Microwave solvent selection

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... 

The results obtained to date have led to the conclusion that microwave radiation causes no degradation of the extracted compounds, unless an excessively high temperature arises in the vessel. However, a specific effect of microwaves on plant material has been found. Microwaves interact selectively with the free water molecules present in the gland and vascular systems, leading to rapid heating and temperature increase, followed by rupture of the walls and release of the essential oils into the solvent. Similar mechanisms are suspected in soils and sediments, where strong, localized heating should lead to an increase in pressure and subsequent destruction of the matrix macrostmcture. 

Solvent selection. One of the most important considerations when designing amicrowave-assisted reaction is whether or not a solvent is actually needed for the reaction. Some reactions will not be successful under solvent-free conditions however, since the solvents are typically disposed of at the end of reactions, the elimination of solvents from chemical reactions is a step forward when designing sustainable chemical reactions. The vast majority of molecules containing functional groups will have a dipole moment and absorb microwave irradiation without the addition of a solvent. If a solvent is required for the success of the reaction, a minimal amount of solvent should be used. These near-solvent-free reactions still significantly reduce the amount of solvent used by the synthesis. 

Chapter 1 provides an introduction to modem synthetic chemistry with emphasis on topics such as solvent selection, catalyst choice, and a discussion of common challenges associated with functional group synthesis. This chapter also includes a detailed discussion of how to adapt conventionally heated reactions to microwave-assisted versions. 

For the microwave-assisted experiments, both solvents were replaced by 1,2-dichlorobenzene, as it couples very effectively with microwaves (loss-tangent (tan 5) at 20 °C 1,2-dichlorobenzene 0.280 as compared to 0.101 for chlorobenzene). Diels-Alder reactions of 3-methoxy or 3-phenyl pyrazinones with DMAD were performed at a pre-selected maximum temperature of 200 °C, whereas the intramolecular reaction of alkyne tethered pyrazinone required a higher temperature (220 °C). The yields obtained under microwave irradiation are comparable with those obtained under conventional conditions, while for the dihydrofuropyridinone the yield was improved from... 

The solvent, microwave energy applied, and extraction time selected are the main parameters controlled in MAE. The user should use proper extraction vessels and equipment in MAE because very high pressures can be achieved and explosions may result if appropriate precautions are not taken. 

Microwave-assisted extraction (MAE) of analytes from various matrices using organic solvents has been operative since 1986 [128], In this process microwave energy is used to heat solvents in contact with a solid sample uniformly and to partition compounds of analytical interest from the sample matrix into the solvent. The way in which microwaves enhance extraction is not fully understood. The main factors to consider include improved transport properties of molecules, molecular agitation, the heating of solvents above their boiling points and, in some cases, product selectivity. 

The principle of FMW involves the heating of both the solvent and the matrix by wave/matter interactions. The microwave energy is converted into heat by two mechanisms dipole rotation and ionic conductance. The heating is, therefore, selective with only polar or moderately polar compounds susceptible. Due to the use of low microwave energy the structure of target molecules remains intact. 

The advantages associated with the MAP technology as compared to conventional and automatic Soxhlet methods are considerable (Table 3.33). In MAP high sensitivity and selectivity by fractionation are achieved using different extraction media with similar, or better, linearity and reproducibility parameters. One of the principle features of the process is the lower temperatures observed in the microwave-extracted materials in contrast to volumetric heating usually experienced in traditional solvent procedures. These lower temperatures... 

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. 

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