I Jerry F. Janssen
Yellow
Antioch College Ohio 45387
Springs,
II
The Photoisomerization of Azobenzene A tlc experiment for the undergraduate organic laboratory
Although molecular photochemistry is a growing field in organic chemistry, few undergraduate laboratory experiments employing photochemical transformations have yet been- described. The experiment described below is one which is easily performed and the equipment needs are well within the budget requirements of most undergraduate laboratory programs.' Hartley (1) discovered in 1937 that azobenzene was equilibrated to a mixture of trans-azobenzene and the then unknown cis-azobenzene by the action of sunlight. A later study by Fischer, Frankel, and Wolovslcy ((2 indicated that the composition of the equilibrium mixture depended upon the wavelength of incident radiation. Irradiation of benzene solutions of azobenzene with light of 365 mp wavelength produced mixtures containing the thermodynamically less stable cis-isomer in amounts greater than 90%, an excellent example of the principle of optical pumping (3). Procedure
Each student prepares six thin layer chromatographic plates on microscope slides using the rapid technique described by Peiffer (4). The glass slides are thoroughly washed with soap and water, rinsed with distilled water and then methanol and stood on end to dry. A suspension of 35 g of Silica Gel G (e.g., "Adsorbosil" available from the Applied Science Laboratories, P.O. Box 440, State College, Pennsylvania, 16801) in 100 ml of chloroform is prepared prior to the laboratory period and stored in a wide-mouth glass stoppered bottle. The student dips the slides in the suspension two a t a time, back to back holding them with forceps. The slides are used to stir the suspension which tends to settle, and are then withdrawn with a smooth motion. The speed with which the slides are withdrawn controls the thickness of the silica gel layer; rapid withdrawal leads to thin layers. The slide pair is separated and layed on a paper towel to dry. After all six slides have been treated in this manner, each is held for five seconds in the steam emanating from a steam bath to set the plaster of Paris hinder contained in the silica layer. Slides which are not so treated have layers which are more sensitive to mechanical shock. Prior to use the silica layer is activated by heating the slide for ten minutes over a hot plate. A square piece of wire gauze with the corners bent downward serves as a tray upon which the slides can be supported over the hot plate. A number of slides can be prepared by one 1 Adapted by permission from H m s ~ : G., , "Thin-Layer Chromatography with Adsorbents Woelrn," M. Woelrn, Esehwege, Germany.
student in a few minutes by this method and they can be stored in a microscope slide box in a dessicator for later use. A solution containing 1 g of commercial azobenzene (Eastman Organic Chemicals #704) in 50 ml of benzene is prepared prior to the laboratory period and is stored in a brown glass stoppered bottle. The student places a small spot of this solution on each of two prepared slides about 1 cm from the bottom of the slide and centered. This spotting is done using an ordinary melting point capillary into which the solution is drawn about 5 mm by capillary action. One slide thus prepared is placed in the locker where it is protected from light. The other is exposed to a source of ultraviolet light. If the source of light is suulight, the slides should be left for one hour. (Sunlight through windowglass is an acceptable light source; this was the light source used by Hartley in his initial investigation of the reaction.) If an ultraviolet source is used, good results are obtained after irradiation of the slide for shorter periods. A convenient and inexpensive uv source is a pair of germicidal "fluorescent" lamps mounted in a desk lamp and placed in the hood with proper eye protection for the students. After the slide has been irradiated, it is developed in a chamber containing 3: 1 cyclohexane-benzene ( V / V ) to a depth of cm. A strip of paper towel lining the inner wall of the chamber serves to maintain a saturated atmosphere in the chamber as the development proceeds. A beaker or wide-mouth bottle capped with a watch glass serves as an excellent developing chamber. After the solvent front has migrated to within 1 cm of the top of the silica layer on the plate, the position of the front is marked with a scratch and the plate is removed from the developing chamber. The unirradiated plate is similarly treated and the two plates are compared. The R, values of any spots observed are measured and recorded. Results
On each plate two spots of yellow compound will be seen. One spot near the starting point is due to the more polar (dipole moment 3.0 D) cis-azohenzene; the spot with an R,value of about 0.7 is due to the nonpolar (dipole moment 0.0 D) trans-azobenzene. The relative areas of the two spots on both plates is noted. (Depending upon the previous history of the azobenzene sample used, the cis-azobcnzene spot may be absent on the unirradiated plate.) The student is asked to propose a mechanism for the photoinduced isomerization of azobenzene and to discuss the relationship between these experimental observations and the common textVolume 46, Number 2, February 1969
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1 17
Ultraviolet spectra of cis- and trans-ombenzene [cyclohexane rdutiond.
book statement that rotation about a double bond is hindered. (A discussion of the mechanism of the photoisomerization of trans-stilbene is found on pages 1071-74 of Roberts and Caserio (5). The mechanism of the photoisomerization of azohenzene is still a subject of inquiry and the student may he interested in reading the studies by Zimmerman, et al. (6) and Fischer (7).) For the more enterprising student who wishcs verification of the identity of the two spots, a simple extension of the experiment may be performed. A plate is prepared by placing five or six spots of azobenzene solution horizontally across a tlc plate about 2 em from the bottom of the plate. These spots will spread slightly, forming a band of the compound about 1 cm wide.
1 1 8 / Journal o f Chemicol Educofion
This plate is irradiated and developed as before. Working in subdued light, the separated bands are scraped from the plate into separate 1 0 ml beakers by means of :t spatula. Five ml of ethyl ether is poured into each beaker to dissolve the azobenzene and the clear solution is filtered to remove the silica. Each solution is evaporated to dryness over a steam bath (avoid excessive heating). The residue is taken up in cyclohexane and the ultraviolet spectrum of each solution is obtained. Following the procedure outlined above, the author obtained the spectra illustrated in the figure which compare closely with those reported for cis- and t~ans-azobenxeneby Zimmerman (7). Since sub-milligram quantities of material are being handled, the student must exercise care throughout the experiment described immediately above and may have to repeat it to obtain satisfactory results. Literature Cited (1) HARTLIIY, H., Nature, 140, 281 (1937). (2) F I S C H ~ E., , FRANKEL, M., AND WOLOVSKY, R., J. Chem. Phgs., 23, 1367 (1955). (3) K \ N , R . O., "Organic Photochemistry," McCraw-Hill Co., New Yark, 1966, p. 20. (4) P W F F ~ J. ,J., Mikrochim. Acta, (19621, 520. ( 5 ) Ilon~,:n~s, J. U., A N D C A S ~ I OM, . C., "Baaic Principles of Orgenie Chemistry," W. A. Benjamin Co., New York, 1964, pages 1071-1074. ( 6 ) ZIMMERMAN, G., CHOK,LUE-YUNG,AND PAIK, UN-JIN,J . Am. Chtm. Sac., 80,3528 (1958). (7) FISCHER, E., J . Am. Chem. Soe., 90,796 (1968).
