Microwave-transparent solvents

Principles and Characteristics Pare et al. [475] have patented another approach to extraction, the Microwave-Assisted Process (MAP ). In MAP the microwaves (2.45 GHz, 500 W) directly heat the material to be extracted, which is immersed in a microwave transparent solvent (such as hexane, benzene or iso-octane). MAP offers a radical change from conventional sample preparation work in the analytical laboratory. The technology was first introduced for liquid-phase extraction but has been extended to gas-phase extraction (headspace analysis). MAP constitutes a relatively new series of technologies that relate to novel methods of enhancing chemistry using microwave energy [476]. 

The microwave rays travel freely through the microwave-transparent solvent (relative to the leaves) and reach the leaves. The latter - like many other food-related materials - are made of a multitude of pocket-hke cavities that are defined by the cells, glands, vascular vessels, and the like, all of which contain different chemical species and different levels of water. The microwaves interact selectively with the free water molecules and cause locahsed heating that give rise to a sudden non-uniform elevation in temperature with more pronounced effects where... 

Some ionic liquids are soluble in nonpolar organic solvents and can therefore be used as microwave coupling agents when microwave-transparent solvents are employed. For example, in Diels-Alder reactions, when adding ionic liquids to toluene, the temperature can reach 195 °C within 150 s of irradiation in contrast to 109 °C without ionic liquids [24]. Leadbeater et al. used this method to increase the rate of the Diels-Alder reaction (Scheme 11.1) (see Chapter 7 of this book). 

Fortunately, differences in microwave absorptivity generally have little impact commercial monomode units are able to heat effectively just about any pure solvent. Furthermore, as reactions generally have multiple components such as acid, base, or metal catalysts, and one or more reactants, reaction mixtures will often heat much more efficiently than the solvent alone. Finally, in the extreme cases where microwave units are unable to heat reactions due to poor substrate or solvent absorp-tivities, additives can be utilized that allow the bench chemist access to any solvent system. Most commonly, ionic liquids or reusable inserts such as silicon carbide or WeflonT have been used when microwave transparent solvents such as toluene or hexane must be employed for a particular reaction. 

The Microwave-Assisted Process (MAP ) technology uses microwaves, and solvents that are relatively transparent to microwaves, to extract chemicals from various matrices based on the temperature differential between the solvent and the target compound. According to the developers, the technology is applicable to soils and wastes containing polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), total petroleum hydrocarbons (TPH), and other organic compounds. 

Pressurized MAE in closed vessels This technique employs a microwave-transparent vessel for the extraction and a solvent with a high dielectric constant (electrical permittivity). Such solvents absorb microwave radiation and can thus be heated to a temperature exceeding solvent boiling points under standard conditions. Boiling does not occur, however, because the vessel is pressurized. This mode of operation is very similar to ASE—the elevated pressure and temperature facilitate extraction of the analyte from the sample. 

Atmospheric MAE system This second technique employs solvents with low dielectric constants. Such solvents are essentially microwave-transparent they thus absorb very little energy, and extraction can therefore be performed in open vessels. The temperature of the sample increases during extraction because it usually contains water and other components with high dielectric constants the process is thereby enhanced. Because extraction conditions are milder, this mode of operation can be used to extract thermolabile analytes. 

Figure 3.9. In the oven cavity is a carousel (turntable or rotor) that can hold multiple extraction vessels. The carousel rotates 360° during extraction so that multiple samples can be processed simultaneously. The vessels and the caps are constructed of chemically inert and microwave transparent materials such as TFM (tetrafluoromethoxyl polymer) or polyetherimide. The inner liners and cover are made of Teflon PFA (perfluoroalkoxy). The vessels can hold at least 200 psi of pressure. Under elevated pressures, the temperature in the vessel is higher than the solvent s boiling point (see Table 3.11), and this enhances extraction efficiency. However, the high pressure and temperature may pose safety hazards. Moreover, the vessels need to be cooled down and depressurized after extraction.

A Teflon autoclave was used in the transformation of aryl iodides to aryl phosphonates, useful precursors to aryl phosphonic acids, in a study made by Villemin [105]. With a domestic microwave oven, the reaction times were successfully shortened compared to those from classic heating from 10 hours to 4-22 minutes. Aryl iodides exhibited good reactivity while bromides gave lower yields and triflates very slow reactions (Scheme 36). It is interesting to note that the reactions were implemented with short reaction times in the nonpolar solvent toluene, which is essentially microwave-transparent [5]. 

To investigate these findings further the authors determined heating rates of the employed multimode instruments and the Discover unit, once again using toluene as solvent. After 10 min irradiation at a constant maximum power output for each microwave reactor, different final temperatures were measured (Fig. 17). Furthermore, it could be shown that the observed differences in temperature are not only related to the different heating efficiencies of the instruments but also to the specific vessel material [27]. Usually the vessel material itself is not completely microwave-transparent and therefore it is at least partially responsible for heating of the irradiated solvent via conventional thermal conduction [42]. 

Roberts and Strauss, 2005). As was described earlier, an added advantage to microwave chemistry is that often no solvent is required. In recent years, many commercial reactors have come on the market and some are amenable for scaling up reactions to the 10 kg scale. These new instruments allow direct control of reaction conditions, including temperature, pressure, stirring rate and microwave power, and therefore, more reproducible results can be obtained. For most successful microwave-assisted reactions, a polar solvent that is able to absorb the energy and efficiently convert it to heat is required, however, even solvents such as dioxane that are more or less microwave transparent can be used if a substrate, coreagent or catalyst absorbs microwaves well. In fact, ionic liquids have been exploited in this field as polar additives for low-absorbing reaction mixtures. 

The literature indicates that a significant proportion of microwave reactions have been carried out in an organic solvent. The solvent choice is dependent on the dipole properties of the reactants. If none of the reactants couples with microwaves, a solvent which does is needed. Some of the most used solvents that can be categorized as those essentially transparent to microwaves and those which absorb, and thus heat rapidly, are identified in Table 1. 

The assistance of microwave heating has been also proposed to accelerate RCM reactions using classic ruthenium-based catalysts. The reaction can be rapidly conducted in either ionic liquids, such as [bmim][BF4], or in a micro-wave transparent solvent such as dichloromethane. 

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