Gulmini, M., Ostacoli, G., Zelano, V and Torazzo, A. (1994) Comparison between microwave and conventional heating procedures in Tessier s extractions of calcium, copper, iron and manganese in a lagoon sediment. Analyst, 119, 2075-2080. 

Microwave and Conventional Heating Systems. A General Electric Space Maker 500 Watt Microwave Oven at 2450 MHZ was used in the preparation of all microwave samples. A standard General Electric Hotpoint electric range or a hot water bath was used to prepare conventional heated samples. 

Strecker Aldehyde Formation. Formaldehyde, acetaldehyde and iso-butyraldehyde were formed by Strecker degradation of glycine, alanine and valine, respectively. Relative concentrations of aldehydes produced by microwave and conventional heating to comparable temperature is shown in Table I. Significantly higher concentrations were observed for microwave heated samples. 

Table I. Strecker-Aldehydes Produced by Microwave and Conventional Heating of Amino Acids and Diacetyl...

Figure 3 Microwave-assisted organic synthesis, (a) A domestic kitchen microwave with local "hot spots." (b) A single-mode microwave reactor for organic synthesis, (c) Comparison of surface temperature between microwave and conventional heating.

Differences between chemistry observed with microwave and conventional heating can often be attributed to the different transfer rate or spatial distribution of heat. Once appropriate temperatures are known in various parts of the system, conventional laws of thermodynamics or kinetics commonly apply. Such cases may be called microwave specific effects. However, there are also cases where it is proposed that an additional effect operates, perhaps through the action of the electric field 23 such effects are commonly called nonthermal or athermal microwave effects. Although their existence is controversial in fluid phases,6,24-26 there is a strong body of evidence supporting such effects in solid phases.27-29... 

Albi, T., Lanzon, A., Guinda, A., Leon, M., and Perez-Camino, M.C. 1997. Microwave and Conventional Heating Effects on Thermoxidative Degradation of Edible Fats. J. Agric. Food Chem. 45 3795-3798. 

The proposal of some authors on the operation of nonthermal effects is still controversial [120-122]. In the literature microwave effects are the subject of some misunderstanding. Most mistakes was recorded when authors considered that microwave effect means specific effect , i.e. a nonthermal effect. For that reason, let us discuss the matter in more detail. In heterogeneous catalytic reactions, differences between reaction rates or selectivity under microwave and conventional heating conditions have been explained by thermal effects. 

VUlamiel, M., Castillo, M.D., San Martin, C., and Corzo, N. 1998. Assessment of the thermal treatment of orange juice during continuous microwave and conventional heating. /. Sci. Food Agric. 78, 196—200. 

In the biofuels arena, the energy efficiencies of microwave and conventional heating have been compared in the preparation of biodiesel. To be viable, biodiesel production must be conducted on a very large scale and also must be energy efficient over the entire process. Calculations indicate that while microwave heating in batch... 

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