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PBr3 cause inversion?

Discussion in 'MCAT Study Question Q&A' started by csx, Sep 4, 2014.

  1. csx

    csx
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    I think Chad says PBr3 does not cause inversion (in like one text book under rare conditions it does however) while my content review book says it does cause inversion.

    does anybody know?
     
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  3. acac

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    Its an SN2 reaction, causes inversion of stereochemistry. Maybe looking at the mechanism will help, see attached.

    The only SN2 reaction that does NOT cause inversion of stereochemistry at the electrophillic carbon is the reaction of alcohols with Toluenesulfonyl chloride (TsCl)
     

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  4. justadream

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    @acac

    Why doesn't the latter reaction you refer to cause inversion?
     
  5. Czarcasm

    Czarcasm Hakuna matata, no worries.
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    It's part of the mechanism, definitely nothing you'd need to know aside from inversion taking place. You should also know that to retain stereochemisttry (no inversion) you use a sulfonate ester like TsCl.
     
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  6. acac

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    yea another way to state that would be:

    - SN2 reactions occurring at the electrophillic carbon causes inversion of stereochemistry (Alcohols with PBr3, PCl3, SOCl2)
    look at step 2 of the attached picture above

    - SN2 reactions not occurring at the electrophillic carbon retains stereochemistry (Alcohols with TsCl)
    in this mechanism, the electrophillic carbon is not where the bimolecular step takes place, but rather the sulfur of the sulfonate ester (the sulfur atom is also an electrophile).
     

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