Specific Nonthermal Microwave Effects

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]. 

More interesting fundamentally is the very strong specific nonthermal effect of microwaves, as evidenced by comparison with classical heating. This effect grows as ester reactivity falls. 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

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... 

Microwave dielectric heating was initially categorized by thermal effects and nonthermal effects. Thermal effects are those which are caused by the different temperature regime which can be created due to microwave dielectric heating. Nonthermal effects are effects, which are caused by effects specifically inherent to... 

Differences between chemistry observed with microwave and conventional heating can often be attributed to the different transfer rate or spatial distribution of heat. Once appropriate temperatures are known in various parts of the system, conventional laws of thermodynamics or kinetics commonly apply. Such cases may be called microwave specific effects. However, there are also cases where it is proposed that an additional effect operates, perhaps through the action of the electric field 23 such effects are commonly called nonthermal or athermal microwave effects. Although their existence is controversial in fluid phases,6,24-26 there is a strong body of evidence supporting such effects in solid phases.27-29... 

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. 

As another consequence of the assumptions above, it might be foreseen that specific nonthermal MW effects could be important in determining the selectivity of some reactions. When competitive reactions are involved, the GS is common for both processes. The mechanism occurring via the more polar TS could, therefore, be favored by use of microwave radiation (Scheme 4.7). 

With the same type of molecule, nonthermal specific microwave effects were apparent in the Wilkinson complex-catalyzed orthoalkylation of ketimines [103], which occurred via initial transimrnation (Eq. 18) ... 

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. 

The reactions have also been performed under solvent-free conditions with considerable success and almost quantitative yields of the products [35a]. For example, the last entry in Table 7.2 was improved to 100% when irradiation was performed for 3 min. at 50 °C. Under solvent-free conditions, the existence of nonthermal specific microwave effects has been proposed on the basis of comparison with control reactions performed using conventional heating. 

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). 

What are microwave effects Microwave effects are usually effects which cannot be achieved by conventional heating. These microwave effects can be regarded as thermal or nonthermal. Thermal effects result from microwave heating, which may result in a different temperature regime, whereas nonthermal effects are specific effects resulting from nonthermal interaction between the substrate and the microwaves. 

The proposal of some authors on the operation of nonthermal effects is still controversial [120-122]. In the literature microwave effects are the subject of some misunderstanding. Most mistakes was recorded when authors considered that microwave effect means specific effect , i.e. a nonthermal effect. For that reason, let us discuss the matter in more detail. In heterogeneous catalytic reactions, differences between reaction rates or selectivity under microwave and conventional heating conditions have been explained by thermal effects. 

The second group supposes that during microwave irradiation a specific effect of microwave activation appears that leads to an increase of reaction rates for which the bulk temperature of the reaction mixture is inadequate to explain. Such effects are the so-called the nonthermal microwave effect or specific microwave effect. 

Microwave-mediated Leuckart reductive amination of carbonyl compounds was carried out (Loupy et al, 1996). They observed a strong specific (nonthermal) activation effect of microwaves. The yields obtained were excellent (75-97%) within short reaction time compared to conventional harsh conditions. 

The essential questions raised by the assumption of athermal or specific effects of microwaves are, then, the change of these characteristic terms (free energy of reaction and of activation) of the reaction studied. Hence, in relation to previous conclusions, five criteria or arguments (in a mathematical sense) relating to the occurrence of microwave athermal effects have been formulated by the author [25], More details can be found in comprehensive papers which analyze and quantify the likelihood of nonthermal effects of microwaves. This paper provides guidelines which clearly define the character of nonthermal effects. 

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