multiple reactions simultaneously, or in scaling up reactions that require identical
heating conditions.
1.2.5 SELECTIVE HEATING
Selective heating is based on the principle that different materials respond differently
to microwaves. Some materials are transparent to microwaves where as others
absorb these radiations. Therefore, microwaves can be used to heat a combination
of such materials, for example, the production of metal sulfide with conventional
heating requires weeks because of the volatility of sulfur vapors while rapid heating
of sulfur in a closed tube results in the generation of sulfur fumes, which can cause
an explosion. However, in microwave heating, since sulfur is transparent to microwaves,
only the metal gets heated. Therefore, reactions can be carried out at a much
faster rate with rapid heating, without the threat of any explosion.
The ability of metal powders to couple to microwave radiation has been used to
accelerated a range of solid-state reactions. Although metal objects cause extensive
arcing within a microwave cavity, metal powders couple in a more conventional
manner with the microwave radiations and experience high heating rates. This property
has been used for the synthesis of metal chalcogenides and metal cluster compounds
of the Group 5 and 6 elements.
1.2.6 ECOFRIENDLY CHEMISTRY
Reactions conducted through microwaves are cleaner and more environmental
friendly than conventional heating methods. Microwaves heat the compounds directly
and therefore, use of solvents in the chemical reaction can be reduced or in
some cases even eliminated. A method was developed to carry out a solvent-free
chemical reaction on sponge like material (alumina) with the help of microwave
heating. The reactants are absorbed on alumina and exposed to microwaves. It was
observed that they react at a faster rate than conventional heating. The use of microwaves
has also reduced the extent of purification required for the end products of
chemical reactions.
1.2.7 GREATER REPRODUCIBILITY
Reactions with microwave heating are more reproducible as compared to conventional
heating because of uniform heating and better control process parameters.
The temperature of chemical reactions can also be easily monitored. This is of particular
relevance in the lead optimization phase of the drug development process in
pharmaceutical companies.