Nonthermal effects

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Reports of sterilisation (qv) against bacteria by nonthermal effects have appeared, but it is generally beheved that the effect is only that of heating (164). Because microwave heating often is not uniform, studies in this area can be seriously flawed by simplistic assumptions of uniform sample temperature. 

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. 

A series of 1,4-dithiocarbonyl piperazines has been synthesized from aldehydes, piperazine, and elemental sulfur under the action of microwave irradiation and solvent-free conditions. An important nonthermal effect of the radiation was revealed [72] (Eq. 19). 

Nonthermal Effects of Microwaves in Organic Synthesis Nucleophilic aromatic substitutions... 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

Since there appeared to be strong evidence for a nonthermal effect in this type of reaction, we repeated the reaction of o-phenylenediamine 34 (Scheme 4.13, Rj = R2 = H) with ethyl acetoacetate 35 (R = CH3) [19], which was one of the reactions reported by Soufiaoui [53] to give the diazepine only on MW heating. However, when the same reaction mixtures were heated forlO min with the same temperature profile, almost identical yields of the diazepines were obtained by MW and classical heating. Later, this was also found to be the case in the reaction of 34 with ethyl benzoylacetate 35 (R = Ph). 

However, the possibility of the participation of nonthermal effects in MW-assisted reactions in nonpolar solvents is still an open question. Loupy et al. [55] observed an increase in yield and purity of the diazepine 36, in the reaction of ethyl acetoacetate with o-phenylenediamine using monomode MW reactor with focused MW heating, when compared with conventional heating with the same temperature profile. 

Because observed rate enhancements are usually small, or zero, nonthermal effects do not seem to be important in MW heated reactions in homogeneous media, except possibly in some reactions of polymers and reactions in nonpolar solvents. Relatively few studies have been conducted on MW-assisted reactions of polar reactants in nonpolar solvents. Also, since there is some disagreement as to whether or not these reactions are accelerated significantly by MW, in comparison with conventionally heated reactions at the same temperature, more research on the effect of MW irradiation on the rates of these reactions is required. Nonthermal effects may, however, explain the more substantial MW rate enhancements in solvent-free reactions on solid supports [44] (see Chapt. 5) and solid state reactions [68, 69]. 

The effect of solvent on regioselectivity was attributed to nonthermal effects, which are favored in nonpolar solvents and under solvent-free conditions, where products formed via more polar transition states would be expected to predominate. 

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. 

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. 

A 6- to 48-fold rate enhancement was observed for this reaction. The authors suggested that a nonthermal effect might account for this enhancement [56],... 

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). 

These microwave effects can be regarded as thermal. The proposal of some authors on the operation of nonthermal effects is still controversial (see Chapt. 3). 

L. Perreux, A. Loupy, Nonthermal Effects of Microwaves in Organic Synthesis. In Microwaves in Organic Synthesis, A. Loupy (Ed.), Wiley-VCH, Weinheim, 2002. 

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