Question

Figure 10.2 Diels-Alder reaction with anthracene and maleic anhydride. Background: Reaction The Diels-Alder reaction is a powerful method for synthesizing a six- membered ring by reaction of a conjugated diene with an alkene (see Figure 10.1). Figure 10.1 Diels-Alder reaction. diene dienophile (alkene) cyclohexene It is surprising at first glance that this reaction is so favorable since we are disrupting an "aromatic ring. If we calculate the difference between the resonance stabilization of the start-ing material and product (anthracene has a resonance energy of 84 kcal/mol, two benzenes contribute 2 x 36 = 72 kcal/mol) we see only a loss of 12 kcal/mol of resonance energy. The formation of the two new sigma bonds in the product more than compensates for this loss, explaining the favorability of this reaction The reaction as written above with unsubstituted alkenes and dienes is to be done, forcing reaction conditions (high temperature and pressure are needed). If a diene held in the s-cis conformation and a dienophile bearing electron-withdrawing groups are used, the reaction conditions can be much milder. Pre-Lab Preparation Reading from Manual Procedures for running the reaction The reaction in this experiment has the aromatic compound anthracene as the "diene." In anthracene, the diene is constrained to the s-cis conformation in the center ring. The dienophile is maleic anhydride, which bears electron withdrawing carbonyls on both vinyl carbons You should be able to draw the product of the reaction between a general diene and dienophile. You should be able to draw the correct chemical structure of compounds and solvents used in this experiment when given their name. This reaction proceeds in boiling xylene (see Figure 10.2)

1. What is the objective of this experiment?
2. The reaction solvent for this experiment is xylene. What property of this solvent makes it a better choice than toluene or benzene?
3. Draw the orbitals involved in the 4+2 electrocyclization for this reaction. Draw the structures of the reactants as best you can. It may be appropriate to abbreviate the structure of anthracene.
4. IR is not provided for this reaction product. Why is IR not useful for monitoring the reaction or characterizing the product?

Answer 1

The main aim of this experiment is to synthesize Diels Alder product (9,10-dihydroanthracene-9,10-a,ß- succinic anhydride) via Diels Alder reaction. The reaction involves the heating of the mixture of anthracene and maleic anhydride in the presence of xylene as a solvent as shown in the reaction below. Here, anthracene acts as the diene and maleic anhydride as the dienophile. xylene solvent (3) heat anthracene-1 mw = 178 mp = 216 maleic anhydride - 2 mw = 98 mp = 60 Diels-Alder Product (4) mw = 276 mp - 230-300 range Figure 1. Reaction scheme for the formation of Diels Alder product. Benzene (80 °C) and toluene (110 °C) have low boiling points as compared the xylene (144 °C). This reaction requires higher temperature for the progress of the reaction. Xylene (dimethylbenzene) is used as a high boiling temperature solvent so that the reaction will proceed quickly. Thus, xylene is a better choice for this reaction. Following, is the diagram involving the bonding interaction of orbitals of diene and dienophile in Diels Alder reaction. The secondary interactions in this diagram is not shown but is in fact involved during the reaction. yo X The structures of the reactants are already shown in the reaction scheme (Figure 1). The Diels Alder product obtained in this reaction has higher melting point as compared to the reactants (see Figure 1). So, determining the melting point of this product will give sufficient information for the progress of the reaction and the formation of the product. This is the reason why IR data is not taken. However, recording the IR data for this product will give addition information regarding the formation of the product. Taking IR data is useful because it will give information such as presence of carbonyl groups.