May 26, 2015
Diels Alder Reaction
Diels Alder Reaction is the reaction of a diene with a species capable of reacting with the diene, the dienophile. A diene is a hydrocarbon that contains two carbon double bonds, while a dienophile is an electron-deficient alkene. The Diels-Alder is also called a [4+2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne. The product of the Diels-Alder reaction is usually a structure that contains a cyclohexene ring system.
The mechanism of the reaction involves a cyclic flow of electrons in a single step in which the diene, supplies 4 π electrons and the alkene or alkyne, also called the dienophile, supplies 2 π electrons. In this process, two new sigma bonds which link the former dienophile to the diene and one new pi bond between the former double bonds of the diene are formed. The reaction involves only pi bonds in the reactants; no sigma bonds are broken. Due to the concerted mechanism, two kinds of selectivity are evident in Diels-Alder reaction products. The stereochemistry of the dienophile is retained in the product. If the substituents of the dienophile are cis, they remain cis in the product. If they are trans, they remain trans in the product. If the diene is cyclic, and if the dienophile has cis substituents, then two stereochemical outcomes are possible. The endo isomer is more sterically hindered than the exo. In spite of this, the endo tends to predominate in cases where the R substituents have a system that is conjugated with that of the dienophile double bond. During cyclization favorable interaction between the substituent system and that of the diene overcomes the steric hindrance and thus favors the endo isomer over the exo. Electron withdrawing groups on the dienophile and electron-donating groups on the diene can speed up this reaction. The dienophile is the part of the reaction that must be “hungry” for electrons from the diene. By having electron withdrawing groups on the dienophile, the dienophile’s alkene (or alkyne) is stressed by having to overly “share” electrons with the electron withdrawing groups. Even though s-trans conformation of dienes is more stable due to steric reasons, the s-cis conformation is needed to carry out the Diels-Alder reaction. In this experiment, cis-norbornene-2,3-endo-dicarboxylic anhydride, commonly called carbic anhydride, will be synthesized from cyclopentadiene and maleic anhydride. Cyclopentadiene, which is obtained from a light oil distilled from coal, is a very reactive diene and dienophile, so it readily dimerizes at room temperature to form “dicyclopentadiene”. However, this Diels-Alder reaction is reversible by “cracking” dicyclopentadiene using slow distillation and keeping the cyclopentadiene cold. At the boiling point of dicyclopentadiene, equilibration with the monomer is rapid; the monomer, being more volatile, may be removed from the dimer. Cyclopentadiene reacts with itself by a Diels-Alder reaction to make a dimer of cyclopentadiene. A dimer is the product of a self-addition reaction. Pure cyclopentadiene is obtained by “cracking” the dimer. In this case, cracking means heating the dimer until it undergoes a reverse-Diels-Alder reaction. So a reverse Diels-Alder reaction converts the product of the forward reaction back into the reactants of the forward reaction.
The side product that are formed are:
Maleic anhydride reacts with cyclopentadiene to form cis-norbornene-5,6-exo-dicarboxylic anhydride Two cyclopentadiene molecules dimerize to form the endo form of dicyclopentadiene
Glass stirring rod
Ligroin (Diethyl ether)
1.0 g of maleic anhydride and 4.0 mL of ethyl acetate was added to an Erlenmeyer flask. The flask was swirled to...
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