Diels-Alder Reaction
Laboratory procedure used was written by Dr. John Barbaro of the University of Florida. http://www.chem.ufl.edu/~barbaro/2211L/diels-alder/da-proc.html
The Diels-Alder reaction is probably the most familiar example of a reaction type known as a cycloaddition reaction, in which the conjugated p-systems of two reactants join to generate a new ring. The reactants in the Diels-Alder reaction are a 1,3-diene and an alkene called the dienophile. The carbon-carbon double bond in the dienophile is usually conjugated with an electron-withdrawing group such as a carbonyl, a nitrile or a nitro group. Conjugate (or 1,4) addition of the double bond of the dienophile to the diene produces a new six-membered ring. A more detailed discussion of the Diels-Alder reaction can be found in chapter 15 of your textbook, and you should read those sections of your text for additional background material.
The Diels-Alder reaction appears to occur in a single step without the intervention of radicals, carbocations, or other intermediates. It is a powerful method for the construction of cyclohexene rings, and has become such a mainstay of organic synthesis that its discoverers, Otto Diels and Kurt Alder, were awarded the Nobel Prize in Chemistry in 1950 for their discovery of this reaction.
Diels-Alder Reaction (Diene Synthesis).
FIRST CITATION: O. Diels and K. Alder, Ann. 460, 98 (1928); 470, 62 (1929) Ber. 62, 2081, 2087 (1929)
Stereoselectivity:
· The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.
· Addition is syn on both components (bonds form from same species at the same time)
· This is shown by the examples below:
cis-dienophile gives cis-substituents in the product.trans-dienophile gives trans-substituents in the product.
If both substituents on the diene are Z, then both end up on the same face of the product
If substituents on the diene are E and Z, then they end up on opposite faces of the product
Cyclic dienes can give stereoisomeric products depending on whether the dienophile lies under or away from the diene in the transition state. The endo product is usually the major product (due to kinetic control)
(dienophile under or in = endo) /
Diene and dienophile staggered with respect to each other gives the exo product
(dienophile exposed or out = exo)
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch10/ch10-5-2.html VERY COOL LINKS!!!!!
Reaction of Anthracene with Maleic Anhydride
The Diels-Alder reaction in this experiment uses an unusual cyclic diene–the central ring of anthracene. Benzene and naphthalene do not take part in typical Diels-Alder reactions because the p-electrons of the diene are part of an aromatic ring; if cycloaddition were to occur the aromaticity of that ring would be lost. In anthracene however, as you will find if you draw the structures of the possible resonance contributors, all three rings cannot have benzenoid character simultaneously so the resonance energy per ring is lower than that of benzene and naphthalene (resonance energies: benzene, 36 kcal/mole; naphthalene, 60 kcal/mole; anthracene, 84 kcal/mole). Diels-Alder addition across the center ring of anthracene is possible since it leaves two intact benzene rings in the adduct.
Techniques
In this lab you will learn how to perform two valuable laboratory techniques that will be used throughout the semester: heating a solution under reflux conditions and vacuum filtration.
For refluxing a solution, you will use the ground-glass joint glassware found in the kit in your drawer. The ground-glass joints enable all of the various pieces of glassware in the kit to connect together snugly, which allows you to build a wide variety of set-ups from a few pieces of glassware. Make sure that all of the ground-glass equipment in your kit has the same size joints (i.e., the same taper) or else the pieces will not fit together properly.
One problem with ground-glass joints is that they have a tendency to freeze or stick together. This problem can be avoided by applying a thin film of silicone grease to the inner (male) member of the joint before connecting the two pieces. After slipping the two pieces of glassware together, rotate the joint to distribute the grease evenly. Do not use too much grease on the joints since the excess might contaminate your sample.
Safety and Waste Disposal
Xylene is flammable, so keep it away from flames and hot plates.
Experimental Procedure
Place 1.0 g of anthracene and 0.5 g of maleic anhydride in a 100-mL round-bottom flask. Add 15 mL of xylene and a boiling chip. Attach a reflux condenser to the flask. It might be necessary to run cooling water through the condenser; an air-cooled condenser can be sufficient when using a high-boiling solvent such as xylene on a small scale. Heat the mixture under reflux for 30 minutes. At the end of the reflux period, cool the flask to room temperature and then chill it in an ice bath for about ten minutes to complete the crystallization of the product.
Collect the crystals by suction filtration on a small Büchner funnel and wash the solid with 15-20 mL of ice-cold xylene. Allow it to dry on a piece of filter paper. Record the weight of the product and calculate the percent yield for the reaction. Don’t worry about getting a melting point (lit. mp 261-262 oC) for your product.
POSTLAB QUESTIONS:
1. Explain why a 4M solution of maleic anhydride is made in CH2Cl2 and not in water. Show the reaction between maleic anhydride and water.
2. Give structures for the major Diels-Alder product of the following reactions:
1,3-cyclohexadiene and tetrachloroethene
1,3-cylcohexadiene and fumaric acid, the trans isomer of maleic acid