E. G. Nosh Ferris State College Big Rapids, Michigan 49307
I
I
A Novel Photochemistry Experiment Using
4
Several recent articles in this Journal (1-4) have pointed out the need for laboratory experiments in organic photochemistry. Although some laboratory manuals (e.g., (5, 6 ) ) offer photochemical experiments, these have been limited to a few examples primarily using sunlight as the energy source. A more broadly based photochemical study would require a photochemical reactor with the corresponding expense for large classes. We would like to present a two-step experiment incorporating a novel, but general, photochemical reaction that is rapid, does not require special equipment, and which yields an interesting product worthy of further study. The first step of this experiment is a Diels-Alder reaction that can be used to compare the reactivity of cyclopentadiene (Ia) and hexachlorocyclopentadiene (Ib).
Dieb-Alder ~e"ctio"
IIIa,b
When this reaction is carried out as a thin film of compound (IIIb) on an aluminum foil-backed glass plate, the photocyclization is complete in 2-3 hr of direct sunlight. The progress of this facile reaction can be conveniently followed visually by a color change from yellow to grey or off-white, by thin layer chromatography, or by spectroscopy. For example, compounds (IIIa) and (IIIh) have carbony1 absorption a t 1660 cm-1 while the cage compound VI absorbs 1755 cm-'. The high melting points of compounds (Va) and (Vb), 240°C and 300°C are indicative of the symmetry of the cage structure. (7) Experimental
Caution: Benzaquinone is a skin irritant and lachwmator. In the case of cyclopentadiene (Ia), the reaction is exothermic and the addition must he done carefully to avoid formation of the diadduct. With hexachlorocyclopentadiene (Ib) the reaction is endothermic (1 hr of reflux in Odichlorohenzene a t 170-175°C) and only the monoadduct (IIIb) is formed. Quite clearly, the halogen substitution has resulted in an electron deficient diene requiring a higher activation energy for the (4 + 2) cycloaddition. The yellow crystalline compounds (IIIa) and (IIIb) are stable if stored in the dark and if acidic or basic conditions are avoided. In the presence of acid or base a n enolization takes place (eqn. (2)) to the corresponding hydroquinones (IVa,h).
7
?"?
YH
Hexachloroeyclopentadiene has toxic properties. Students
using these compounds should wear gloves and all weighing6 should be done in a hood. Compound IIIb. A solution of 2.54 g (0.01 MI of hexachlorocyelapentadiene and 1.08 g (0.01 M )of p-benzoquinone in 10 ml of o-dichlarobenzene is refluxed for 1 hr. The resulting solution is cwled and then filtered giving 2.25 g of yellow crystals, mp 190195°C. This product is generally satisfactory for the next step. The compound may be purified by recrystallization from chloroform. Compound Vb. 1.0 g of compound (W)is dissolved in about 10 ml of chloroform and slowly poured onto an 8 x 8-in. glass plate with aluminum foil backing. To be most effective, a very thin film of (IIIh) should remain an evaporation of the chloroform. After 3 hr of direct sunlight the crystals become grey and are scraped off the plate. The compound melts, with decomposition. at 300°C. Literature Cited
--
Na,R=H b,R = C1
IIIa,R = H b.R=CI
Examination of models of compounds (IIIa) and (IIIb) with the endo-stereochemistry reveals that the twodouble bonds are in close proximity to each other. Exposure of the yellow crystals of compound (IIIb) to sunlight results 2) photocyclization to form the in a facile internal (2 cage compounds (Va,b), eqn. (3).
+
Jsnssen.J. F., J. CHEM. EDUC.. 46. 117 119691. Borak, R. E., and Alvarer, V. E., J. CHEM. EDUC., 47,589 119701. Evans. R. F., J. CHEM. EDUC.,48.168119711. VanDeMark. M.R., andKumler. F. F.J. CHEM.EDUC.50.512119731. Fieser. L. F., "Experiments in Orsanie Chemistry," 3rd Ed.. 0. C. Heath, n...~oo., " ~ ~ ,= n u ",a, ? 161 M a h r i ~J. R., snd Neckcrs, D. C.. "Laboratory Exporimenfr in Organic Chemin~ t r y ; ' ~ ~ i n h o ~ d c o r pN. . ~ W ori it. , 9 6 8 , ~138. (71 Cmkmn. R. C., Ccundwell.E., andHudec, J.. C h e m lnd.. IW3-4,119681.
111 (21 131 14) (51
.""".
.
Presented in part at the 160th National Meeting of the American Chemical Society, Chicago, Illinois, September 1970.
Volume 51, Numberg, September 7974
/
619
