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Cope Elimination

  • Page ID
    39383
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    When a tertiary amine oxide bearing one or more beta hydrogens is heated, it is converted to an alkene. The reaction is known as Cope elimination or Cope reaction, not to be confused with Cope rearrangement.

    eg:

    Copeelimination1.png

    The net reaction is 1,2-elimination, hence the name Cope elimination.

    copeelimination2.png

    mechanism: Cope elimination is an intramolecular E2 reaction. It is also a pericyclic reaction.

    copeelimination3.png

    Intermolecular E2 reactions occur preferentially from the conformation of the substrate in which the leaving group and the beta hydrogen abstracted by the base are antiperiplanar, which is not possible in intramolecular E2 reactions in which the base is built into the leaving group because the basic atom is too far away from the beta hydrogen anti to the leaving group. Intramolecular E2 reactions occur preferentially from the conformation of the substrate in which the leaving group and the beta hydrogen abstracted by the base are synperiplanar. The basic atom and the beta hydrogen abstracted by it are closest to each other in this conformation.

    eg:

    copeelimination4.png

    mechanism:

    copeelimination5.png

    Cope elimination is regioselective. Unlike intermolecular E2 reactions, it does not follow Zaitsev’s rule; the major product is always the least stable alkene, i.e., the alkene with the least highly substituted double bond.

    eg:

    copeelimination6.png

    This trend is most likely due to the fact that the less highly substituted β-carbon bears more hydrogen atoms than the more highly substituted one; at a given moment, in a sample of the substrate, there are more molecules in which a hydrogen atom on the less highly substituted beta carbon is synperiplanar to the leaving group than there are in which a hydrogen atom on the more highly substituted beta carbon is.


    This page titled Cope Elimination is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Gamini Gunawardena via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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