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Diels-Alder Reaction

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Category: Science

Autor: anton 09 November 2010

Words: 895 | Pages: 4

THEORY:

The Diels-Alder cycloaddition reaction was discovered by Otto Diels and Kurt Alder and is very useful in the synthesis of polycyclic compounds. The Diels-Alder reaction can be described as: [4+2] cycloaddition- a diene with 4 π electrons + 2π electrons from the dienophile; a pericyclic concerted reaction- meaning the reaction occurs in a single step (no intermediates) and involves a cyclic redistribution of bonding electrons.

In order for a Diels-Alder reaction to take place the diene must be a conjugated system oriented in the s-cis conformation; s-trans dienes can undergo the reaction only if there configuration permits free-rotation around one of the –C=C double bonds such that they can assume the s-cis conformation. Diencs locked in the s-cis configuration will react faster than those able to assume both s-cis and s-trans configurations; those locked in the s-trans conformation will not react.

The reaction is favored (proceeds faster) by the presence of electron withdrawing groups (ie Nitriles, Amines, Carboxylic acids, esters, aldehyde/ketone etc) in the dienophile and electron donating groups (ie methyl ether, alkyl, etc.) in the diene. Diels-Alder reactions are steriospecific (cis-alkenes form cis substituted and trans-alkenes form trans-substituted); this is called syn addition, the configuration of dienophiles is always retained in the product. The diene and dienophile react in such a way that the endo product is formed rather than the exo product.

TECHNIQUES:

Refluxing is a purification technique used when you need to heat a solution of reactants for a lengthened period of time in order to complete the reaction. It differs from distillation in that the “distillation and collection of the distillate are carried out in the same flask” (Nauman, 37). The solution is placed in a round bottom flask with a condenser placed vertically over it. As the flask is heated, the solvent boils and vapors rise in the condenser. The vapors are then condensed into liquid and they flow back into the same flask. In this experiment the condenser column was fitted with a drying column packed with calcium chloride (CaCl2) because the reagents / rxn are sensitive to air (esp H20 (g); the CaCl2 reacts with the vapor essentially dehydrating it. The temperature remains relatively constant and stirring is not needed because the two boiling chips that were placed in the mixture create enough agitation to ensure even boiling.

This experiment required two separate recrystalizations. After refluxing for the allotted time the distillate was poured while hot, to maximize the amount of adduct transferred, into an Erlenmyer flask and allowed to cool to room temperature before placing it an ice bath (We don’t Want Broken Glass and Product contaminating our ice bath!). While hot, the solvent keeps the solute dissolved, in a easily transferable liquid state, but loses it solvating ability at cooler temperatures, thus the adduct crystallizes. After its ice bath the solution needs to be suction filtered to separate it from the xylenes (solvent). With the apparatus setup, hose connected and filter paper wet with solvent and aspirator running, the solution can be poured semi-slowly into the buchner funnel. The crystals tend to cling to the bottom and sides of the flask so a spatula will probably be needed to scrape them out. Washing the flask and crystals with ice- cold xylenes completes this process. The resulting dried product was recrystalized using hot ethyl acetate. A hot plate was used to boil ethyl acetate to be poured into a heating beaker with a mixture of EtoAc and adduct until the product dissolved. A sequential suction filtration completed this recrystalization.

A Mel-Temp apparatus was used to determine the melting point of our product. Comparing the known melting point of a pure compound to that of your product is a quick and inexpensive, compared to x-crystalography, way to determine the purity of a compound. The Mel-Temp apparatus consists of a metal heating device that is controlled by a voltage dial; a thermometer, and a magnifying glass through which the user can view the sample melt. The temperature range extremes are recorded from when temperature increases occurs at a rate of 2-3°C/min until the sample is completely melted.

DISCUSSION:

We obtained 4.129 grams of anthracene and 2.022 grams of maleic anhydride in the initial steps of our procedure. The theoretical yield produced from the reaction was calculated to be 5.69722 grams of 9,10-Dihydroanthracene-9,10-α,β-Succinic Acid Anhydride. The actual yield obtained was 2.903 g, thus the % yield was 50.955 %. The actual and percent yields were indeed lower than they should have been due to a spillage of an unknown amount of reagent when setting up the reflux apparatus and less than 100% collection of the crystals between transfers. Our product’s melting point ranged from ~267-273°C indicating a relatively pure compound with a degree of impurity no doubt. The impurities could be correlated to a contaminant in the reagents or over-exposure to air.

CONCLUSION:

We would have liked to have seen the % yield come out higher; a comparison

of the % yield of the reaction performed ideally (perfectly) would give us a better inclination as to our degree of efficiency. Even though our melting point maxima indicated impurity, it was encouraging that the minima was very close to that of the pure compound.

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