Rationalization of microwave effects

There are distinct advantages of these solvent-free procedures in instances where catalytic amounts of reagents or supported agents are used since they provide reduction or elimination of solvents, thus preventing pollution at source . Although not delineated completely, the reaction rate enhancements achieved in these methods may be ascribable to nonthermal effects. The rationalization of microwave effects and mechanistic considerations are discussed in detail elsewhere in this book [25, 193]. A dramatic increase in the number of publications [23c], patents [194—203], a growing interest from pharmaceutical industry, with special emphasis on combinatorial chemistry, and development of newer microwave systems bodes well for micro-wave-enhanced chemical syntheses. 

Perreux, L. and Loupy, A., A tentative rationalization of microwave effects in organic synthesis according to the reaction medium, and mechanistic considerations, Tetrahedron, 2001, 57, 9199-9223. 

We have proposed in this review a rationalization of microwave effects in organic synthesis based on the effect of the medium and on mechanistic considerations. The most suitable conditions in which there is any chance of checking specific not purely thermal effects are ... 

It should be obvious from a scientific standpoint that the question of microwave effects needs to be addressed in a serious manner, given the rapid increase in the use of microwave technology in chemical sciences, in particular organic synthesis. There is an urgent need to remove the black box stigma of microwave chemistry and to provide a scientific rationalization for the observed effects. This is even more important if one considers safety aspects once this technology moves from small-scale laboratory work to pilot- or production-scale instrumentation. 

Many of these reports are, however, based on inaccurate or unfounded comparisons with classical conditions which do not enable unequivocal conclusions to be drawn about the effects of microwaves. For this reason, apparent contradictions and controversies have appeared in the literature [5-7]. To try to rationalize all of these results it is necessary to propose a plausible interpretation of effects based on accurate... 

In early literature, there were many claims of a specific microwave effect responsible for the observed rate accelerations. Later experiments showed some of these early reports to be artifacts,while others are debatable or hard to explain. An attempt to rationalize a possible specific microwave effect has been published by Perreux et al. Most of the reports on specific effects, however, can be rapidly dismissed due to poor temperature control. These inaccuracies in temperature measurements often occur when performing the reactions in domestic ovens, with microtiter plates or on solid supports, where there are inherent difficulties in measuring the temperature accurately. Even with today s specialized equipment, it is very difficult to capture the true temperature of a reaction performed on a dry solid support or in a continuous flow system. 

The effect of microwave irradiation in chemical reactions can be attributed to a combination of the thermal and nonthermal effects, i.e. overheating, hot spots, and selective heating, and nonthermal effects of the highly polarizing field, in addition to effects on the mobility and diffusion that may increase the probabilities of effective collisions. These effects can be rationalized considering the Arrhenius law [36, 37] and result from modification of each of the terms of the equation k = Aexp -AGyRT)... 

A strong dependence of the [HF] content on the structural arrangement has been found to occur. Such a phenomenon has been rationalized by the structural impact of the hydroxyl groups. The coupling of sol-gel and microwave synthesis has also shown the effect of the temperature on the kinetics of fluorination. 

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