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Reverse of optical isomerism in a reaction

  1. Sep 14, 2008 #1
    1. The problem statement, all variables and given/known data

    when chiral 2-bromobutane reacts with aqueous hydroxide ions, the optical activity of the product is reversed. Suggest why the optical isomerism is reversed.


    3. The attempt at a solution

    i have no idea... but i don't know if the fact that the hydroxide ion will attack the electron deficient carbon (from C - Br) away from the Br atom to avoid repulsion may lead to the answer...

    thanks!
     
  2. jcsd
  3. Sep 14, 2008 #2

    Borek

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    Staff: Mentor

    You are on the right track. Try to imagine how the reaction proceeds. Molecule model may come handy.
     
  4. Sep 14, 2008 #3
    ermmm well i tried to imagine but i still can't see how a dextro or leavo enantiomer could become the opposite?!

    because the Br- will go and the OH- will just attach to the electron deficient carbon.

    does that have something to do with the SN2 mechanism?
     
  5. Sep 14, 2008 #4

    Borek

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    Yes.
     
  6. Sep 14, 2008 #5
    i had a look at the mechanism but i can't find how there could be a reversal in chirality. a hint would be well appreciated.

    thnks
     
  7. Sep 14, 2008 #6

    Borek

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    You start with R1, R2, R3 and Br. You end with R1, R2, R3 and OH. During reaction R1, R2 and R3 are first "made flat" (when there exist intermediate, 5 substituded carbon) and then they are "switched" to the other side, as if they were reflected in the mirror.
     
  8. Sep 14, 2008 #7
    so you mean that when the carbanion forms a trigonal bipyramidal structure, the R1, R2 and R3 lie on one plane with the Br and OH perpendicular to that plane.

    but how does the switching occur?! does it occur after the Br- leaves?

    thnks
     
  9. Sep 14, 2008 #8

    epenguin

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    I remember an old textbook (Ingold) likened it to an umbrella inverting. Imagine a 3-ribbed umbrella - not a perfect analogy but may help visualise. :smile:
     
  10. Sep 14, 2008 #9

    Borek

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    That's how it looks. Not sure if its OK to call it carbanion, I am called Mr.pH, not Mr.Organic :wink:

    For the OH- to attach to carbon, R1, R2 and R3 must move to make place, then when Br- leaves, they have new place where they can move - at this moment they have to change symmetry. They behave like a hat turned inside out.
     
  11. Sep 14, 2008 #10

    Borek

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    Pretty good example :smile: Somehow I missed the moment you posted the answer.
     
  12. Sep 14, 2008 #11
    niice analogies lol.... thank you borek and epenguin.....

    i can get the picture now:)
     
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