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Triphenylmethanol vs methanol acidity

Discussion in 'MCAT Study Question Q&A' started by ponyo, Aug 8, 2011.

  1. ponyo

    ponyo 人魚姫
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    EK1001 Ochem #631:

    Which of the following would be most acidic

    A. Ethanol
    B. Benzophenone
    C. Methyl benzoate
    D. Triphenylcarbinol

    Structure of D:
    [​IMG]

    The solution says that D is most acidic b/c "it has extensive resonance stabilisation of the conjugate base's negative charge".

    This seems strange to me because unlike in the case of phenol, the anionic charge would not be at a benzylic position. In addition, wouldn't the three phenyls, by virtue of high electron density, stabilize a slight partial positive charge at the benzylic carbon, such that the anion is actually disfavored? Isn't this why phenylmethanol has pretty much the same pKa as methanol?

    Of course once I tried to corroborate this with real data things got really complicated:

    According to wikipedia, the pka of D is "16-18".

    Yet the literature seems to suggest it's actually a strong acid with a pka of -6.6:
    http://onlinelibrary.wiley.com/doi/10.1002/jctb.503310162/pdf
    http://www.sciencedirect.com/science/article/pii/0021951765900941

    Or in some cases, 12.7
    http://www.jstage.jst.go.jp/article/bcsj/84/1/82/_pdf
    http://www.chemicalbook.com/ProductMSDSDetailCB3335936_EN.htm

    But definitely lower than ethanol, which is about 16.

    I am utterly confused...
     
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  3. tn4596

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    i think they have relatively the same pka, cuz you cant take away the phenylic hydrogen...and the C-O carbon is saturated so no resonanace is formed if the conjugated base is formed. I guess just a bad question from EK.
     
  4. plzNOCarribbean

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    I think your over thinking everything and making it more complicated. I think they are going for trends, as opposed to exact pKa values.

    Normally, a single hydroxyl group attached to an aromatic ring has a Pka of ~10. normal alcohols, like in the case of ethanol in this example have a pKa ~ 16. From this, they want you to deduce that the added aromatic ring lowers the Pka (which makes sense) because it contributes to the stability of the conjugate base. The fact that the electron are free to move through the pi system is entropically favorable (its always favorable to spread charge around) and so by virtue, they stuck 3 aromatic rings to the OH group in the problem to make it very evident that the resonance effects of the aromatic ring result in the added acidity to of compound. D seems like the logical answer when you think of it this way.
     
  5. tn4596

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    Yes, phenol have a pka of 10. That is because the resonance structure is viable in phenol, in phenyl methanol, that is a different situation, there is no resonance because of the extra carbon.
     
  6. ponyo

    ponyo 人魚姫
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    You missed the fact that there's no resonance stabilisation into the three rings because the anionic charge is not at the correct position...
     
  7. muhali3

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    It definitely would not be stabilized by resonance, but the phenyl groups are more electron withdrawing than hydrogens. I think the stabilization would be more a result of the inductive effect.
     

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