Diels-Alder Reaction: Experiment
Written by Vijay
In this experiment, cyclopentadiene and maleic anhydride were reacted in solution to produce a single organic product in crystal form. Cyclopentadiene is composed of a five-carbon ring where two of the carbon-carbon bonds of the ring are double bonds; the double bonds are separated by a single σ bond, making the double bonds conjugated. Maleic anhydride is has a five-membered ring with ring’s oxygen atom bonded to two acyl groups (R-C=O), and where the R-group is the remaining two ring carbons in a double bond. A 4+2 cycloaddition occurs between the electron-rich diene, cyclopentadiene, and the electron-poor dienophile, maleic anhydride. The result is a six-membered ring bridged by cyclopentadiene’s single sp3 hybridized carbon, and with the rest of the maleic anhydride as substituents to the ring.
Percent yield observed during the first week of experimentation was 89.9% of the crystalline product; pipetting solution away from the crystal product and air drying was the method of purification. For the second week of experimentation, the percent yields observed were 56.5% and 57.8% respectively for Samples A and B respectively. Sample A was purified by vacuum filtration but otherwise prepared under the same procedure as the sample from Week 1. Sample B was purified by vacuum filtration, but also included an additional 0.2 mL of petroleum ether in the reaction solution prior to addition of cyclopentadiene. Vacuum filtration appeared to lower yield because a purer product of lesser mass is prepared. The Week 1 product likely has a higher percent yield figure due to being incompletely dried by pipetting and air drying. Increasing the amount of petroleum ether reduces the solubility of the final product in solution; reducing the final product’s solubility causes an equilibrium shift to crystallize more of the final product. The change from Sample A to Sample B thus increased the yield by reducing solubility of crystalline product. Changing the nonpolar solvent petroleum ether to hexane caused the largest effect on yield based off the Wiki data. Petroleum ether is composed of a mixture of larger alkanes (more than six carbons), whereas hexane is only composed of six-carbon alkanes. The non-polar product should experience greater solubility in petroleum ether than in hexane due to the greater Van der Waals forces in the petroleum ether. Since the petroleum ether is composed of larger alkanes, there are more intermolecular forces than are present in hexane; this consequence means that the environment of petroleum ether is slightly more soluble for polar molecules. Increased solubility of the product indicates a decreased yield, as less of the final product will crystallize. Nonetheless, the yield of product is still low due to the following: losses due to the mechanism of transfer and cyclopentadiene forming dicyclopentadiene. The first issue could be solved by devising a method of transferring the filtered and dried crystal product from the filtration funnel or beaker in a manner that conserves the product. This issue may be no more than practicing the technique of transferring product and finding the most efficient way of doing so. The second issue, a reaction of cyclopentadiene with another molecule of cyclopentadiene via Diels-Alder mechanism, removes reactant for the desired reaction from solution; this issue can be solved by mixing the reaction solution well, adding cyclopentadiene slowly, and keeping the cyclopentadiene at low temperature to prevent dimerization.
A sample of the Week 1 product was tested by melting point analysis and found to have a melting point range of 152°C-155°C. The expected melting point value for the adduct is 164°C-165°C. The reaction has gone to completion because the melting point of the observed product is well above the melting points of the reactants and solvents involved; if the reaction had not gone to completion, the melting process would have begun much earlier due to maleic anhydride melting. Since the observed melting point is lower than the expected melting point, the melting point may appear lower due to the presence of impurities. Any impurities that lead to a solution of the product and impurities cause a depressed melting point.
Cyclopentadiene is a good diene for the Diels-Alder reaction because of its substitution and its locked s-cis conformation. Due to cyclopentadiene being substituted, the diene can draw on the electron density of the methylene in between the diene. Also, being in a ring, the cyclopentadiene is locked into the s-cis conformation that facilitates the Diels-Alder Reaction. Maleic anhydride functions well as a dienophile because of its electron-withdrawing groups and relatively small size. Electron-withdrawing groups (the acyl groups) remove electron density from the dienophile’s double bond, which enhance its reactivity with electron-rich dienes. Maleic anhydride’s small size also ensures that collisions will yield reactions often. Endo is favored over exo due to the reaction being under kinetic control; kinetic control indicates that the lower activation energy is favored. If the reaction were thermodynamically controlled, the overall more stable product, the exo product, would be the major product.