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dat destroyer organic #109

Discussion in 'DAT Discussions' started by reecij, Aug 4, 2011.

  1. reecij

    2+ Year Member

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    Hello,

    So i'm super confused on this problem I was tempted to just skip it and hope I didn't see it on my actual DAT in 2 weeks but knowing my luck...... It'd be the first question in the Orgo section. I don't even know where to begin.... I was trying use my notes from Chad's videos... Is it a Micheal Addition? In the solutions manual how did they get A and B? what does the -40 have to do with the problem? I'm just confused!!! where do you start counting for the 1,2 vs 1,4 addition?

    If anyone could help me with this problem I'd greatly appreciate it
     
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  3. PooyaH

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    You need to know that addition of HCl or HBr to a conjugated compound like for example butadiene could be either a 1,2 addition or 1,4 addition depending on the conditions. If the condition is under thermodynamic control where the temperature is at 50C you'd get a 1,4 addition. When the condition is under kinetic control where the temperature is at -40C then you'd get a 1,2 addition.
     
  4. LaughingGas

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    First markovnikov reaction of H-Cl with the double bond.: double bond attacks the H+ of HCl, at first, forming carbocation. When this carbocation forms, b/c the compound had a conjugated double bond (double bond separated b a single bond), you can draw resonance structure, shifting the carbocation. That means you have two places to attack with the Cl-.
    1,2 Addition (before carbocation shift) = Kinetic
    1,4 Addition (after carbocation shift) = Thermodynamic
    1,2 Addition has lower Energy for Transition state and higher energy for the product, compared to 1,4 addition, having higher energy for transition state and lower energy for product.
    You can understand why its kinetic ( easier to overcome the hill for the lower energy of transition state) and why its called thermodynamic ( product is thermodynamically more stable, lower energy).
    Therefore if the condition as the problem is stated, low T, although the product stability is higher for 1,4 addition, because the condition given (-40) only permits the reaction to overcome the transition state of the 1,2 addition (1,4 addition has higher transition state), you get 1,2 addition product, which is having H+ and Cl- reacting on the same double bond.
     
  5. reecij

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    Thanks so much the solutions make so much more sense after you explained it :thumbup:
     
  6. EpicDentist

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    What version of the Destroyer do you have? I have 2011 and #107 says: Complete the reaction.
     
  7. PooyaH

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    No one's talking about #107.
     
  8. asdfray

    asdfray currently at work
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    For the A structure, isn't the carbocation on a tertiary structure and also stabilized by resonance? (maybe there's something I'm not seeing) Shouldn't the A structure be preferred in both cases of kinetics and thermodynamics?

    Is it a general rule that under certain conditions (in this case, low temp) it's always going to go for 1,2 addition or just the fastest product that can form? Same question goes for thermodynamics (always 1,4 addition?)


    Sorry, newbie question :thumbup:

    NM! I got it! ty =)
     
    #7 asdfray, Mar 17, 2012
    Last edited: Mar 19, 2012

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