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CHEM302 S09 CA3key

CHEM302 S09 CA3key - CHEM302 Spring 2009 Dr Ruder 1 Class...

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CHEM302, Spring 2009 Dr. Ruder 1 Class Activity 3 Diels-Alder Reaction Cycloaddition of a diene and an alkene gives a six membered ring. Model 1 : Diels-Alder Reaction The Diels-Alder reaction is a cycloaddition of a diene with an alkene (dieneophile) to form a six membered ring. Key information about the Diels Alder Reaction is listed below: R + heat R 1 2 3 4 5 6 6 5 4 3 2 1 diene (Nu-) dieneophile (E+) Questions 1. For the reaction shown in Model 1. (a). What role does the diene play ( nucleophile or electrophile)? What role does the dieneophile play? electrophile (b). What bonds were broken to form the product and what bonds were formed? (use the numbers given to indicate bond location). Bonds 1-6, 4-5, and 2-3 were formed. Bonds 1-2 and 3-4 were broken. (c). The Diels-Alder reaction occurs by a concerted mechanism. What does this mean? Concerted means that all bonds are broken and formed at the same time. (d). Draw curved arrows to show the bonds formed and broken in the Diels-Alder reaction above. R + heat R 1 2 3 4 5 6 6 5 4 3 2 1 diene (Nu-) dieneophile (E+) R 6 5 4 3 2 1 (e). Using dotted lines to show partial bonds, draw the transition state for the Diels-Alder reaction. See above (f). How many π -electrons are involved in the Diels-Alder reaction? 6 2. The dieneophile in a Diels-Alder reaction is typically either a mono- or disubstituted alkene. (a). Since the dieneophile is the electrophile, what kind of substituent (electron donating or electron- withdrawing) would enhance the electrophilic nature of the dieneophile? Explain. Electrophiles are electron –loving, so electrophiles are electron deficient. If the dieneophile is the electrophile, then it would need to be electron deficient. Any group that takes electrons away from the double bond would make it electron deficient. Thus electron-withdrawing groups are more likely to enhance the electrophilic nature of the dieneophile.
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