In early days, it was recognized that microwaves could heat water in a dramatic
fashion. Cooking food with microwaves was discovered accidentally in the 1940
s. Domestic and commercial appliances for heating and cooking of foods began to
appear in the 1950 s. The appliance called “Radarange” appeared in the market in
1947, and it was proposed for food processing. The first domestic microwave oven
was introduced by Tappan Stove Company on 25 October 1955 but the widespread
use of domestic microwave ovens occurred during the 1970s and 1980s. The first
application of microwaves irradiation in chemical synthesis appeared in 1986.
The term ‘microwave’ is inseparably linked in our modern society to the rapid
heating or warming of foodstuffs. What is fascinating from a chemical synthesis
standpoint is the dynamic range of temperatures afforded by modern laboratory microwave
instrumentation. To date, most of the efforts have been focused on elevated
temperature transformations or reactions requiring heating. Synthetic transformations
unachievable through conductive heating have recently been realized using
microwaves as the energy source. Low temperature reactions via microwave energy
have been recently introduced with the key point being that gentler reaction conditions.
Gentler reaction conditions are especially important with respect to biochemical
applications, where the preparation of peptides (Murray et al., 2005; Murray
and Gellman, 2006), peptoids and oligosaccharides (Brun et al., 2006), etc. are of
interest.
Heating water in a closed vessel well above its boiling point produces supercritical
water. In this situation, the form of water is less polar and thus, it is more
effective in dissolving organic substrates. In addition, the increased use of water in
industrial settings is a popular notion in terms of green chemistry, as water is not
only low cost but environmentally more benign than any other traditional organic
solvent and when used in this context, it provides facile separation of the solvent,
organic reactants and products.
2.1 MICROWAVES AS ENERGY SOURCE
Microwave radiations are basically electromagnetic radiation, which are widely
used as a source of heating in organic synthesis. Microwaves have enough momentum
to activate reactants to cross the energy barrier and lead a reaction to completion.
Microwaves occupy a place in the electromagnetic spectrum between infrared
waves and radio waves. They have wavelengths between 0.01 and 1 m, and operate
in a frequency range between 0.3 and 30 GHz. The typical bands for industrial applications
are 915 ± 15 and 2450 ± 50 MHz. The wavelength between 1 cm and 25
cm are extensively used for RADAR transmissions and the remaining wavelength
range is used for telecommunications.
The entire microwave region is therefore not available for heating applications
and the equipment operating at 2.45 GHz, corresponding to a wavelength of 12.2