Diels-Alder Reaction

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 vapors are then condensed into liquid and they flow back into the same flask. In this experiment the condenser column was fitted with a drying column packed with calcium chloride (CaCl2) because the reagents / rxn are sensitive to air (esp H20 (g); the CaCl2 reacts with the vapor essentially dehydrating it. The temperature remains relatively constant and stirring is not needed because the two boiling chips that were placed in the mixture create enough agitation to ensure even boiling.

This experiment required two separate recrystalizations. After refluxing for the allotted time the distillate was poured while hot, to maximize the amount of adduct transferred, into an Erlenmyer flask and allowed to cool to room temperature before placing it an ice bath (We don't Want Broken Glass and Product contaminating our ice bath!). While hot, the solvent keeps the solute dissolved, in a easily transferable liquid state, but loses it solvating ability at cooler temperatures, thus the adduct crystallizes. After its ice bath the solution needs to be suction filtered to separate it from the xylenes (solvent). With the apparatus setup, hose connected and filter paper wet with solvent and aspirator running, the solution can be poured semi-slowly into the buchner funnel. The crystals tend to cling to the bottom and sides of the