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21.9: The Claisen Condensation Reactions of Esters

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    183144
  • Because esters can contain alpha hydrogens, they can undergo a condensation reaction similar to the aldol reaction called a Claisen Condensation. In a fashion similar to the aldol, one ester acts as a nucleophile while a second ester acts as the electrophile. During the reaction a new carbon-carbon bond is formed. The product is a β-keto ester. A major difference with the aldol reaction is the fact that hydroxide cannot be used as a base because it could possibly react with the ester. Instead, an alkoxide version of the alcohol used to synthesize the ester is used to prevent transesterification side products.

    Claisen Condensation

    The Claisen condensation reactions of methyl acetate and methyl propanoate are shown as examples.

    Example: Claisen Condensation

     

    Claisen Condensation Mechanism

    1) Enolate formation

    2) Nucleophilic reaction

    3) Removal of leaving group

    Crossed Claisen Condensation

    Claisen condensations between different ester reactants are called Crossed Claisen reactions. Crossed Claisen reactions in which both reactants can serve as donors and acceptors generally give complex mixtures. Because of this most Crossed Claisen reactions are usually not performed unless one reactant has no alpha hydrogens.

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    Example: Crossed Claisen Condensation

    Dieckmann Condensation

    A diester can undergo an intramolecular reaction called a Dieckmann condensation.

    Example: Dieckman Condensation

    Mechanism

    dieckmanncondensation2.png

    1. Dieckmann, W. Ber. Dtsch. Chem. Ges. 1894, 27, 102–103.

     

    Exercise

    13. Draw the bond-line structures for the products of the following reactions.

    ch 23 sect 9 exercise.png

    Answer

    13.

    ch 23 sect 9 exercise solution.png

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