Diels Alder Reaction

The Diels-Alder Reaction is a reaction used in organic chemistry that builds rings very efficiently (1), this cycloaddition process allows for the stereoselective formation of cyclohexene rings possessing as many as four contiguous stereogenic centers (3). This reaction occurs without intermediates, in a single step, which explains the stereospecificity due to substituents not being given the chance to “switch around”. (1)These six membered rings are synthesized by reacting a dienophile with a diene, forming two new carbon-carbon sigma bonds in the form of the ring. The dienophile used in this experiment was Maleic anhydride (a cyclic dienophile) and the diene used was cyclopentadiene (a cyclic diene). Due to the fact that more than one stereoisomer may form, it is important to know that the major product will be the one with the transition state for the endo stereochemistry while the minor product will be for the exo stereochemistry.

The reason for this is because the reaction favors the product that involves the maximum overlap of pi electrons in its transition state, which in this case, is the endo isomer. To ease the concerns of orbital overlap between the HOMO (Highest Occupied Molecular Orbital) of cyclopentadiene with the LUMO (Lowest Unoccupied Molecular Orbital) maleic anhydride, reactants are used which possess electron withdrawing groups on the dienophile (maleic anhydride) and electron donating groups on the diene (cyclopentadiene). (2) Side reactions in this experiment include the reaction of cyclopentadiene with itself as it will dimerize to form dicyclopentadiene if kept at room temperature as well as the exo isomer of the product. Reaction Mechanism:

Experimental Procedure
To begin this experiment, a fractional distillation was set up in order to distill dicyclopentadiene. A 10 mL conical vial was filled with 5 mL of dicyclopentadiene and then heated to a steady boiling point (40-42 C). Cyclopentadiene was then placed on ice while the remaining reactant was prepared.. Maleic anhydride was then obtained (0.2 g) and dissolved in 1 mL of ethyl acetate, followed by addition of 1 mL hexanes to this mixture. 0.2 mL of the cyclopentadiene was finally added and reaction was observed with the formation of the crystals. Following the formation of the crystals, the solvent was removed via Pasteur pipette and then crystals were washed with cold petroleum ether. Crystals were then scraped onto a piece of filter paper to dry and then weighed. Finally, melting point was obtained and percent yield was calculated. Table of Chemicals

Cyclopentadiene

Melting Point: -40 C
Toxicity: Irritant, do not inhale
Maleic anhydride

Melting Point: 52.8 C
Toxicity: Irritant, flammable

Cis-Norbornene-5,6-endo-dicarboxylic anhydride

Melting Point: 165 C
Toxicity: Corrosive, health hazard
Results
5 mL of distilled dicyclopentadiene enabled my partner and I to obtain 0.2 mL of cyclopentadiene for the reaction. 0.2 g of Maleic acid was obtained and the product yielded was 0.168 g of Cis-Norbornene-5,6-endo-dicarboxylic anhydride. The melting point of this product was 138 C. The percent yield was then calculated: .1572 g / 66.1 g = .00238 mols cyclopentadiene

.2 g / 98.06 g =.00203 mols maleic anhydride (Limiting Reagent) .168 g / 164.16 g = .00102 mols cis-norbornene-5,6-endo-dicarboxylic anhydride .00102 mols / .00203 mols = 50.25% Yield
Discussion
In comparison to the literature value of the melting point for cis-norbornene-5,6-endo-dicarboxylic anhydride, 165 C, our actual melting point was much lower at 138 C. There are many reasons why this may have occurred some of which include flaws in the distillation process, flaws in controlling cyclopentadiene from dimerizing, not allowing ample time for reaction to occur, and insufficient drying of crystals following ether wash.

Our percentage yield for this reaction was also quite low at only 50.25 %. It is imaginable that this could be due to not being able to full amount of crystals that were yielded because some remained stuck in the vial. If left at room temperature, cyclopentadiene will react with itself (dimerize) to form dicyclopentadiene, which is a side reaction with this reaction. In order to avoid this, cyclopentadiene is kept on ice to prevent dimerization. The desired product would have been one that yielded crystals with a melting point much closer to the literature value of 165 C, ranging between 161-169 C, this would have meant a pure sample. Therefore, the desired product has not been attained in this experiment. Conclusion

The aim of this experiment was to perform a Diels-Alder reaction between cyclopentadiene and maleic anhydride and identify the product. Although this is what my partner and I did, it is very obvious based upon our results that the product obtained is impure based upon melting point values. Many of the techniques performed in this lab can be applied to other situations, the distillation apparatus is used quite often in organic chemistry labs as well as determination of melting point and percent yield. For this reason, it is beneficial that my partner and I know how to overcome the issues faced in this experiment in order to make sure the following experiments are performed with better technique in these areas. The desired product of a pure cis-norbornene-5,6-endo-dicarboxylic anhydride has not been attained, despite the impurity of our product I feel that the overall aspect of the experiment performed was understood very well and product purity could have been improved had the above flaws discussed been fixed.

References

1. Weldegirma, Solomon. “Diels-Alder Reaction: Synthesis of cis-Norbornene-5,6-endo- dicarboxylic anhydride.” Experimental Organic Chemistry. Print. 2. Myers, Kyle,. Roark, James. Diels-Alder Synthesis of Exo-Norbornene-cis-5,6-

Dicarboxylic Anhydride for Organic Chemistry Laboratory Instruction. Web.

<http://www.unk.edu/uploadedFiles/academics/gradstudies/ssrp/Myers.pdf>