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Electrophilic Alkene Addition Mechanism

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  • Page ID
    1137
  • Mechanism Overview

    The electrophilic Alkene Adddition reaction involves two sequential steps. The red arrows are curved or curly arrows showing electron movement.

    Step 1 (slow)

    step1_a.gif

    The alkene pi (π) electrons react with a source of electron deficient H. The most stable carbocation is formed. Occasionally the first-formed carbocation undergoes a rearrangement to a more stable carbocation. Formation of the carbocation is the rate limiting (slowest) step in the mechanism.

    Step 2 (fast)

    step2_a.gif


    The electron deficient carbocation reacts with an electron rich X- ion.

    Examples

    Reactant Reagent(s) and conditions
    (yield: %)
    Product
    (fraction formed: %)
    Comments.
    but1ene.gif HBr
    arrow.gif
    acetic acid
    (90%)
    2brbutane.gif Regioselective: Markovnikov

     

    Note the bromine attached to the C-2 position from the more stable secondary carbocation

    methylpropene.gif HBr
    arrow.gif
    acetic acid
    (90%)
    2brmepropane.gif Regioselective: Markovnikov
    Note the bromine attached to the C-2 position from the more stable tertiary carbocation
    HCl
    arrow.gif
    (100%)
    1cl1mecyclopentane.gif Regioselective: Markovnikov
    Note the chlorine attached to the C-1 position from the more stable tertiary carbocation
    cyclohexene.gif KI
    arrow.gif
    H3PO4, 80ºC
    (90%)
    icyclohexane.gif  
     
    HCl
    arrow.gif
    3me2clbutane.gif (40%) + 2me2clbutane.gif (60%) Rearrangement occurs
    (hydride migration)

    +

    The first-formed carbocation is secondary, some of this carbocation reacts with Cl- but a hydride migration converts this into a more stable tertiary carbocation from which the majority of the product is derived.
    HCl
    arrow.gif
    33me2clbutane.gif (17%)
    23me2clbutane.gif (83%)
    Rearrangement occurs
    (methyd [CH3 with 2 electrons] migration)

    +

    The first-formed carbocation is secondary, a methide migration converts this into the more stable tertiary carbocation.

    A small amount of the secondary carbocation reacts with Cl- before rearranging, but the majority rearranges before reacting.

    Contributors