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BDE and Acidity TBR Ochem Passage 1

Discussion in 'MCAT Study Question Q&A' started by doy, 05.12.14.

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  1. doy

    doy 2+ Year Member

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    Hi Guys,
    TBR Passage 1 Question 1 Says.
    What bond dissociation energy would you expect for bond between carbon-1 and hydrogen and the only carbon-carbon single bond in H-C(triplebond)C-CH3?
    I understand that with the trends in the table given, H-C(sp3 C) is 88, H-C(sp2 C) is 108 so H-C(sp) must be higher than 108.

    But doesn't that contradict acidity?
    The more s character, the easier it is to break the bond with hydrogen. So shouldn't the bond between an H-C that is sp hybridized be weaker and thus the bond dissociation energy less than an sp2 C and sp3 C?

    Or am I comparing two different concepts and I should look at conugate base strength rather than acidic trends here?
    Thanks!
     
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  3. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    The more s character, the more difficult it is to break a bond. Consider the bond energies you provided: (H)-s-sp3-(C); the sp3 only has 25% s characteristic, 75% p characteristic. It's longer than what you'd expect from a pure s-s bond. Likewise, sp2 is about 33% s characteristic. Having more s character, the bond is shorter (and stronger) than an sp3-s and therefore, has a higher BDE. Basically, the more s character, the stronger and shorter the bond, the more difficult it is to break. As an analogy, consider what happens when you shorten a long stick. If you break it in half - then try to break it again, ...and again, and again, it gets more and more difficult. This might help you remember.

    As far as acidity is concerned: a H of a sp C is more acidic than an sp2 C, which in turn, is more acidic than an sp3 C. This might seem contradicting at first since you would expect an sp-s to be the strongeset bond and therefore the most difficult bond to break, but the explanation for this is no different than any other acid. The stronger an acid, the more stable it's conjugate base. As it turns out, losing a hydrogen on sp has a more stable conjugate base because the resulting negative charger is on average closer to the nucleus (where positive charge resides) and this helps to stabilize it (therefore making the acid stronger and more likely to dissociate).
     
  4. doy

    doy 2+ Year Member

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    Thank you for your explanation. Reading further in the TBR book, I ran into a passage that cleared some things up for me more. Perhaps this could be helpful for others:
    BDE is based on Heteroloytic cleavage (radical formation) while acid strength is based on homolytic cleavage(+ and - charge formation), therefore acidic and bond dissociation energy shouldn't be compared.
     
  5. Teleologist

    Teleologist 2+ Year Member

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    This is wrong. For example, NaOH does not dissolve in water to give hydroxyl radicals but rather hydroxide ions.
     
  6. doy

    doy 2+ Year Member

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    I accidently missed up the Homolytic and Heterolytic terms. My bad.
    But what I meant is BDE is based on Homolytic cleavage (radical formation) while acid strength is based on heterolytic cleavage(+ and - charge formation).
    http://en.wikipedia.org/wiki/Bond-dissociation_energy
     
  7. Teleologist

    Teleologist 2+ Year Member

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    That's more like it.
     
  8. Teleologist

    Teleologist 2+ Year Member

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    Your post is good up to this sentence. I would say that the effective negative charge stabilization afforded by higher s-character does not make the acid more likely to dissociate but rather makes the acid's conjugate base less potent - and thus less likely to reassociate with hydrogen protons again.
     
  9. Czarcasm

    Czarcasm Hakuna matata, no worries. 2+ Year Member

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    I appreciate the insight, but I prefer to view it differently. It's more likely to dissociate because it's conjugate base is "less potent" or more stable. They go hand in hand. Sure you can look at it from the reverse perspective as well, but regardless, it means the same thing. Stability of the conjugate base ultimately determines acid strength.
     
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  10. Teleologist

    Teleologist 2+ Year Member

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    Ah, good insight. Like a waterfall, we go from up to down (not the other way around). I read your post very narrowly - i.e without thinking about free energy considerations.
     
  11. BerkReviewTeach

    BerkReviewTeach Company Rep & Bad Singer Exhibitor 7+ Year Member

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    THIS!

    That is exactly the point of the MCAT and the learning that comes from it. When you realize the same concept can be viewed from multiple perspectives, you are ready for their exam.

    Whether you consider an sp-hybridized carbon as holding onto elections more tightly than an sp3-hybridized carbon, and thereby being less basic (less of an electron pair donor) or you consider an sp-hybridized carbon bonded to H as pulling electron density from the H more than an sp3-hybridized carbon pulls electron density from the H, and thereby making it more acidic (better proton donor), it's the exact same foundation concept. Seeing that is a sign of a great understanding of the concept.
     
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