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Diels-Alder Reaction

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  • May 2006
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THEORY:
The Diels-Alder cycloaddition reaction was discovered by Otto Diels and Kurt Alder and is very useful in the synthesis of polycyclic compounds. The Diels-Alder reaction can be described as: [4+2] cycloaddition- a diene with 4 π electrons + 2π electrons from the dienophile; a pericyclic concerted reaction- meaning the reaction occurs in a single step (no intermediates) and involves a cyclic redistribution of bonding electrons.

In order for a Diels-Alder reaction to take place the diene must be a conjugated system oriented in the s-cis conformation; s-trans dienes can undergo the reaction only if there configuration permits free-rotation around one of the –C=C double bonds such that they can assume the s-cis conformation. Diencs locked in the s-cis configuration will react faster than those able to assume both s-cis and s-trans configurations; those locked in the s-trans conformation will not react.

The reaction is favored (proceeds faster) by the presence of electron withdrawing groups (ie Nitriles, Amines, Carboxylic acids, esters, aldehyde/ketone etc) in the dienophile and electron donating groups (ie methyl ether, alkyl, etc.) in the diene. Diels-Alder reactions are steriospecific (cis-alkenes form cis substituted and trans-alkenes form trans-substituted); this is called syn addition, the configuration of dienophiles is always retained in the product. The diene and dienophile react in such a way that the endo product is formed rather than the exo product.

TECHNIQUES:
Refluxing is a purification technique used when you need to heat a solution of reactants for a lengthened period of time in order to complete the reaction. It differs from distillation in that the "distillation and collection of the distillate are carried out in the same flask" (Nauman, 37). The solution is placed in a round bottom flask with a condenser placed vertically over it. As the flask is heated, the solvent boils and vapors rise in the condenser. The...