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Homework Help: Organic Chemistry Mechanism Question

  1. Apr 25, 2009 #1
    1. The problem statement, all variables and given/known data
    I am given isobutane as my starting material, Br2 and benzoyl peroxide as my radical initiators. My final product is the isobutane with Br attached instead of H.

    I have to design the mechanism. I have tried what seems like an endless combination of mechanism attempts, and can't seem to get it. I do not have to include the termination steps, only the initiation and propagation steps.

    3. The attempt at a solution
    -I homolyzed the Br atom and the benzoyl peroxide at the same time in step one.
    -In step two, I took one of the benzoyl peroxide radicals and attempted to remove the H from isobutane creating a radical off the alkyl group.
    -In step three, I homolyzed another Br2 molecule added it onto the isobutane radical.
    -In step four, I didn't know what to do.

    Evidently I have my steps all screwed up. I don't know when I'm supposed to homolyze the peroxide and then what I'm supposed to do with it. Any help would be greatly appreciated.
  2. jcsd
  3. Apr 25, 2009 #2
    i never came across benzoyl peroxide. I would suggest that it would break Br2 into Br+ and Br-. The Br+ is accepted by the benzene ring (electrophilic substitution rxn) and the Br- would attack the hydrocarbon as a radical.

    but i really don't know.
  4. Apr 28, 2009 #3


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    In isobutane you have three methyl groups and one methine. That is a 9:1 ratio of primary substitution vs tertiary. One would think that you would get a 9:1 ratio of the 1-bromo product relative to the 2-bromo product but remember that bromine is less reactive than chlorine in free radical radical reactions and is much more selective.

    Benzoyl peroxide can be thought of as a free radical initiatiator (like you would think of heat or light in that regard). Which bond (C-H vs. Br-Br) is most likely to homolytically cleave in the first step of your mechanism?

    Hint: Look at the bond energies of these two types of bonds.
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