Microwave frequency The Use of Microwave Irradiation in
Organic Synthesis
Microwave heating refers the use of electromagnetic waves ranges from 0.01m to 1m wave length of certain frequency to generate heat in the material. These microwaves lie in the region of the electromagnetic spectrum between millimeter wave and radio wave i.e. between I.R and radio wave. They are defined as those waves with wavelengths between 0.01 metre to 1meter, corresponding to frequency of 30GHz to 0.3GHz.
Microwave effect vs. thermal effect
Although the first publications concerned with the possibilities of use of microwave irradiation in organic synthesis appeared in 1980’s and in polymer chemistry even earlier at the end of 1960’s, the sudden growth of interest in the application of microwave irradiation in almost all fields of chemistry took place at the end of 1980s. Nowadays, there is hardly find any reaction that has not been attempted under microwave conditions. The application of microwaves in chemistry is therefore so attractive that form the very beginning it was realized that a number of chemical process can be carried out with a substantial reduction in the reaction time in comparison to conventional process. Reactions that usually take many hours or days, under influence of microwave irradiation can be run in considerably shorter time of several minutes or even seconds .These phenomena are not fully understood yet; however, there are two groups of theories that are proposed to explain the reduction of the reaction time under microwave conditions in comparison with process under conventional conditions.

MICROWAVE SYNTHESIS- A GREEN TECHNOLOGY
The term “green chemistry” is defined as “the invention, design and application of chemical products and processes to reduce or to eliminate the use and generation of hazardous substances”. Green chemistry can diminish the need for other approaches to environmental protection. Ideally, the application of green chemistry principles and practice renders regulation, control, clean-up, and remediation unnecessary, and the resultant environmental benefit can be expressed in terms of economic impact. Historically, chemists thought that compounds react only in the liquid state or if dissolved. This has made solvents common in chemical syntheses, however, many compounds used as solvents were found to be environmentally unfriendly. The problem associated with waste disposal of solvents has been overcome by performing reactions without a solvent under microwave irradiation (MWI). Coupling of MWI with the use of mineral-supported catalyzed reactions, under solvent-free conditions, provides clean chemical processes with the advantage of enhanced reaction rates, higher yields, greater selectivity, and greater ease of manipulation. These expeditious and solvent-free approaches involve the exposure of neat reactants to MWI in conjunction with the use of supported reagents or catalysts35.
Microwave reactors in Organic Synthesis (Multimode reactors):
In the multimode version, the walls of the relatively large reactor space reflect the irradiation, spreading it throughout the entire microwave cavity. Domestic microwave ovens as multimode reactors are the most common instruments used in organic synthesis. Since they are comparatively inexpensive and readily available, using domestic microwave cavity has done a lot of satisfying organic synthesis. In household microwave ovens only time and the power irradiation during this time can be varied as reaction parameters. Thus, the temperature is undetermined and increases steadily during irradiation. A possible but insufficient method to control the temperature or pressure is the on and off switching of the microwave field within a given time interval. In modern laboratory microwave systems, however, computer controls, which allow setting of the attainable temperature or pressure as limiting parameters, are state of the art. This feature is important with regard to safety aspects of handling chemicals and is also crucial for both reproducibility and the scale-up of reactions. Assistant Lecturer. Ekhlas Hammadi