MICROWAVE ASSISTED SYNTHESIS…

Objectives : Range of Microwave Microwave Evolution Instrumentation Introduction Range of Microwave Microwave Evolution Instrumentation Interaction of MW with Molecules Choice of Solvent Kinetics of Reaction (about Product) Applications Hoffman Elimination Hydrolysis Oxidation of Toluene

What is Microwave ? Microwave radiation is a form of energy, relatively low in the electromagnetic spectrum. As like other waves it consist of its own characteristic property.

Electromagnetic spectrum Light comparison Name Wavelength Frequency(HZ) Photon Energy (eV) Gamma ray less than 0.02 nm more than 15 EHz more than 62.1 keV X-Ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 keV – 124 eV Ultraviolet 10 nm – 400 nm 30 PHz – 750 THz 124 eV – 3 eV Visible 390 nm – 750 nm 770 THz – 400 THz 3.2 eV – 1.7 eV Infrared 750 nm – 1 mm 400 THz – 300 GHz 1.7 eV – 1.24 meV Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 meV – 1.24 µeV Radio 1 mm – 100,000 km 300 GHz – 3 Hz 1.24 meV – 12.4 feV

Microwave Representation…

Microwave Evolution… In 1986, Gedye and Giguere/Majetich found the application of Microwave irradiation to their reactions, as opposed to “conventional” heating techniques, such as heating a reaction on a hotplate, enabled more rapid product formation.

Instrumentation…

Interaction of MW with Sample… Microwave irradiation falls below x-rays, UV, visible, and infrared in the energy spectrum, with a frequency of 300 to 300,000 megahertz (MHz). Because microwave irradiation is at such a low frequency, below that of x-rays, UV, and even infrared, it will only cause bonds to rotate, not break.

Diagrammatic Representation…

Choice of Solvent… Every solvent and reagent will absorb microwave energy differently. They each have a different degree of polarity within the molecule. A solvent have a stronger dipole to cause more rotational movement in an effort to align with the changing field. Most organic solvents can be broken into three different categories ,

Solvent Chart…

Kinetics of Reaction…

Applications… General applications of Microwave Chemistry are, Microwave Assisted synthesis using Water. Microwave Assisted synthesis using Organic Solvents. Solvent free Reactions.

Microwave Assisted synthesis using Water… Hoffman Elimination : Quaternary Ammonium salts are heated at high temperature and the Yield is low. Use of Microwave irradiation has led to high yield.

Hydrolysis… Hydrolysis of Benzyl Chloride : Hydrolysis of Benzyl Chloride with water in Microwave oven gives 97% yield of Benzyl alcohol in 3 min. Whereas conventional method takes about 35 min.

Oxidation of toluene : Oxidation of toluene by conventional method takes 10 – 12 hr. but by Microwave irradiation the reaction completed in 7 min. with 40% yield.

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