Ernest F. Silversmith and Fay C. Dunson Moraan State Coilecle Baltimore, Mavland 21239
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Photochemiml and Thermal Interconversion of ds and trans Isomers An organic laboratory experiment
Modem laboratory manuals for first-year organic chemistry frequently contain a n experiment in photochemistry. These experiments include the photoreduction of benzophenone (1-3), the photodimerization of trans-cinnamic acid (4). the photobromination of chloroform (51, and the photoisomeriz&ion of trans-azohenzene (1, 6, 7). We have had g o d success with the photoisomerization of tram-
1,4-d~phenyl-2-butene-l,4-dione (trans-1,2-dibenzoylethylene) to the cis isomer. The experiment has two advantages over the one with azobenzene. First, the cis isomer can be thermally reconverted to the trans, showing that the latter is thermodynamically more stable. Second, the student can readily obtain gram quantities of both isomers for further testing, such as the determination of melting points and the running of spectra. The photochemical conversion of trans-1,4-diphenyl-2butene-1,Cdione to the cis isomer in solution was apparently discovered by Paal and Schulze (8a), who had earlier assigned cis and trans structures to the colorless and yellow isomers, respectively (8b). These assignments were supported by the work of Conant and Lutz (9). The photoisomerization has also been described by a number of other workers (10). We found that a 2-3 hr irradiation period gives satisfactory results. Extremely long periods should be avoided, as this may lead to other reactions (IOc, IOd, 11, 12). The thermal conversion of cis to trans, using a variety of conditions, is also known (8a. 9, 13). We found that heating with ethanolic hydrochloric acid works well. The isomers can be readily identified by their colors, melting points (see Procedure), and infrared and ultraviolet spectra (14). The experiment provides a fine example of the photochemical conversion of a thermodynamically more stable compound to a less stable one (the rough analogy to photosynthesis can he pointed out). The more sophisticated students might wish to speculate as to why the ultraviolet absorption maximum and the carbonyl band in the infrared are a t higher wavelengths for the trans isomer than for the cis (14).
568
/ Journal of Chemical Education
Procedure The tram-1,4-diphenyl-2-buGne-1.4-dione was purchast?d from Aldrich Chemical Company; it can be used as is or puri~fiedby bailing 5 g with 250 ml 95% ethanol and 1 ml concentratsd HCI for 5 min, adding 100 ml water, boiling, decanting while hot and coaling. This yields 3.5 g canary yellow crystals, m.p. 11:3-115T (renorted (IOa) mm. 111°C). Using a 125-mlErlenmeyer flask, 1.00 g of the trans isomer was dissolved in 60 rnl 95% ethanol by heating. The flask was corked loosely and irradiated for 2-3 hr by a 275-W sunlamp at a distance of 2-3 in. The mixture was allowed to cool and the solid cis isomer was recovered by vacuum filtration. Washing with a small amount of ethanol and air drying yielded 0.60 g (60%) of colorless crystals, m.p. 137-138'C (reported (10a)m.p. 13S°C). A mixture of 0.20 g cis isomer, 10 ml 95% ethanol, and 1 drop concentrated HC1 was boiled in a test tube for 5 min. Ten milliliters of water was added and the mixture was cooled and filtered to give the trans isomer, m.p. 113-115'C. in quantitative yield. The infrared spectra can be conveniently run on 0.5 M chlomform solutions, using 0.1 mm cells. The carbonyl bands appeared at 6.12and 6.22p for the cis and trans isomers, respectively. Literature Cited (I) Helmbmp. G. K.. and Johnson, W. H., Jr., "Selected Experiments in Owganie Chemistry," (2nd Ed.), W. H. Fneman and Co., San Francisco, 1968, pp. 32 and 171. (2) Brew&r. R. Q., Vanderwed C. A,. and MeEvan. W. E., "Unitized Exporimenrr in Organic Chemistry;' (3rd Ed.), Van Nmtrand Reinhold Co., New York. 1970. p. 187. (3) Mohring, J. R., and Nceken. D. C., "Labaratmy Experiments in Organic Chemis. try,"ReinhddBmk Com. NevYork. 1968. p. 138. (4) M o m , J. A,, and Dalmrrle, D. L.. "Experimental Methods in Organic Chemistry." W. B. Ssunden Co., Philadelphia. 1971. p. 161. 151. h n b l a f t . D. H.. and Davis. G. T.. "Laborston, Course in Oreanic Chemistrv," ~ l l ~ ~I"=..d ~ o~s t o~IWI, i ~ p. nzoi. . ( 6 ) Brieger, G., "A Laboratory Manual for M o d m Organic Chemiaw," Harper and Row, NawYork, 1969, p. 153. (71 Fishel. D. L.. and Fort, R. C.. "Modern Experimental Organic Chemistry," The Me~millanCo.,NeuYork. 1971. p. 179. (81 (a) Pad. C., and Schulze, H., E e l , 35. 168 (1902): (b) Pad. C.. and SehulZe, H., Be,., 33.3795 119W). (91 Conant. J. 6.. and Lutz. R.E.. J. A m a Chem. Soe., 45.1303 (1923). (10) (a1 Kolbr, H., Pssternsk, R., and Halban, V. H., H d u . Chim, Acts., 23. 512 (1946): lb) Cauzzo. G.. Mazmcstto. U., and Foffani. A,, Bull Soc. Chem 0.1p, 71. 838 (19621: ( 4 Zimmerman. H. E..Dun. H. G. C.. Lewis. R. G.. and Bram, S., J A ~ D I Chem. Sac. 84. 4149 (19621; (dl Zimmerman, H. E.. and Hull, J. J., J. Amer Chem. Sor.. 92.6515 (19701. (11) Zimmorman, H. E.. Dorr. H. G., Givens, R. S.. and Levis, R. G., J . Amrr. Chem. Sac., 69.1863(19671. (121 Griffin, G. W.. and0'Connell.E. J., J A m w C k m . Sac.. 84,4148i1962). (13) (a) Lutz, R. E., and Bailey, P. S., J. Amer C k m . Soc., 67. 2229 (19451: (bl Lutz, R. E., end Gilloapie. J. S., Jr., J. Amsr Cham. Soc., 72. 244 (1950); (rl Campan d i , M., Marrucato, U., and Toffani. A,, Ann. Chim. (RomsJ. 54, (3-41. 195 11W). (111 Kuhn, L.P.,Lutz,R. E.,andBsuer,C.R., J Amer Chrm Sor.. 72.5058(19501.
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