Diels-Alder Reaction Between Anthracene and Maleic Anhydride AICI3 or heat Anthracene Maleic Anhyrdide Diels-Alder Adduct Background. The Diels-Alder reaction is one of the most useful reactions in all of organic chemistry. Usually, an electron-rich diene is reacted with an electron-poor dienophile resulting in the formation of a new six-membered ring. There are no ionic or radical intermediates in this reaction. Instead, bond-breaking and bond-making occurs at the same time; the reaction is said to be concerted. Electrons flow from the Highest Occupied Molecular Orbital (HOMO) of either reactant to the Lowest Unoccupied Molecular Orbital (LUMO) of the other reactant. The smaller the energy gap between the HOMO and LUMO, the less energy is needed to move the electrons from the HOMO to the LUMO, and the faster is the reaction. In fact, the role of electron-withdrawing substituents on the dienophile is to lower the energy of the LUMO, thus decreasing the HOMO-LUMO energy gap, and making the Diels-Alder reaction kinetically feasible. Benzene and related aromatic compounds do not undergo Diels-Alder reactions. However, anthracene, which consists of three fused benzene rings, does undergo Diels- Alder reactions, although the kinetic barrier is quite high. One of the reasons for the high kinetic barrier is the fact that as a result of the pi-bond of one of the anthracene rings reacting, this ring will no longer be aromatic. It has been estimated that at room- temperature, it would take 4,800 hours to react anthracene with the excellent dienophile (low-lying LUMO) maleic anhydride! There are two ways of accelerating such slow Diels-Alder reactions: One possibility is to simply reflux the solution in a solvent with a high boiling point. This is in fact what we will be doing in our laboratory experiment. Another method that has become very important was first discovered by Prof. Eaton (then at Harvard) and coworkers.' A Lewis acid such as AlCl, is added to the reaction mixture. It forms a complex with the dienophile making it much more electron-deficient and lowering the energy of its LUMO, thereby lowering the overall activation energy needed for the reaction. The first Lewis-acid catalyzed Diels-Alder reaction studied was in fact the reaction of anthracene with maleic anhydride. Eaton reported that addition of one to two equivalents of AlCl, led to a complete reaction within minutes at room-temperature instead of the 4,800 hours needed without the catalyst. Since then, many different Lewis- acid catalysts have been used to accelerate Diels-Alder reactions. Much research is still being undertaken with chiral Lewis acids as this may lead to the selective formation of only one stereoisomer.? In this laboratory, we will study the classic reaction between anthracene and maleic anhydride described in Eaton's now famous paper. We do not want to wait 4,800 hours in the lab for the reaction to go to completion, but instead of using AICI, to speed up the reaction, we will be using a high-boiling solvent, namely xylene. This is somewhat safer than using AICI, which reacts violently with water even when only small amounts are present. We will then use 'H NMR spectroscopy (not reported in Eaton's paper as it was not yet widely available in 1960) to analyze the product and determine if the middle ring of the anthracene molecule is indeed the one that has reacted with the dienophile. Prior to the lab, you will need to calculate the amount of maleic anhydride needed to react with 1.0g of anthracene. You will not be allowed to start the experiment without this calculation in your notebook. Experimental Procedure Spot the starting materials on TLC plate, labeling them appropriately. We will be using 1.0g of anthracene, and an equimolar amount of maleic anhydride. Prior to the lab, you will need to calculate the amount of maleic anhydride needed. Set up a reflux apparatus consisting of a heating mantle, 100mL round bottom flask, and a reflux condenser. Put approximately 30mL of xylene into the round-bottom flask, 3-4 boiling chips, then add the anthracene and maleic anhydride. Attach a drying tube filled with drierite to the top of the condenser, but make sure that the outlet of the tube is not closed – otherwise you would be heating a closed system which would result in an explosion! Connect the hoses for cooling water to the reflux condenser. Reflux the mixture for one hour, then let it cool down and remove the condenser. Place the reaction flask on an ice bath to precipitate the product. Vacuum filter the solid and wash the solid several times with 10mL portions of pentane. Air dry the product by keeping it on the filtration apparatus for approximately 10 minutes. Weigh the crude product to calculate the yield. Spot the product on the same TLC plate and develop it in TLC chamber using methylene chloride as an eluting solvent. Analyze your TLC and check if you need to recrystallize the product. If you indeed need to do that, then use methylene chloride. Waste disposal: Organic filtrates should be placed in the organic waste container.