dielsalder - Diels-Alder Condensation Reaction Organic...

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Diels-Alder Condensation Reaction Organic Chemistry II CHM2211 Otto Diets and his pupil Kurt Alder received the Nobel Prize in 1950 for their discovery and work on the reaction that bears their names. Its great usefulness lies in its high yield and high stereospecificity. A cycloaddition reaction, it involves the 1,4-addition of a conjugated (Diene in the s -cis conformation to an alkene in which two new σ (sigma) bonds are formed from two π (pi) bonds. The adduct is a six-membered alkene ring. The (Diene can have the two conjugated bonds contained within a ring system, as with cyclopentadiene or cyclohexadiene, or the molecule can be an acyclic diene that must be in the cis conformation about the single bond before reaction can occur. This reaction is not polar in nature in that no charged intermediates are formed. Neither is it a radical reaction, because no unpaired electrons are involved. It is instead known as a concerted reaction or one in which several bonds in the transition state are simultaneously made and broken. When a cyclic diene and a cyclic dienophile react with each other as in the present reaction, more than one stereoisomer may be formed. The isomer that predominates is the one that involves maximum overlap of π electrons in the transition state. The transition state for the formation of the endo isomer in the present reaction involves a sandwich with the diene directly above the dienophile. To form the exo isomer, the diene and dienophile would need to be arranged in a stair-step fashion. But the reaction is also subject to steric hindrance, especially when the difference between the
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electron-withdrawing and electron-donating characters of the two reactants is not great. R. B. Woodward and Roald Hoffmann formulated the theoretical rules involving the correlation of orbital symmetry, which govern the Diels-Alder and other electrocyclic reactions. Cyclopentadiene is a commonly used reactant in Diels-Alder reactions and is obtained from the light oil from coal tar distillation but exists as the stable dimer, dicyclopentadiene, which is the Diels-Alder adduct from two molecules of the diene. Because a bottle of cyclopentadiene actually contains the dimmer, it must be converted
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This note was uploaded on 09/12/2010 for the course CHEM 3371 taught by Professor Jamesmorris during the Spring '10 term at Georgia Institute of Technology.

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dielsalder - Diels-Alder Condensation Reaction Organic...

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