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TBR Ochem Phase III Passage 4 Question 21

Discussion in 'MCAT Study Question Q&A' started by avenlea, 05.13.14.

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  1. avenlea

    avenlea 5+ Year Member

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    Q: Which of the following compounds can deprotonate C5H6?
    A: H3CCOCH3
    B. H3CCH2SH
    C. H3CO-
    D. H2CNO2-


    Why is it not D? There's a nitrogen in there, and I feel like I'm missing something big. I get how to figure it out from PkAs but just for my general knowledge about basicity I'm curious.

    Thanks!
     
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  3. SuperSneaky24

    SuperSneaky24

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    Well it's definetely C, as the methoxy group is already electron-donating, and adding a negative charge into the mix makes it extremely unhappy and want to deprotonate something.

    As for H2CNO2-, it's not going to be a good base because the NO2 group is electron-withdrawing via resonance. When you make the carbon negative by removing the hydrogen, its negative charge is reduced and stabilized by the NO2 group by its side.
     
  4. Teleologist

    Teleologist 2+ Year Member

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    I agree, C would be a good answer. We have a negative formal charge on the oxygen in C as well as a negative partial charge on oxygen in the molecule in C. This makes the oxygen very nucleophilic (and hence likely to deprotonate something). And like the other poster said, in D, we have an electron withdrawing nitrogen. Negative charge is spread out over the multiple oxygens and in addition the nitrogen. Too many cooks in the kitchen with D.
     

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