William J. Sheppard Swarthmore College Swarthmore, Pennsylvania
A Diels-Alder Reaction Experiment
Although some recent elementary organic chemistry 1 a b o r a t . 0 ~texts include the ~ i e l s Alder reaction,' we wanted one which would illustrate the reversible nature of the reaction and which could be completed in one afternoon laboratory session. With this in mind, the original method of Diels and Alder for the reaction of cyclopentadiene with maleic anhydride2 was modified for use in the elementary organic laboratory. Endo Bicycl0[2,2,1]hept&ene-2,3dicarboxylic anhydride is prepared by the Diels-Alder
"Dicyclopentadiene"
Cyclopentadiene
reaction of cyclopentadiene with maleic anhydride in benzene, followed by recrystallization from mixed solvent. Cyclopentadiene is prepared from commercial dicyclopentadiene by reversal of the Diels-Alder reac-
Cyclopentadiene Maleic Anhydride
endo-Bicyclo[2.l.l]hept - 5 - ens - 2,3dicarhovylic anhydride
For example, ROBERTSON, G. R.,AND JACOBS, T. L., "Labar:ttory Practice of Organic Chemistry," 4th ed., Macmillan, New York, 1962, pp. 299-300. 1 DIELS,O., AND ALDER,K., Ann., 460,98 (1928).
40 / Journal of Chemical Education
The ExDerimenl
Set up a fractional distillation assembly, for example, a side-arm distillmg tube loosely packed with brass scouring sponge. The receiver should be cooled with an icebath to reduce the tire hazard. Place 15 ml of practical or technical grade dicyclopentadiene in a 200 ml round-bottom flask, attach, and heat moderately. Drive off the cyclopentadiene which forms on heating as fast as possible without flooding or foaming into the column, but do not exceed 6S°C a t the top of the column. Collect 7 ml of the distillate: no forerun need be collected. If the distillate is cloudy, dry it with a few grams of calcium chloride and decant. Weigh 6.0 g of maleic anhydride and grind it to a tine powder with mortar and pestle; reweigh the powder after grinding if necessary. Avoid touching the powder or breathing t,he dust since it can irritate the skin and respiratory tract. Suspend in a 125 ml Erlenmeyer flask the maleic anhydride in 25 ml of dry benzene and cool, but do not freeze in an icebath (Note A). While still cooling, add, with swirling, a solution of 6.0 ml of cyclopentadiene and 5 ml of benzene (Note B). Swirl for several minutes-until a pasty precipitate appears. Warm the flask for 5 min on the steambath, then remove and add 30 ml of limoin. b~ 60-75°C (Note C). Swirl in the icebath u n 4 thoroughly chilied, then filter with pressthe crystals and rinse first the reaction flask and then the crystals with 5 ml of ice-cold ligroin, Remove the crude adduct and dace it in the same flask previously used (Note D). Dissolve in the minimum amount of boiling ethyl acetate on the steambath (Note E). Remove from the bath and add the minimum amount of ligroin needed to produce a few crystals. Heat on the steambath until the crystals dissolve. (If a few small crystals remain in spite of several minutes boiling, filter hot (Note F).) Allow the solution to cool slowly to room temperature, then cool the flask in the icebath and filter with suction. Press down the crystals and rinse with 5 ml of ice-cold ligroin. Spread the crystals on a tared watchglass and allow to dry in the air. When the odor of solvent is gone, record the weight and bottle quickly to avoid reaction with atmospheric moisture. Determine the melting point. (Diels and Alder reported 164-165'C; this v ~ i c a student l yield is 70% work. 167-16S°C.) A t ". .Note A. Benzene straight from the drum is usually good enough. If not, it can be dried by distilling from it the water azeotrope. That is, distil until the distillah is not cloudy (about 5% of the total) and use the pot material. Note B. The reaction is quite exothermic. If the materials are mixed with no solvent, the reaction goes with explosive violence. However, if the solutions mentioned here are handled according to the directions there is no hazard. If the two solutions are poured together without swirling and without cooling,
the benzene will boil and foam over. When this happens a transitory yellow color is observed, which is probably due to the pi complex that forms initially. For a better illustration of this phenomenon, use benzene solutions of anthracene and tetra~yanaethylene.~ Arote C. Ligroin, petroleum ether, or mixed hexanes, all hoiling 6&75'C, may be used. Note D. As an alternative a t this point, the corresponding diarid can be made and recrystallized as follows. Add 20 ml of water and heat to boiling. After a few minutes the solid will melt and mmtly dissolve. If necessary, add more water to complete solution. Allow the solution t o cool to room temperature, then cool in the icebath. Filter the crystals by suction, wash with 5 ml of ice-cold water and dry in an oven a t llOSC on a trtred watchglass. When i t reaches constant weight, record the yield and determine the melting point. Diels and Alder reported 177-179'C; this work, 182-185°C with decomposition.) Note E. All low boiling solvents should be bested on the steambath, not over an open flame. Ethyl acetate has a tendency to superheat; a boiling chip, t o be removed later, is advisable. Note F . This material is the acid formed when the anhydride reacts with water. Comments
This experiment went well unless the student got water into the reaction. In this event, the alternative conversion to the diacid, Note D, is recommended. Both the anhydride and the diacid form very pretty crystals. A simplified mechanism involving the principle of maximum overlap of r bonds had been presented i n lecture and was driven home in the prelaboratory discu~sion.~The preparation of cyclopentadiene by SCHIMELPFENIG, C. W., J. CHEM.EDUC., 36. A379 (1959). 'For s. detailed discussion see WOODWARD, R. B., AND KATZ, T . J., Tetmhedron, 5 , 70; Tetrahedron Letters, 5, 19 (1959).
the dedimerization helped to illustrate the reversible nature of the reaction, which had been mentioned in the lecture. The students were also introduced to endo-ezo isomerism, which they comprehended readily. They were interested in the nomenclature of the bicyclic compounds, but were not required to learn it. Before starting work on this (or any other experiment), each student made a table in his laboratory notebook listing the amount of each material used, the number of moles used, physical constants such as melting point, boiling point, solubility (to the extent available in the usual handbooks). Likewise the physical constants of the products would be recorded if available. This allows the student to see whether the reagents are used in the stoichiometric ratio or not. He is expected to know why the particular ratio was chosen. The physical constants help the student to understand why the procedural details are chosen as they are. Special projects starting with this compound might include the complete conversion to the cis-cyclopentane1,2,3,4-tetracarhoxylicacid by potassium permanganate oxidation. The double bond is quite reactive and might be hydrogenated by the use of palladium and tetrali, thus avoiding the use of pressure a p p a r a t u ~ . ~ For the student with only a few minutes' time, a search of Beilstein for the physical constants involved would be a useful introduction to this important literature source. For a larger literature problem, a search for the method of proof of structure should he interesting. WILEN, S. H., A N D KREMER,C. B., J. CBEH. EDUC., 39, 209 (1962).
Volume 40, Number I, Junuory 1963
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