Chemistry Reference
In-Depth Information
Microwaves are not only useful as alternative methods of heating; there is also a good body
of evidence to suggest they can cause specific molecular activations [84]. Microwave
processing has now been shown to be effective at pilot scale and at large continuous
processing scale, for example in waste treatment [85]. Microwave radiation technology is
established in the food industry. It has also been suggested as a method of gasifying food
wastes [86]. Many research articles describe the use of microwave radiation to assist chemical
reactions: calculations show that microwave-assisted organic chemical reactions can be
considerably more energy efficient than reactions using conventional heating [87]. This is
another example of PI and should ideally be combinedwith flowsystems due to the limitations
of microwave penetration and in order to allow optimum continuous operation [88].
There are a number of methods by which microwaves are thought to enhance
reactions [79]:
1. Using a polar solvent to achieve rapid heating rates and elevated temperatures (greater
than the boiling point). This leads to conventional-style reactions with rate enhance-
ment through increased temperature. Heating is directly of the solvent, and therefore
there are no losses or time lag due to heating of the vessel.
2. Specific activation of polar or ionic compounds in the presence of nonpolar medium.
These highly energized reactants will then react with statistical probability when they
collide.
3. Acceleration of proton- or ion-catalysed reactions, due to the microwave electro-
magnetic field interacting with the dipole moment of dissociatable molecules through
enhancement of the disoctiation equilibrium.
4. Activation of the active centres of heterogeneous catalysts (so-called 'hot spots'). It
must be noted that this is a little-understood reaction and that no detailed explanation
or theory has been proposed regarding it.
Furthermore, due to the instantaneous nature of the heating, microwaves offer a major
advantage in controllability over conventional heating.
Microwave reactions have been successfully demonstrated for many different organic
reactions [89-91], including metal mediated catalysis [92], cycloadditions [93], hetero-
cyclic chemistry [94], rearrangements [95], electrophilic and nucleophilic substitution [86]
and reduction [96].
Case Study: Microwave-assisted Organic Synthesis.
The vast majority of synthetic
transformations that include organic materials and thermochemical conversions have
been attempted with microwave heating. In many cases, improved yields or reaction times
have been observed.
2.6.2.3 Summary: List of Green Principles Used in Microwave-assisted Organic
Synthesis
(3) Less Hazardous Chemical Synthesis.
(5) Safer Solvents and Auxiliaries.
(6) Design for Energy Efficiency.
(9) Catalysis.
(11) Real-time Analysis for Pollution Prevention.
(12) Inherently Safer Chemistry for Accident Prevention.
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