cis-and trans-Stilbenes: Chromatographic Separation and Photochemical lsomerization Samuel G. Levine, Kathleen Doubek Barboriak, and Ho S. Cho North Carolina State University, Raleigh, NC 27695 Although the preparative chromatographic separation of geometric isomers usually taken ul b e ~ anonlrivial challence even in most research laboratories, we have found that the-senaration of cis- and trans- (Z- andE-I stilbenes can be ~~-~ made the basis for a timely experiment in the undergraduate oreanic chemistrv laboratorv. The experiment is performed about midway in the first semester of the course, coinciding with the student's introduction to cis-trans isomerism in hislher study of alkenes. The separation is accomplished on a column of alumina. Elution with hexane provides cis-stilbene (mp -5 "C) and suhsequent elution with a much smaller volume of hexaneethyl acetate furnishes the less mobile trans isomer (mp 124 O C ) . The two isomers show the same relative mobility by thin layer chromatography (TLC) on silica gel (hexane eluant; cis, Rr 0.27, trans, Rr 0.20) and this analytical method mav be used to follow the column chromatoera~hicsevaration. Chemical identification of the indivijuai isomers is accomplished by a photochemically induced equilibration experiment in which the liquid isomer is converted to a mixture that consists almost entirely of the solid isomer. The latter should, then, he considered the more stable, thus, trans. ~~
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insert cut 73265 Apparatus and Materials The chromatography columns, prepared from 18-mm-0.d. Pyrex tuhing, were 300 mm long and constricted a t one end. A cotton plug wedged into the constriction servedto support the column of alumina. The flow rate was controlled by a screw clamp on ashort length of amber latex tubing attached to the constricted end. The adsorbent, Fisher 80-200 mesh "nontoxic" alumina, was purified in 400 g batches by washing with ethyl acetate and drying a t 110 "C. This procedure was necessary in order to achieve good separation. Plates for TLC were 7.5- X 2.5-cm Baker-flex silica gel IB2-F. After development, the plates were viewed under short-wavelength ultraviolet light. ExDerimental Procedure The column was prepared by slowly adding 20 g of alumina to the eottnn-nlweed ehromatoeraohv tube which was half filled with ..... .--.hexane. A solution containing 50 mg each of cis- and trans-stilhene in 0.5 mL of toluene and 2.0 mL of hexane was carefully loaded onto ~~
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the column. The flow of hexane was continued at a moderate rate nntil el~jtion of the cis .~ .~~ .~~ ~~-isomer was comolete (TLC). Aooroximatelv .. 160rnl.of heaane wasrequircd. I.ater rernwalofsolvcnt r~sultedin the recovery uf:N-:K rng ofrmstillrenp that was pure hy the criteria dTLC andlH NMH, thesignals fi,r the vinyl protonsofthecisand trans isomers appear at 6.55 and 7.11 ppm, respectively. Further elution of the column with 20% ethyl acetatehexane led to the recovery of 35-40 me of crvstalline trans-stilhene that was in some cases e&.aminated-witha trace (TLC) of the cis isomer. Recrystallization from a small amount of hexane gave beautiful crystals that were pure by TLC and gave a satisfactory melting point. Preparatory to the isomerization experiment, the recovered cisstilhene, in 1-2 mL of hexane was filtered through a Pasteur pipet containing about 1 em of silica gel into a 4-mL screw-capvial. After removal of solvent (gentle air or nitrogen jet), 10 drops of stock iodine solution (10 mg/mL in hexane) was added. The vial was then capped and placed on the platform of an overhead projector. The separation of beautifully formed crystals of trans-stilhene began within a few minutes. Confirmatory melting point and TLC experiments may be done on the crystals. The mother liquor was also found to consist mainly of the trans isomer. ~~~~
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Dlscusslon There is a certain historic sienificance to the exveriment described here. The s u c c e ~ s f u l ~ e ~ a r a tof i o cisn and transstilbene on alumina was revorted in 1942 bvL. Zechmeisterl. a pioneer in column chromatography. As one of the very first examples of the column chromatoara~hic .. . separation of colorless substances, it demonstrated the power and generality of the method. Our students' nvpreciation for the technique is reinforced with a later experiment in which a rign nard reaction product is purified by chromatographic separation from the biphenyl byproduct. The student may be asked to speculate as to the higher chromatographic mobility and much lower melting point of cis-stilbene compared to the trans isomer. It seems likely that both effects result from the known2 nonplanarity of the cis isomer (steric hindrance). A detailed student vrocedure for this experiment as well as additional helpful information for the instructor may be obtained by writing to the Secretary of this Department.
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Zechmeister, L.: McNeely, W. H. J. Am. Chem. Soc. 1942, 64, 1919. 2Malda,K.; Muszkat, K. A,; Sherafi-Oxeri,S. J. Chem. Soc. Perkin 11, 1980, 1282.
Volume 65
Number 1 January 1986
79
