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The Diels Alder Reaction of Anthracene and Maleic Anhydride

Lead Author: Amber Sanders Reviewer: Elliott Burnett Editor: Lois Metuge

Introduction The formation of 9,10-dihydro-ethanoanthracene-11,12-dicarboxylic anhydride is the result of a Diels-Alder reaction.1 The purpose of this experiment was to react anthracene, a conjugated diene with 4 pi electrons, with maleic anhydride, a dienophile with 2 pi electrons.1 Successful completion of this reaction in the lab would result in the formation of a cyclic product as seen in Figure 1.1

Figure 1. The basis of a Diels-Alder reaction is the result of combining electrons from a conjugated diene and a dienophile shown here.1 In this reaction, a conjugated diene with 4 pi electrons in a cis formation forms three new bonds with a dienophile, an alkene with 2 pi electrons as seen in Figure 1.1 These three new bonds are characteristically known as two single bonds between two carbons and one carboncarbon double bond.1 As seen above, the transition state consists of orbital overlapping, hybridization changes, and the delocalization of electrons.2 The formation of 9,10-dihydroethanoanthracene-11,12-dicarboxylic anhydride is modelled after the process above and takes place in the presence of xylene and heat as seen in Figure 2.1

Figure 2. Anthracene, a conjugated diene, and Maleic Anhydride, a dienophile, react to form 9,10-dihydro-9,10ethanoanthracene-11,12-dicarboxylic anhydride.1

Table of Reagents3 Compound Melting Point (°C) Anthracene 216-218 Maleic Anhydride 54-56 Xylene -47-13.26 261-262 9,10-dihydro9,10ehtanoanthracene -11,12dicarboxylic anhydride

Boiling Point (°C) 340 202 139 556

Molecular Weight (g/mol) 178.23 98.06 106.16 276.29

Density (g/mL) 1.25 1.48 0.87 1.4

Experimental Initially, a sand bath was placed on top of a stirring plate and was plugged into the Variac controller. The voltage was set to no higher than 40. 0.303 g of anthracene and 0.156 g of maleic anhydride were added to the round bottom flask.1 After the addition of 3.1 mL of xylene and a stir bar, the contents of the round bottom flask were gently shaken to help dissolve the solids within the flask. The round bottom flask was then fitted with a condenser tube and then submerged in the sand bath.1 A piece of wet paper towel was then wrapped around the condenser tube. All solids dissolved before reflux was achieved, indicating the reaction was working; once brought to reflux, the process continued for a total of 30 minutes.1 During the process, 1.5 mL of xylene was placed in a large beaker filled with ice. After reflux, once the product went from yellow to slightly yellow hued, the solution in the round bottom flask was allowed to begin cooling to room temperature. The contents of the flask were then poured into a clean, small Erlenmeyer flask and cooled completely.1 Next, the flask was placed into an ice bath for about 10 minutes and a precipitate was allowed to form. Any additional precipitate that was stuck in the flask was washed with ice cold xylene.1 The precipitate formed was filtered through suction filtration using a Hirsch funnel. Once the solid was completely dry, the final mass was recorded, and the product was submitted for melting point analysis. Percent yield of the final product was then calculated.1 Results The final mass of product, or actual yield, was 0.245 g.4 After the maleic anhydride was determined to be the limiting reagent, it was then used to calculate the theoretical yield as seen in Eq. 1.3 This value was then used to calculate the percent yield, where the actual yield is divided by the theoretical yield as seen in Eq. 2.3 0.156 g M . Anhydride ∗1mol M . Anhydride 1 ∗1 mol C 22 H 22 O 4 98.06 g M . Anhydride ∗276.29C 22 H 22 O 4 1 mol M . Anhydride =0. 44 g 1 mol C 22 H 22 O 4 Eq. 1

0.245 g =0.557 g∗100=55 . 68 % 0. 44 g

Eq. 2

During the experiment, a number of significant color changes were noted. After the addition of xylene, the clear solution turned a pale-yellow color.3 Once the solution was brought to its boiling point, it acquired a clear, bright yellow color. The completion of the reflux portion of the experiment was characterized by a mostly clear solution with a tinge of yellow remaining in the round bottom flask.3 The melting point range of the final product was recorded at 234°C-236°C, indicating that the final product had some impurities.4 Discussion The formation of 9,10-dihydro-9,10-ethanoanhtracene-dicarboxylic anhydride underwent a Diels-Alder reaction in which a conjugated diene with 4 pi electrons, like anthracene, reacted with maleic anhydride, a dienophile with 2 pi electrons in the presence in xylene and heat.1 Xylene acted as a solvent that allowed the reaction to proceed under high temperature conditions. Ideally, stable, unreactive anthracene would have reacted with highly reactive maleic anhydride to form the product mentioned above.1 Maleic anhydride’s high reactivity is due to two electronwithdrawing groups carbon-oxygen double bond groups on each end of the carbon-carbon double bonds.1 The melting point range of 9,10-dihydro-9,10-ethanoanhtracene-dicarboxylic anhydride was recorded at 261-262°C; however, the recorded melting point of the product was around 30°C lower than what was expected. This would indicate impurities were present in the product. There could have been a number of errors that contributed to a low percent yield of 55.68%. The condenser tube could have been placed into the round bottom flask, allowing excess product to escape during boiling in the sand bath. Additionally, an excessive boiling period could have led to extensive evaporation of the product solution. The significant color changes listed above were the only striking observations made during the experiment. Conclusion Anthracene, a conjugated diene, reacted with maleic anhydride, a dienophile, to form 9,10-dihydro-9,10-ethanoanhtracene-dicarboxylic anhydride.1 A theoretical yield of 0.44 g was calculated as seen in Eq.1.3 This was used to calculate a percent yield 55.68%. The product was then submitted for melting point analysis, which had a range of 234-236°C.4 This lower melting point range indicated a presence of impurities in the product formed. The combination of electrons from both the diene and dienophile would have created two new carbon-carbon single bonds and one new carbon-carbon double bond.1 The loss of product could have been due to an improper condenser tube setup or an excessive boiling period. A potential improvement for the lab would be a more secure setup to prevent excess evaporation of product. Additionally, students should take care not to let their product overboil and to properly secure all glassware in the setup.

References 1. Casselman, B. Diels Alder Procedure. https://uab.instructure.com/courses/1532137/files/folder/Diels%20Alder? preview=64054867 (Accessed October 21 2020) 2. Wang, P. 2020 Fall 237-Part-A. https://uab.instructure.com/courses/1532144/files? preview=64158664 (Accessed October 21 2020) 3. Burnett, E.; Metuge, L.; Sanders, A. University of Alabama at Birmingham, Birmingham, AL. Laboratory Notes for CH 238: Organic Chemistry Laboratory I, 2020. 4. Casselman, B. Diels Alder Data Page. https://uab.instructure.com/courses/1532137/pages/diels-alder-data-page? module_item_id=15762159 (Accessed October 21 2020)...