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					Reaction Accelerated by Microwave Radiation in the Undergraduate Organic Laboratory
Shamsher S. Bari, Ajay K. Bose, Ashok G.Chaudhary, Maghar S.Manhas Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 Many preparative reactions that are otherwise attractive are ruled out for use in the und ergraduate organic chemistry laboratory by their long reaction time. Prolonged reaction ti mes lead to student boredom and inactivity. A long reflux time or the requirement that a r eaction stand overnight at room temperature will make it impossible to complete a prepar ative experiment in a single three-hour laboratory period. There have been many recent reports of remarkable decreases in reaction time for reacti ons carried out under microwave irradiation in domestic microwave ovens. The rate increa ses appear to be due mostly to the rapid attainment of high reaction temperature. howeve r, an additional modest acceleration due to some effect specific to the microwave wavelen gths employed has not been completely excluded. We have found that, with the proper choice of reaction solvent, accelerated reactions ca n be carried out safely with ordinary glassware in commercial microwave ovens. The solve nt used must have a dipole moment if it is to absorb microwave radiation. If the solvent a lso has a high boiling point, sealed reactions vessels are not necessary. This eliminates hig h pressures and the danger of explosions. o-Dichlorobenzene(bp180℃)or diglyme(bp162 ℃) can be used as nonpolar solvents, while N,N-dimethylformamide(DMF,bp153℃) work s well when a more polar solvent is required. With a few milliliters of DMF in a large beake r, for example, boiling occurs only during the on portion of the oven's power cycle. Becaus e glass does not absorb microwave energy, the upper parts of the beaker remain cool an d condense the vapors formed. Refluxing is confined to the lowest centimeter of the wall s of the beaker, and the oven's exhaust fan adequately removes the small quantity of vap or not condensed. It is not necessary that the reactants be dissolved at room temperatur e, so only the relatively small amount of solvent required for complete solution at the hig h reaction temperature need be used. This simplifies the reaction workup. Two reactions were selected to test the suitability of microwave acceleration in an under graduate teaching laboratory setting. The Diels-Alder reaction is often included in the unde rgraduate laboratory curriculum; the reaction of anthracene with maleic anhydride would b e a suitable example except that it requires a reflux period of 90 minutes as carried out cla ssically. The preparation of phthalimido derivatives is a standard method for characterizin g amino acids; for glycine, the procedure requires a two-hour reflux in toluene and the us e of a Dean-Stark water trap. With microwave acceleration, both of these reactions are co mplete in one minute, and no special apparatus is needed. The accelerated reactions were carried out as laboratory experiments in the second sem ester of the sophomore Organic Chemistry course. Two microwave ovens(General Electri

c Spacemaster III) were made available in the hood. Weighing out the starting materials a nd carrying out the irradiation required about 30 minutes for a laboratory section of 20 stu dents. The remainder of the laboratory period was available for workup, isolation, purificati on, and recording of the melting points and infrared spectra of the products. Apart form the savings in reaction time, the use of microwave acceleration eliminated th e need for heating mantles or oil heating baths, and reaction flasks and reflux condenser s with ground glass joints. The smaller volume of solvent required contributed to a saving s in cost and diminished the waste disposal problem.

Reaction of Anthracene with Maleic Anhydride A mixture of 1.8g(0.01 mol) of anthracene and 0.98g(0.01mol) of maleic anhydride wa s ground thoroughly in a mortar and then transferred to an 250-mL beaker. After the addit ion of 5 mL of diglyme, the mixture was shaken gently. The beaker was covered with a wa tch glass and placed in the microwave oven. The irradiation was carried out for 90s at a m edium power level (level 5). After the beaker was removed from the oven and allowed to c ool to room temperature, the adduct crystallized out and was collected by suction filtratio n. The product, after washing with methanol(2 x 5 mL) and drying, had mp 258-260℃. Th e average student yield was 80%. Preparation of Phthaloylgycine A mixture of 1.48g(0.01 mol) of phthalic anhydride and 0.75g (0.01 mol) of glycine wa s ground thoroughly in a mortar. The mixture was transferred to a 250-mL beaker an d 5 mL of N,N-dimethylformamide, followed by 0.25 mL of N-methylmorpholine was adde d. The beaker was covered with a watch glass and placed in the microwave oven. The irra diation was carried out for 60s at a medium power setting (level 5). The beaker was remo ved from the oven and, after the mixture had cooled to room temperature, 10 mL of wate r were added. The precipitated phthaloylglycine was filtered and recrystallized from 95% e thanol, mp 192-195℃. The average student yield was 75%.

These experiments on curricular development were aided by a grant from the Howard H ughes Medical Institute.

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