Microwave-assisted Organic Reaction

On the other hand, solvents usually show a decrease in dielectric constant with temperature. Efficiency of microwave absorption diminishes with temperature rise and can lead to poor matching of the microwave load, particularly as fluids approach the supercritical state. Solvents and reaction temperatures should be selected with these considerations in mind, as excess input microwave energy can lead to arcing. If allowed to continue unchecked, arcing could result in vessel rupture or perhaps an explosion, if flammable compounds are involved. Therefore it is important in microwave-assisted organic reactions, that the forward and reverse power can be monitored and the energy input be reduced (or the load matching device adjusted) if the reflected power becomes appreciable. 

Vass, A. Toth, J. Pallai-Varsanyi, E. in Effect of Inorganic Solid Support for Microwave Assisted Organic Reactions, OR 19, presented at the Int. Conf. Microwave Chemistry, Prague, Czech Republic, Sept. 6-11,1998. 

Microwave technology—chemical synthesis applications, 16 538-594 microwave-accelerated solvent-free organic reactions, 16 555-584 microwave-assisted organic reactions in the liquid phase, 16 540-555 Microwave technology, 16 509-537. See also Microwave power Microwave technology— chemical synthesis applications... 

D.E. Pivonka and J.R. Empfield, Real-time in situ Raman analysis of microwave-assisted organic reactions, Appl. 

Caddick, S., Microwave-assisted organic-reactions, Tetrahedron, 1995, 51, 10403. 

Pivonka, D.E. 8t Empfield, J.R. Real-Time In Situ Raman Analysis of Microwave-Assisted Organic Reactions Appl. Spectrosc. 2004, 58, 41 16. 

A new microwave reactor for batchwise organic synthesis is described in Ref. 712 (Fig. 3.5). Its use permits us to carry out synthetic works or kinetic studies on the 20 100 m L scale, with upper operating limits of 260°C and 10 MPa (100 atm). Microwave-assisted organic reactions can be conducted safely and conveniently, for lengthy periods when required, and in volatile organic solvents. The use of water as a solvent is also explored [712]. 

The commercial system is a more expensive way of carrying out microwave-assisted organic reactions. A number of systems are currently available and one such system is manufactured by CEM Microwave Technology Ltd. (www.cem.com). This type of system operates with a rotating carousel, so that a number of reaction vessels can be irradiated and agitated at the same time. The vessels are made of poly(etherimide) with Teflon lining and it is possible to monitor both the internal temperature and pressure of the reaction. 

Figure 4.13 Some microwave assisted organic reactions in NCW.

If the product is needed on a small scale, up to 100 g, the reaction can be scaled out rather than scaled up. The excellent reproducibihty together with automation can easily produce up to 100 g overnight. Bose et al. have described an alternative for minor scale-up where the use of the microwave-assisted organic reaction enhancement (MORE) technique reduces the need for organic solvents and increases atom economy by improving product selectivity and chemical yield, thus, minimizing the need for larger scale-up. 

Real-time in situ Raman analysis of microwave-assisted organic reactions has also been reported [40, 41]. Raman spectroscopy provides a combination of high selectivity along with the ability to conduct analysis directly through the wall of glass reaction vessels and in monitoring the Knoevenegel condensation reaction of salicylaldehyde and benzyl acetoacetate to form 3-acetylcoumarin, observations were made inside the microwave compartment of the synthesizer. 

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