Photochemical and Thermal lsomerization of trans and cisl,2-Dibenzoylethylene: A Microscale Approach An organic chemistry experiment involving the photoisomerization of gram quantities of trans-1,4-diphenyl-2to the cis isomer was described by Silversmith and Dunson in this butene-l,4-dione (trans-1,2-dibenzoylethylene) Journal.' Fessenden and Fessenden, among others, include such an eaperiment in their text.? Mayo, Pike, and Butcher We have introduced into our program an eaperiment have described a microscale photoisomerization e~periment.~ demonstrating the photochemical isomerization of milligram quantities of trans-1,2-dihenzoylethylenetothe cis isomer hy sunlight. Also included is the thermal conversion of the cis isomer to the trans isomer and the evaluation of hoth conversions by thin-layer chromatography. The latter procedures are not seen in some similar microscale experiments. Recoveries of 65-80% are seen a t each stage with overall recoveries of over 50%possible. No special microscale glassware is required, and savings of over $100 for a class of 30 students can he seen over similar conventional experiments. This procedure allows the study of two different mechanisms of isomerization in a little more than one three-hour laboratory period. Procedure is as follows: In a 13- X 100-mm test tuhe 100 mg of trans-1,2-dihenzoylethylene(Aldrich Chemical Co.) is dissolved in 5.0 mL of 95%ethanol with stirring and heat. After cooling, the tube is corked and placed where it will receive direct sunlight for a period of 1to 7 days. The colorless crystals of the cis compound are then isolated using a Hirsch funnel. After weighing, the crystals are returned to the test tuhe, and 4.0 mL of 95% ethanol plus a drop of concentrated hydrochloric acid are added. The tube is heated with stirring until all the solid dissolves and the solution is yellow. After cooling in an ice hath, the yellow trans-1,2-dihenzoylethyleneis recovered using a Hirseh funnel. To evaluate the extent of isomerization, samples of the mother liquor may be taken using disposable micropipets and spotted on a silica gel TLC plate with fluorescent indicator (Kodak 13181). The plate may then he developed with methylene chloride and the spots visualized with ultraviolet light. UV-visible spectroscopy may also be incorporated into this experiment since the wavelength of maximum absorption of the cis isomer is 260 nm while the wavelength of maximum absorption of the trans isomer is 269 nm and it tails into the visible portion of the spectrum.
Presented a t the 21st Great Lakes Regional Meeting of the American Chemical Society, Symposium on the Microscale Organic Lahoratory, June 12,1981. Silversmith, E. F.; Dunson, F. C. J. Chem. Educ. 1973,50,568. 2 Fessenden, R. J.; Fessenden, J. S. Techniques and Experiments for Organic Chemistry; Grant: Boston, 1983: pp 31% 321. Mayo, D. W.; Pike, R. M.; Butcher, S. S. Microscale Organic Laboratory; Wiley: New York, 1986; pp 81-84. Dennls V. Klemm and Alllla Tuncay Indiana University Northwest Gary, IN 46408
Volume 66
Number 6
June 1989
519
