Microwave heating solvent effects

Freitag and John [96] studied rapid separation of stabilisers from plastics. Fairly quantitative extraction (>90% of the expected content) of stabilisers from a powdered polymer was achieved by MAE within 3 to 6 min, as compared to 16 h of Soxhlet extraction for the same recovery. MAE and Soxhlet extraction have also been compared in the analysis of cyclic trimer in PET [113]. On the other hand, Ganzler et al. [128] compared the extraction yields for various types of compounds from nonpolymeric matrices for microwave irradiation with those obtained by the traditional Soxhlet or shake-flask extraction methods. Microwave extraction was more effective than the conventional methods, in particular in the case of polar compounds. As expected, the efficiency of the former is high especially when the extraction solvents contain water. With the high dipole moment of water, microwave heating is more... 

These results provide clear evidence for the existence of selective heating effects in MAOS involving heterogeneous mixtures. It should be stressed that the standard methods for determining the temperature in microwave-heated reactions, namely with an IR pyrometer from the outside of the reaction vessel, or with a fiber-optic probe on the inside, would only allow measurement of the average bulk temperature of the solvent, not the true reaction temperature on the surface of the solid reagent. 

Table 4.3 Microwave heating effects of doping organic solvents with ionic liquids (IL) A and B (data from [63]). a ...

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

We have investigated a number of reactions in polar solvents, most of which had been previously reported to occur more rapidly under microwave heating than classical heating in open vessels, to see if there are any significant MW rate enhancements, which could suggest to the involvement of a specific MW effect [19, 20]. 

The conversion of aryl iodides into aryl phosphonates, a useful precursor to aryl phosphonic acids, was performed in a Teflon autoclave by Villemin [51]. A domestic microwave oven was used for these experiments and the reaction times for classic heating were effectively reduced from 10 h to 4-22 min. The reactivity of iodides was good whereas the use of bromides resulted in lower yields and reactions with tri-flates were very slow (Eq. 11.35). It is notable that the reactions were brought to completion with short reaction times in a non-polar solvent. 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

The importance of hydrogen bonding is apparent from Table 1.1, which indicates that solvents with weak hydrogen-bonding capabilities have much shorter relaxation times. In addition, their lower viscosities reduce the interaction of molecules in the rotational process, which is induced by the microwave coupling and therefore will generally not heat as effectively in a microwave cavity. 

The interactive effect of the polymer and the solvent can be easily demonstrated in a very simple but crude microwave heating experiment, where no consideration is given... 

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

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