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Acidity in organic compounds and hybridization

  1. Jan 28, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-1-28_13-49-39.png

    Label the acid hydrogen in each of the following compounds and arrange them in order of increasing acidity and explain the trend.

    2. Relevant equations

    N/A

    3. The attempt at a solution

    I'm not quite sure what the question means by "label" the acidic hydrogen but I believe the most acidic hydrogen is the one closest to the most electronegative element, correct? I labeled them in the image with red but not sure if this is the conventional way of doing it or if I'm missing something? It just seems strange.

    I would arrange them: C<B<A with A being the most acidic.

    Why? My reasoning is that because the carbon with the triple bond is sp hybridized which means that 50% of its hybrid orbital is "s" character and since "s" orbitals are closer to the nucleus; they have a better capability of handling the negative charge left behind when the proton dissociates and thus forms a more stable conjugate base...same reasoning for the other two...thoughts? Thanks.
     
  2. jcsd
  3. Jan 29, 2015 #2
    I think you've done the labeling part right. Yes, the most acidic hydrogen is the one closest to the most electronegative element.

    For the 'which is more acidic part', I'd suggest you look at the electronegativities of the three carbons. Which carbon atom is more electronegative and why? More the electronegativity, more the acidity.
     
  4. Jan 29, 2015 #3
    Hello,

    Thanks for the reply. What would make a carbon more electronegative? Aren't atoms of the same element equally electronegative? If anything, I guess, wouldn't the triple bond be the most electronegative? If that's the case, why would it be more electronegative? Thanks!
     
  5. Jan 29, 2015 #4
    I studied this 5 years back so these concepts are a bit foggy. But consider a C-C single bond. It has 3 H atoms attached to it. Carbon being more electronegative than hydrogen will pull the 6 electrons (2 from each covalent bond) towards it. It wont pull the electrons

    In case of C=C double bond, the carbon has only 2 H atoms attached to it. So it will pull only 4 electrons towards it.

    By that logic, the single bond carbon (C-C) will be the more electronegative one. Makes sense?
     
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