Reactions Accelerated by Microwave Radiation in the Undergraduate Organic Laboratory Shamsher S. Bari, Ajay K. Bose, Ashok G. Chaudhary, Maghar S. Manhas, Vegesna S. Raju, and Ernest W. Robb 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 imdergraduate organic chemistry laboratory by their long reaction time. Prolonged reaction times lead to student boredom and inactivity. A long reflux time or the requirement that a reaction stand overnight a t room temperature will make it impossible to wmplete a preparative experiment in a single threehour laboratory period. There have been many recent reports (14)of remarkable decreases in reaction time for reactions carried out under microwave irradiation in domestic microwave ovens. The rate increases appear to be due mostly to the rapid attainment of high reaction temperatures. However, an additional modest acceleration due to some effect specific to the microwave wavelengths employed has not been completely excluded. We have found that, with the proper choice of reaction solvent, accelerated reactions can be carried out safely with ordinary glassware in commercial microwave ovens (7, 8).The solvent used must have a dipole moment if it is to absorb microwave radiation. If the solvent also has a hieh boiline ~ o i n t sealed . reaction vessels are not necessary. This &&nates high pressures and the danger of explosions. o-Dichlorobenzene (bp 180 'C) or diglyme (bp 162 'C) can be used a s nonpolar solvents, while N,Ndimethylformamide ( D m , bp 153 T) works well when a more polar solvent is required. With a few milliliters of DNE in a large beaker, for example, boiling occurs only during the on portion of the oven's power cycle. Because glass does not absorb microwave energy, the upper parts of the beaker remain wol and condense the vapors formed. Refluxing is confined to the lowest centimeter of the walls of the beaker, and the oven's exhaust fan adequately removes the small quantity of vapor not condensed. It is not necessary that the reactants be dissolved at room temperature, so only the relatively small amount of solvent required for complete solution at the high reaction temperature need be used. This simplifies the reaction workup. Two reactions were selected to test the suitability of microwave acceleration in a n undergraduate teaching laboratory setting. The Diels-Alder reac-
tion is often included in the undergraduate laboratory curriculum; the reaction of anthracene with maleic anhydride would be a suitable example except that it requires a reflux period of 90 minutes as carried out classically (9).The preparation of phthalimido derivatives is a standard method for characterizing amino acids; for glycine, the procedure requires a two-hour reflux in toluene and the use of a Dean-Stark water trap (10). With microwave acceleration, both of these reactions are complete in one minute, and no special apparatus is needed. The accelerated reactions were carried out as laboratory experiments in the second semester of the sophomore Organic Chemistry course. Two microwave ovens (General Electric Spacemaster 111)were made available in the hood. Weighing out the starting materials and carrying out the irradiation required about 30 minutes for a laboratory section of 20 students. The remainder of the laboratory period was available for worknp, isolation, purification, and recording of the melting points and infrared spectra of the products. Apart from the savings in reaction time, the use of micmwave acceleration eliminated the need for heating mantles or oil heating baths, and reaction flasks and reflux wndensers with ground glass joints. The smaller volume of
m+r>-
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o +
0
H2KCHp.XOH
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solvent required contributed to a savings in cost and diminished the waste disposal problem. Experimental Reaction of Anthracene with Maieic Anhydride
A mixture of 1.8 g (0.01 mol) of anthracene and 0.98 g (0.01 mol) of maleic anhydride was ground thoroughly in a mortar and then transferred to a 250-mL beaker. ARer the addition of 5 mL of diglyme, the mixture was shaken gently. The beaker was covered with a watch glass and placed in the microwave oven. The irradiation was carried out for 90 s a t a medium power level (level 5). After the beaker was removed from the oven and allowed to cool to room temperature, the adduct crystallized out and was collected by suction filtration. The product, after washing with methanol (2 x 5 mL) and drying, had mp 258-260 Z. The average student yield was 80%. Preparation of Phthaloyigycine
Amixture of 1.48 g (0.01 moll of phthalic anhydride and 0.75 g (0.01 mol) of glycine was ground thoroughly in a mortar. The mixture was transferred to a 250-mL beaker and 5 mL ofN,N-dimethylformamide, followed by 0.25 mL of N-methylmorpholine was added. The beaker was covered with a watch glass and placed in the microwave oven. The irradiation was carried out for 60 s at a medium power
setting (level 5). The beaker was removed from the oven and, after the mixture had cooled to room temperature, 10 mL of water were added. The precipitated phthaloylglycine was filtered and recrystallized from 95% ethanol, mp 192-195 'C. The average student yield was 75%. Acknowledgment These experiments on curricular development were aided by a grant from the Howard Hughes Medical Institute. Literature Cited 1. (a) Giguere, R. J.;Bray, T. L.; Dunean, S. M.; Msjetkh. G. l%froh&n Lett. 1886, 27,494fr4948;(b)G@ere,R. J.;Namen,A. M.;Lapez,B. O.;Arepally,A.:Ramos, D. E.: Majetieh, G.: Defanw, J. TpfmhodmnLett.198'7,28,6553-6556: (4Oiguere, R. J. In Orgonfc Synfhosig Thoow and Applimfions: Hudlieky, T , Ed.; JAI Press hc.: Greenwich, CT, 1989:Vol. 1.pp 103-172. 2. (a) Cedye. R. N.; Smith,F E.; Weatswsy, K C. Can, J. Chem. 1988,66. 17-26; ib) Gedye,R.N.; Smith,F;Westaway K C.;AII,H.;Baldbera,L.;laberge,L.;RouseU, J. l%fmhdmnLen.1986,27,279:(dGedye. R. N.;Purnk,W:Weataway K C.Con. J Chorn. 1991,69.706. 3. Alloum, A. B.; l a b i d , B.; fillemin, D. J. J. Charn. Soc,Charn. Cornrnun. 1886,386. 4 . Berlan, J.; Giboreau, P.: Lefervre, 8.; Marchand, C. lbtmhdmn Lett. 1991, 32, 2363-2366. 5. Chen, S. T;Chiou, H. H.; Wang, K T. J. Cham Sae, Cham. Commun. 1990,807. 6. (a) Dutierrez, E.; Loupy A ; Bram, 0 . ; Rui-Hitzky, E. TPfmhodmn Lot* 1999,30, 945: (b) Bra,", c.;Laupy, A,; Majdoub, M.; Dutiemz, E.; Rui-Hitzky, E. Tarohedron 1990,46,5161. 7. Ba8e.A. K.;Manhas,M.S.;GhoshM.;Rsju,VS.;Tabei,IC:Urbanezyk-Lipkowska, 2.Hetemcycks ISSO, 30,741-744. 8. Manhao. M. S.; Ba", S. S.;Raju,V 8.;Ghosh, M.;Bose.A.K: 202ndACSNational Meeting, New York,August 1991, ORGN 195. 9. Dermer. 0.C.; King, J. J A m c Chem. Soe. 1841,63,3232. 10. Bose, A. K; Greer, F;Rice, C. C. J Org Chern. 1938.23.1335-1338.
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