Thermal microwave effects

Most, if not all, microwave biological effects and potential medical appHcations are beheved to be the result of heating, ie, thermal effects. The phenomenon of microwave hearing, ie, the hearing of clicking sounds when exposed to an intense radar-like pulse, is generally beheved to be a thermoelastic effect (161). Excellent reviews of the field of microwave bioeffects are available (162,163). 

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... 

In all cases, besides resulting in good to excellent yields, the microwave-assisted multistep syntheses resulted in much faster reactions compared to earlier published procedures at atmospheric pressure under conventional heating conditions. It is also noteworthy that in some cases the strong thermal effect due to graphite/microwave interaction, can efficiently be used for the synthesis of heterocyclic skeletons, especially benzothiazoles but, in fact, there is no general rule and some reactions performed in the presence of solvent may sometimes be more convenient than the same dry-media conditions. 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Wu et al. [12] used both microwave (MW) and ultrasound (US) methods individually and in combination to examine the combined effect. The rapid thermal effect of MW could be seen on polar chemicals and more OH radicals were produced due to US. Microwave irradiations have shown enhanced degradation effect when applied with sonication in absence of additional catalyst though the rate increased more in presence of H2O2. The rate order was found to be MW-US > MW > US. 

In general, most reactions that can be carried out under thermal heating can be performed and accelerated by microwave irradiation. As discussed in Section 2.2, the efficiency of the microwave heating is highly dependent on the dielectric properties of the reaction mixture. Most results suggesting rate enhancements and improved yields can be explained in terms of simple thermal effects. However, for two main reasons, some reactions may not be suitable for performance in micro-wave reactors ... 

In conclusion, is it necessary to obtain a microwave athermal effect to justify microwave chemistry Obviously not - it is not necessary to present microwaves effects in a scientific disguise. There are many examples in which microwave heating results in specific time-temperature histories and gradients which cannot be achieved by other means especially for solid materials. Hence, rather than claiming nonther-mal effects it is better to claim a means or a tool which induces a specific thermal history. 

The acceleration of reactions by exposure to microwaves results from material-wave interactions leading to thermal effects (which can easily be estimated by temperature measurement) and specific (not purely thermal) effects. Clearly, a combination of these two contributions can be responsible for the effects observed. 

The origin of specific microwave effects is twofold - those which are not purely thermal and a special thermal effect connected with possible intervention of hot spots . 

Alkylation of several substituted benzoic acid salts with n-octyl bromide was performed under solvent-free PTC with excellent yields (95 %) within a very short reaction time [93] (2-7 min). Oil bath heating (A) led to yields equivalent to those produced under the action of microwave irradiation, which thus revealed only thermal effects in the range of temperature used (145-202 °C) (Eq. 39). 

This is a typical representative example of an enhanced microwave-specific effect related to the difficulty of the reaction, which presumably proceeds via a later and later TS. Whereas essentially thermal effects are observed (around 200 °C) with methyl and octyl benzoate, a microwave-specific effect is increasingly apparent with hindered esters and becomes optimal with mesitoyl octanoate (Eq. (43) and Tab. 3.18) [95]. 

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

The existence of results that cannot be explained solely by the effect of rapid heating has led authors to postulate the existence of a so-called microwave effect . Hence, acceleration or changes in reactivity and selectivity could be explained by a specific radiation effect and not merely by a thermal effect. Several reviews have collected synthetic results that have been attributed to the microwave effect [5 b, 38, 39]. 

Photochemical Reactions in the Microwave Field -Thermal Effects... 

Saillard, R., Poux, M., Berlan, J. and Audhuypeaudecerf, M., Microwave-heating of organic-solvents -thermal effects and field modeling, Tetrahedron, 1995, 51, 4033. 

In general, the reasons for rate-enhancements in microwave-assisted transformations in comparison to conventional heating are not always fully understood. Some authors have postulated a specific non-thermal microwave effect for those effects that could not be rationalised as a simple consequence of superheated solvents and higher reaction temperatures. Stadler and Kappe therefore carried out a kinetic comparison of the thermal coupling of benzoic acid to chloro-Wang resin at 80° C, with the microwave-assisted coupling at the identical temperature of 80°C and otherwise identical reaction parameters. However, the reaction rates for the two runs were quite similar and the small observed differences could not be attributed to non-thermal effects. In order to confirm this hypothesis, the authors also carried out coupling experiments with... 

Thermal effects that reduce reaction times and which can be observed for the reactions under microwave irradiation are related to different temperature regime under microwave conditions in comparison to conventional conditions. However, they can result in a seemingly faster course of chemical reactions, the proper temperature measurement and its analysis for the entire sample of the material (i.e., the bulk reaction mixture) leads to the reaction rates that and comparable to reaction rates observed under conventional conditions. Three factors that can cause thermal effects are considered [3] ... 

All the mentioned above examples prove that for the investigated reactions the influence of microwave irradiation was limited to pure thermal effects (i.e., higher pressure and/or bulk temperature of the reaction mixture, development of temperature gradient, overheating of solvents) that can be responsible for seemingly higher reaction rates under microwave conditions. However, a careful consideration of all the factors that influence the reactions under microwave conditions can eliminate most of these effects, there are several of reactions in which the influence of only thermal effects do not suffciently explain the enhancement of yield and rate of the reactions. 

In order to eliminate the influence of temperature on the rate of a chemical reaction, a reaction mixture of phthalic anhydride with amino acids was placed in a block of ice and then irradiated under microwave conditions. Ice was used to cool the reaction mixture because opposite to water it is transparent to microwaves (e 3.2, c" 0.0029 at 25°C for 2.45 GHz) [26]. The reaction product was formed after 3 min. of irradiation while under conventional conditions the reaction was conducted in a boiling toluene solution for 1.5 h. However, since in such a case microwaves interact directly with the reaction mixture and temperature was not monitored during the experiments, it was stated that the increase of the reaction rate is not only due to thermal effects [27]. 

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