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    Aldol Reactions: Trans-4-Nitrochalcone and Others

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    8.88) In a number of cases it is possible to successfully isolate the B-Hydroxyketone intermediate prior to the dehydration that froms the alpha,Beta unsaturated ketone.
    (a) Does the isolation of the B-Hydroxyketone suggest which step in the aldol condesnation is the rate determining step in this case?
    (b)if so, Which one is the rate determining step?
    (c)Suggest what reaction conditions may have a significant impact on determing which step becomes rate determining.
    (d)What Structural changes might lead to a change in the rate determing step?

    8.89) KEtones also undergo the aldo condensation, although a succesful reaction often requires "enhanced" conditions, since the addition involves an unfavorable equilibrium constant. This situation in the reaction in which 4 nitrochalcone is synthesized. With odds against it, why i the rreaction successful in this case?

    8-92) Give the aldol product or products from the following reaction:
    CH3CH2CH2CH2CH2CHO + CH3CH2CH2CH2CHO --base->

    8.94) The mixed aldol reaction between propionaldehyde and acetne gives an 85% yield if 4 hydroxy - 2- hexanone when run in THF at -78 degrees C with lithium diisopropylamide (LDA, a powerful base). The reaction is carried out by first adding the ketone to the base, cooling the solution , and then adding the aldehyde.
    (a) Why does this muxed system give essentially a single product?
    (b) Why is there noself condensation of the acetone?
    (c) Why does the system not rapidly go on to dehydrate?
    (d) Why is the keyone added to the base rather than Vice-versa?

    8.96) If you had obtained both the cis- and trans-chalcone products, adn had purified them by recrystallization, how could you instantly know which one was cis and which one was trans without any further characterization?

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    Solution Summary

    Questions concerning aldol reactions are answered in detail.