Microwave chemistry is the science of applying microwave radiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid. Microwave heating occurs primarily through two mechanisms: dipolar polarization and ionic conduction. Polar solvents because their dipole moments attempt to realign with the oscillating electric field, creating molecular friction and dielectric loss. The phase difference between the dipole orientation and the alternating field leads to energy dissipation as heat. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material. Domestic and commercial microwave systems typically operate at a frequency of 2.45 GHz, which allows effective energy transfer to polar molecules without quantum mechanical resonance effects. Unlike transitions between quantized rotational bands, microwave energy transfer is a collective phenomenon involving bulk material interactions rather than individual molecular excitations. Microwave heating in the laboratory began to gain wide acceptance following papers in 1986, although the use of microwave heating in chemical modification can be traced back to the 1950s. Although occasionally known by such acronyms as MAOS (microwave-assisted organic synthesis), MEC (microwave-enhanced chemistry) or MORE synthesis (microwave-organic reaction enhancement), these acronyms have had little acceptance outside a small number of groups. Microwave chemistry is applied to organic chemistry and to inorganic chemistry.
Microwave effect There are three classes of microwave effects that are divided differently among sources:
Conventional thermal effects, which are the same as conventional heating. Microwave-specific heating effects, which are thermal effects that cannot be (easily) emulated through conventional heating methods. Examples include: (i) selective heating of specific reaction components, (ii) rapid heating rates and temperature gradients, (iii) the elimination of wall effects, and (iv) the superheating of solvents. These tend not to be controversial and invoke "conventional" explanations (i.e. kinetic effects) for the observed effects. Non-thermal microwave effects, which entail some non-thermal coupling of microwave energy directly onto energy modes within the molecule or lattice. Proposed in order to explain unusual observations in microwave chemistry, such effects are controversial. Older literature tends to lump (2) and (3) together into the specific microwave effects, effects that are different from conventional heating. Newer organic literature tends to only consider (2) when using this term. At the boundary of (2) and (3) are selective excitation of specific molecules/functional groups and stationary hot spots.
Specific heating Conventional heating usually involves the use of a furnace or oil bath, which heats the walls of the reactor by convection or conduction. The core of the sample takes much longer to achieve the target temperature, e.g. when heating a large sample of ceramic bricks. Microwave works via dielectric heating: the oscillating electromagnetic field causes molecular dipoles to rotate. Acting as internal heat source, microwave absorption is able to heat the target compounds without heating the entire furnace or oil bath, which saves time and energy. It is also able to heat sufficiently thin objects throughout their volume (instead of through its outer surface), in theory producing more uniform heating. However, due to the design of most microwave ovens and to uneven absorption by the object being heated, the microwave field is usually non-uniform and localized superheating occurs. Microwave flow reactors can overcome some of the limitations that affect performing microwave chemistry on large volumes. Different compounds convert microwave radiation to heat by different amounts. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than others (particularly the reaction vessel). Microwave heating can have certain benefits over conventional ovens:
reaction rate acceleration milder reaction conditions higher chemical yield lower energy usage different reaction selectivities
Binary systems A specific application in synthetic chemistry is in the microwave heating of a binary system comprising a polar solvent and a non-polar solvent obtain different temperatures. Applied in a phase transfer reaction a water phase reaches a temperature of 100 °C while a chloroform phase would retain a temperature of 50 °C, providing the extraction as well of the reactants from one phase to the other. Microwave chemistry is particularly effective in dry media reactions.
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