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Grignard reagent

  1. Dec 6, 2012 #1
    1. The problem statement, all variables and given/known data
    (see attachment)


    2. Relevant equations



    3. The attempt at a solution
    One of Br should be replaced by MgBr but the answer key states that its wrong. I can't think of anything else. :confused:

    Seems like I need to do something with alkyne.
    Any help is appreciated.
     

    Attached Files:

  2. jcsd
  3. Dec 6, 2012 #2

    AGNuke

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    Write it as Br--Mg--R-. Does R- rings a bell? What does the answer key suggests anyway?
     
  4. Dec 7, 2012 #3
    Thanks AGNuke for the help! :smile:
    The answer key suggests that its an allene.
    If i write it as R-, I do get near to the answer but yet i can't seem to make the final compound. See attachment for my attempt. The negative charge and pi bonds form a conjugated system so there will be resonance.
     

    Attached Files:

  5. Dec 7, 2012 #4

    AGNuke

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    What about that negative charge attacking CH2 and Br exiting? I am sure you would get CH2=C=C=CH2. As a matter of fact. It is a method to prepare otherwise hard to prepare cumulene.
     
    Last edited: Dec 7, 2012
  6. Dec 7, 2012 #5
    I still don't get it. :uhh:

    Care to explain a bit more.
     
  7. Dec 8, 2012 #6

    AGNuke

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    Lets number the C-atoms in the diagram. The C-atom attached with Br-Mg be 1 and that last atom attached with Br be 4.

    There are lone pair on C-3. They can act as a nuclophile and can attack C-4. On C-4, lone pairs from C-3 are coming and Br is going. This is SNi. And we get Butatriene.
     
  8. Dec 9, 2012 #7
    How is it SNi? As far as I remember, this mechanism is found only during the reaction of R-OH with SOCl2 and there is one more reaction where this mechanism is found but it is definitely not related to what you have said. I think its related to Neighbouring Group participation but I would like to confirm it.
     
  9. Dec 9, 2012 #8

    AGNuke

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    Substitution Nucleophilic Internal. It is also linked to many forms of NGP.
     
  10. Dec 11, 2012 #9
    I know what's the meaning of SNi but I can't agree with you until you give me some references.
     
  11. Dec 11, 2012 #10

    AGNuke

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    Wikipedia?
     
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