CH242 diels alder.09

CH242 diels alder.09 - Chemistry 242 Experiment #1...

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Chemistry 242 Experiment #1 Diels-Alder Reaction of a Conjugated Diene in Eucalyptus Oil Introduction The Diels-Alder reaction is a powerful tool in organic synthesis because of its ability to make unsaturated rings. A conjugated diene that is s-cis can undergo this reaction. The “s” probably stands for sigma-cis, which is not a cis arrangement in the classical sense, since there is rotation around the sigma bond. Sometimes, however, rings may lock the diene into an s-trans formation, in which case there is no Diels-Alder reaction. These conjugated dienes may undergo cycloaddition reactions with certain double bonds to make cyclohexenes and related compounds. The Reaction is named for Otto Diels and Kurt Alder who received the 1950 Nobel Prize for this discovery. The simplest Diels-Alder reactin is between 1,3-butadiene and ethylene to produce cyclohexene. + diene dienophile Diels-Alder adduct (product) The mechanism for the reaction, which can be described as “4 + 2 cycloaddition,” begins when the diene assumes the higher energy but more reactive s-cis conformation. As the diene and dienophile (the mone-ene) come together, the two terminal carbons of the diene meet the two carbons in the dienophile. What follows is a concerted 1 H 3 C H 3 C CH 3 H CH 3 H H CH 3 H CH 3 Faster Diels Alder Rate No Diels Alder s-cis s-trans
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rearrangement of bonds (see lecture text p326-327). Electron donating groups on the diene and electron withdrawing groups on the dienophile make the reaction proceed easier. Maleic anhydride, is an example of a good dienophile because it contains two electron withdrawing carbonyl groups. Maleic anhydride was one of the original dienophiles studied by Diels and Alder. The reaction of maleic anhydride with 1,3- butadiene is shown below. O
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CH242 diels alder.09 - Chemistry 242 Experiment #1...

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