Comparison of microwave and conventional heating

These differential heating effects may also be used in solid-state reactions involving metal powders that are made into a fluidised bed by a counter stream of gas. The metal particles interact very strongly with the microwaves and rapidly heat, whereas the gases are transparent to microwaves therefore, the reaction is induced by a very selective interaction between the metal particles and microwaves. 

Therefore, for large samples it is possible to develop quite large temperature gradients and it is necessary to introduce electronic and mechanical perturbations to even out the field pattern. There are well-determined methods for evening out the field pattern in this way. However, the procedures are complicated because the sample itself perturbs the field pattern and therefore there may not be a universal solution to the problem. 

In summary, it is apparent that the application of dielectric heating to chemical reactions may result in different reaction rates and product distributions, and chemists 

However, definitive proof for these proposals are difficult to obtain because of the difficulties in temperature measurement alluded to above and that the reaction might be occurring under non-equilibrium conditions, where the conventional rate expressions and transition state theory assumptions may not be valid. 

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P. Colombo, G. A. Danko, and R. Silberglitt, Production of SiC and SiOC from preceramic polymers comparison of microwave and conventional heating, in Ceramics - Getting into the 2000s, Part C (Section F), Techna Sri, Faenza, 1998, pp. 353-360. 

Bacsa B, Horvati K, Bosze S et al (2008) Solid-phase synthesis of difficult peptide sequences at elevated temperatures a critical comparison of microwave and conventional heating technologies. J Org Chem 73 7532-7542... 

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. 

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

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.

A very detailed comparison for every single step of the whole sequence (linking of a pyrazinone to the solid support, Diels-Alder reaction with acetylenic dienophile and cleavage of a formed pyridinone from the resin) was made between microwave irradiation and conventional heating conditions [115]. 

DEVECE C, RODRIGUEZ LOPEZ J N, FENOLL L G, TUDELA J, CATALA J M, DE LOS REYES E and GARCIA CANOVAS F (1999) Enzyme inactivation analysis for industrial blanching applications comparison of microwave, conventional, and combination heat treatments on mushroom polyphenoloxidase activity , J g-nc Food Chem, 47 (11) 4506-11. 

Raner, K.D., Strauss, C.R., Vyskoc, F. and Mokbel, L., A comparison of reaction-kinetics observed under microwave irradiation and conventional heating, J. Org. Chem., 1993, 58, 950. 

Salvatorelli, G., Marchetti, M. G., Betti, V., Rosaspina, S., and Finzi, G. 1996. Comparison of the effects of microwave radiation and conventional heating on Bacillus subtilis spores. Microbios 87 169-174. 

Knoevenagel condensation under the action of microwave irradiation has been widely studied. It has also been used for the synthesis of heterocycles in accordance with Scheme 10.80 [36], At 160-170 °C under the action of focused irradiation (40-60 W), condensation occurs within 45 s to 4 min, without solvent, without base, and without catalyst. Unfortunately, no comparisons were made with conventional heating and classical methods. 

Loupy et al. studied possible speciflc MW effects in irreversible Diels-Alder reactions with acetylenic dienophiles under solvent-free conditions [73]. Strict comparisons of microwave irradiation and conventional heating were conducted and substantial speciflc kinetic MW effects were observed for the reaction of 3-carbomethoxy-2-pyrone with acetylenic compounds (Scheme 11.22). This were in... 

Thermal analyses of other dental polymers have also been reported. The compositions of waxes have been studied by differential thermal analysis (DTA) [64-66] and DMA [67], Conventional DSC and DMA recently showed the superiority of microwave-processed denture base resins to those processed by the slower traditional dental laboratory heating regimen [68], In contrast, comparisons of Tg and other relevant properties for a soft denture liner that was processed by the dental laboratory procedure and a more convenient chairside procedure indicated that both procedures yielded equivalent results [69],... 

In order to reach a conversion of styrene of 70 %, constant heating in an oil bath for as long as 6 h was required in comparison with only 8.3 min in the microwave oven. Calculated values of Rp (mol/Js) for both processes showed that the rate of microwave-assisted emulsion polymerization of styrene was higher by a factor of 26 than the reaction activated by conventional heating (Fig. 6) [39]. Molecular weight distribution and polydispersities of the polystyrene samples obtained under microwave and conventional conditions were similar for both activation methods and depended on the initiator concentration. 

Fig. 8. Comparison of the typical hydrodynamic radius distribution f(Rh) of the polystyrene nanospheres prepared microwave irradiation and conventional heating method. Reprinted from (1997) Macromolecules 30 6388 [42] with permission...

The characterization of the final product shows that the resins prepared by the microwave and conventional methods have comparable properties. In particular, GPC analysis showed that all the E-M epoxy resins have a similar molecular weight and molecular weight distribution. The main advantage of such a process performed in the microwave reactor was the reduction of the reaction time under microwave conditions (40 min, 160 °C) in comparison to thermal heating (80 min, 160 °C) [117]. 

Danko, G.A., Silberglitt, R Colombo, P Pippel, E., and Woltersdorf, (. (2000) Comparison of microwave hybrid and conventional heating of preceramic polymers to form silicon carbide and silicon oxycarbide ceramics./. Am. Ceram. Soc., 83, 1617. 

Recently, Fustero et al. [44] reported an efficient microwave-assisted inter-molecular aza-Michael reaction catalyzed by 9-amino-9-deoxy-epi-hydroquinine (20mol%) with pentafluoropropionic acid co-catalyst (Scheme 21.19). Chiral five-and six-membered aza-heterocycles, for example, piperidines and pyrrolidines, were synthesized with high enantioselectivities. CycUzation using microwave irradiation at 60 °C allowed a reduction of reaction time (e.g., from 20 to 1 h) and led to comparable results in terms of yield and enantioselectivity to the conventional method at room temperature. Unfortunately, no comparison of microwave heating with conventional oil bath heating at 60 °C was presented. 

Figure 1. Comparison of heating mechanism in microwave and conventional sintering methods.

A new series of 2,4-disubstituted-2,3-dihydro substituted-1,5-benzodiazepine derivatives was synthesized by the condensation of o-phenylenediamine and various 1-(4 -substituted phenyl)-3-(3-mono-, or 4-mono-, or 3,4-di-, or 3,4,5-tri-sub-stituted phenyl)-2-propene-l-one under microwave irradiation. The modification of coupling microwaves with the solvent-free technique is an easy, rapid, efficient and convenient protocol for the synthesis of 1,5-benzodiazepines. The microwave irradiation is better over the classical reflux heating because the solid-supported reactions gave better yield (58-86%) and these reactions were completed within 4-6 min in comparison to hours under conventional heating. However, it was observed that long exposure time to the microwave irradiation resulted in decomposition of compounds to dark tarry mixtures (Sharma and Joshi, 2012). 

The use of microwave irradiation for this reaction, compared to conventional thermal heating, was investigated. Chloroform used as solvent under the conventional heating did only allow a temperature of 60 °C and a direct comparison between the two methods is therefore somewhat unfair imder these circumstances. Nevertheless, the microwave-assisted method is attractive and proved useful for both primary and secondary amines resulting in highly substituted pyrazolo ring-fused pyridones 40 in 68-86% yields within only 10 min. 

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