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Nucleophile question

Discussion in 'MCAT: Medical College Admissions Test' started by UCLAstudent, Apr 13, 2004.

  1. UCLAstudent

    UCLAstudent I'm a luck dragon!
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    I was just doing a passage on kinetics, and one problem asked what would happen to the rate if CH3SH (the nucleophile) was replaced with methanol (for an SN2 reaction). The solution says that the reaction will occur more slowly. It says that methyl CH3SH is a stronger nucleophile than methanol because CH3SH is a stronger base. I am having trouble understanding why. I have always been told that CH3SH is a stronger ACID than methanol because of the larger radius; if this is the case, shouldn't it be less basic than methanol? Also, S is less electronegative than O. Can someone please help me out? I know the chances are slim that this would be on the MCAT, but I would like to know the answer for sanity, if anything. Thanks. :)
     
  2. brodaiga

    brodaiga Senior Member
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    According to the TPR materials a nucleophiles' nucleophilicity is inverse to its electronegativity because a more electronegative atom would want to hold onto its electrons instead of donating for covalent bonds, I.E. this explains the order of the strong bases (HF is a weak base, etc...), so S is less electronegative than O making it a better nucleophile...yeah, but I had that CH3SH was acidic in origin?
     
  3. Mahi

    Mahi Member
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    CH3SH would be a stonger nucleophile in a protic solvent because it is larger and more polarizable. In a protic solvent the O in methanol would latch onto H in hydrogen bonding, making it able to attack and less nucleophilic. In an aprotic solvent, the reverse is true, the more electronegative, the more nucleophilic. I think this is right...
     
  4. Mahi

    Mahi Member
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    i meant LESS able to attack and less nucleophilic
     
  5. babinski bob

    babinski bob Senior Member
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    The answer to the question is right but their reasoning is incorrect.

    CH3SH is a better nucleophile because it's more polarizable than CH3OH.

    When comparing nucleophiles down a COLUMN, polarizability dominates
    When comparing nucleophiles across a ROW, basicity dominates.

    In fact, CH3SH is a weaker base than CH3OH. THe pKa of thiols = 10 whereas the pKa of an alcohol is 16-18. Therefore, thiols are less basic than alcohols.

    Hope this helps. YOu should send a letter to the publisher informing them of the mistake.
     
  6. blz

    blz Senior Member
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    lol babinski is Mr Nucleophile :D
     
  7. UCLAstudent

    UCLAstudent I'm a luck dragon!
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    Indeed, he attacks these threads as a good nucleophile would. I didn't even attach a positive charge to this thread! ;)

    Thanks for your help, everyone!
     

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