Nonthermal microwave effects

The CMR and MBR are capable of operation in the presence of organic solvents. They have extended the useful operating temperature range for low-boiling organic solvents and have facilitated the development of new chemical processes that require moderate temperatures. Although debate continues to rage on the existence or otherwise of so-called nonthermal microwave effects [74—76], the principles of micro-wave chemistry now are well understood [33, 34]. 

Significant rate accelerations and higher loadings are observed when the micro-wave-assisted and conventional thermal procedures are compared. Reactions times are reduced from 12-48 h with conventional heating at 80 °C to 5-15 min with microwave flash heating in NMP at temperatures up to 200 °C. Finally, kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) rather than to a specific nonthermal microwave effect [17]. 

Solvent-free benzoylation of aromatic ethers has been performed under the action of microwave irradiation in the presence of a metallic catalyst, FeCl3 being one of the most efficient [101]. With careful control of the temperature and other parameters, nonthermal microwave effects have not been observed either in terms of yields or isomeric ratios of the obtained products (Eq. 49). 

A nonthermal microwave effect was not observed when identical temperature gradients were produced by classical heating and microwave irradiation and if the reaction temperature was strictly controlled. 

The short reaction time (1 min, 160 °C) in the benzoylation of anisole was probably a result of large temperature gradients rather than a nonthermal microwave effect. 

Other microwave-assisted SPOS processes reported in the literature are summarized in Scheme 12.8. The addition of isocyanates to amines bound to Wang resin, for example, was studied both under conventional conditions at room temperature and under the action of microwave irradiation in open vessels by use of a monomode instrument. By monitoring the progress of the addition by on-bead FTIR it was demonstrated that the microwave procedure proceeded significantly faster than the reaction at room temperature (12 compared with 210 min) [38], The temperature during the microwave irradiation experiment was not determined, however, so it is unclear if any nonthermal microwave effects were responsible for the observed rate-enhancements (Scheme 12.8a) [38]. 

Nonthermal Microwave Effects - Intersystem Crossing in Radical-recombination Reactions... 

Strauss, C.R., Microwave-assisted reactions in organic synthesis are there any nonthermal microwave effects Comments, Angew. Chem. Int. Ed., 2002, 41, 3589. 

While microwave-assisted reactions are well established in current synthetic methodology, nonthermal microwave effects have been shown not to be a factor in the observed rate enhancement with the ring-closing metathesis of 274 to form the azepine 275 (88% conversion 20 min, 100 °C) in this case the mthenium catalyst 276 was used (Equation 38) <2003JOC9136>. 

Microwave-assisted synthesis in general is likely to have a tremendous impact in the medicinal/combinatorial chemistry communities because it shortens reaction times, improves final yields and purities, and can carry out reactions that previously were thought impossible to do. It should be stressed that in general the rate enhancements seen in microwave-assisted synthesis can be attributed to the very rapid heating of the reaction mixture (flash heating) and the high temperatures that can be reached, rather than to any other specific or nonthermal microwave effect.40... 

The literature review of microwave-assisted or induced pyrolysis of plastics follows. In this section special attention is paid to the reactor configurations used, comparing them with the configurations found on more conventional pyrolysis equipment. The most important findings produced from this research are presented, including product yield, characteristics and composition. An analysis is presented to assess whether in any example there is evidence for nonthermal microwave effects promoting the pyrolytic reactions. 

Scientific studies have found that the differences between microwave and conventional pyrolysis go beyond the obvious difference in the source of heat. Other differences arise from the very high rates of heat transfer from the microwave-absorbent to the waste, the amount heat received by the primary pyrolytic products once they leave the absorbent bed and the highly reducing environment. These three aspects have been shown to have an important effect in the final products since they modify the extent of secondary and tertiary reactions. Moreover, the scientific studies have shown that a nonthermal microwave effect in these processes is unlikely to exist. Tests have showed the potential of the microwave-induced pyrolysis process for the treatment of real plastic-containing wastes and it is believed that a commercial process could be developed, for example, to recover clean aluminium from plastic/aluminium laminates. Other materials, in particular tyres, coal and medical wastes are very good candidates to be treated/recycled using microwave pyrolysis and there have been a considerable number patents filed with this goal in mind. 

The observed rate accelerations and sometimes altered product distributions compared to classical oil-bath experiments have led to speculation on the existence of specific or nonthermal microwave effects. " Historically, such effects were claimed when the outcome of a synthesis performed under microwave conditions was different from that of the conventionally heated counterpart. When reviewing the present literature, it appears that most scientists now agree that in the majority of cases the reason for the observed rate enhancements is a purely thermal/kinetic effect. Even though for the industrial chemist this discussion seems largely irrelevant, the debate on microwave effects is undoubtedly going to continue for many years in the academic world. Today, microwave chemistry is as reliable as the vast arsenal of synthetic methods that preceded it. Microwave heating not only reduces reaction times significantly, but is also known to reduce side reactions, increase yields, and improve reproducibility. 

The recent literature on microwave-assisted chemistry has reported a multitude of different effects in chemical reactions and processes and attributed them to microwave radiation. Some of these published results cannot be reproduced, however, because the household microwave ovens employed often have serious technical shortcomings. Published experimental procedures are often insufficient and do not enable reproduction of the results obtained. Important factors required for qualification and validation, for example exact records, reproducibility, and transparency of reactions/processes, are commonly not reported, which poses a serious drawback in the industrial development of microwave-assisted reactions and processes for synthesis of fine chemicals, intermediates, and pharmaceuticals. Technical microwave devices for synthetic chemistry have been on the market for a while (cf a.m. explanations) and should enable comparative investigations to be conducted under set conditions. These investigations would enable better assessment of the observed effects. It is, furthermore, possible to obtain a better insight into the often discussed (nonthermal) microwave effects from these experiments (Ref. [138] and Chapter 4 of this book). Technical microwave systems are an important first step toward the use of microwave energy for technical synthesis. The actual scale-up of chemical reactions in the microwave is, however, still to be undertaken. Comparisons between microwave systems with different technical specifications should provide a measure for qualification of the systems employed, which in turn is important for validation of reactions and processes performed in such commercial systems. 

Microwave-assisted solvent-free reactions have been used by Jenekhe [146] to synthesize quinoline derivatives. An important specific nonthermal microwave effect has been observed compared with conventional heating (Eq. 60). This MW effect is consistent with mechanistic considerations, because the rate-determining step is the internal cyclization depicted in Eq. (60) resulting from nucleophilic attack of the enamine on the carbonyl moiety occurring via a dipolar transition state. 

Significantly, microwave processing frequently leads to noticeable reduction in reaction times and higher yields. The rate improvements observed may simply be a consequence of the high reaction temperatures that can be achieved rapidly by use of this nonclassical heating method, or may result from the involvement of so-called specific or nonthermal microwave effects (cf Chapter 4 in this book). 

Reddy PM, Huang YS, Chen CT, et al. Evaluating the potential nonthermal microwave effects of microwave-assisted proteolytic reactions. J Proteomics. 2013 80 160-70. doi 10.1016/j. 

The ongoing debate distinguishes between thermal microwave effects and nonthermal microwave effects [3,8,10]. The passionate discussion goes on, but it is beyond the scope of this book. 

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