Microwave Assisted Reactions Under Benign Conditions

Abstract

Microwave chemistry has an edge over conventional heating methods for conducting chemical reactions, and is emerging as the preferred technology for performing chemical reactions. Microwave technology enable chemists to achieve cleaner and more efficient chemical reactions with higher yields, compared to conventional heating methods. The possibility of performing reactions in a very short time period by direct interaction of microwave energy with the reaction mixture as opposed to the indirect transfer of energy by utilizing an oil bath certainly can be considered “green”, not only because of the reduced energy consumption, but also it enable us to do the reaction using water as solvent or even under neat conditions. Water behaves as pseudo organic solvent at high temperature. During the present study, Mannich base, benzyl alcohol, Diels-Alder adduct, alkyl thiocyanates have been synthesized by the use of domestic microwave oven and the reactions were complete in very short interval as compared to thermal conditions. These reactions were studied by the use of microwave irradiations under solvent-less condition or by the use of water as benign solvent. Mannich base has been synthesized by using secondary amine and hydrogen chloride in water instead of using hydrochloride salt of secondary amine (which requires anhydrous conditions) in the domestic microwave oven, followed by amine exchange with another amine using ethanol as solvent. Another reaction viz., Diels Alder adduct was studied under microwave radiation and was complete within 3 minute as compared to conventional heating method, which required 2 hour of refluxing. Cross Cannizzaro reaction of aldehyde leads to two products, one is alcohol and other as acid. Reaction of benzaldehyde, paraformaldehyde and barium hydroxide octahydrate in microwave leads to benzyl alcohol. Reaction has been done in solvent less condition. Other substitution reactions studied are by the use of alkyl halides (bromooctane and benzyl chloride) and potassium thiocyanate in aqueous media without the use of phase transfer catalyst and were complete within 15- 20 minutes under microwave irradiation.