Microwave effect

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. 

Since the early days of microwave synthesis, the observed rate accelerations and sometimes altered product distributions compared to oil-bath experiments have led to speculation on the existence of so-called specific or non-thermal microwave effects. Historically, such effects were claimed when the outcome of a synthesis per- 

Essentially, one can envisage three different possibilities for rationalizing rate enhancements observed in a microwave-assisted chemical reaction [12]  

---

There is sfill some dispufe about how microwave irradiation accelerates reactions. Besides the generally accepted thermal effects, one beheves that there are some specific (but also thermal) microwave effects, such as the formation of hot spots . There is still some controversy about the existence of non-thermal (athermal) microwave effects. At the present time, new techniques such as coohng while heating are being investigated and the problem of upscahng... 

Clearly, a combination of two or all three contributions may be responsible for the observed phenomena, which makes the investigation of microwave effects an extremely complex subject. 

In addition to the above mentioned thermal/kinetic effects, microwave effects that are caused by the unique nature of the microwave dielectric heating mechanisms (see Section 2.2) must also be considered. These effects should be termed specific... 

Scheme 2.4 Involvement of non-thermal microwave effects in Diels—Alder cycloaddition reactions.

Microwave effects are still the subject of considerable current debate and controversy, and the reader should be aware that there is no agreement in the scientific community on the role that microwave effects play, not even on a definition of terms. 

For many transformations, the reaction times are in fact significantly shorter than the Arrhenius equation would predict, probably because of the additional pressure that is developed, or arguably due to the involvement of microwave effects (see Section 2.5). 

The same Suzuki couplings could also be performed under microwave-heated open-vessel reflux conditions (110 °C, 10 min) on a ten-fold larger scale, giving nearly identical yields to the closed-vessel runs [33, 35], Importantly, nearly the same yields were obtained when the Suzuki reactions were carried out in a pre-heated oil bath (150 °C) instead of using microwave heating, clearly indicating the absence of any specific or non-thermal microwave effects [34],... 

Most importantly, microwave processing frequently leads to dramatically reduced reaction times, higher yields, and cleaner reaction profiles. In many cases, the observed rate enhancements may be simply a consequence of the high reaction temperatures that can rapidly be obtained using this non-classical heating method, or may result from the involvement of so-called specific or non-thermal microwave effects (see Section 2.5). 

---