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First Row BDE's: Why is H-NH2 lower than H-CH3?

  1. Oct 2, 2016 #1
    Here are some first row BDE's (in kcal/mol):
    H-CH3 = 105 <---> [H]+ [CH3]-
    H-NH2 = 103
    H-OH = 119
    H-F = 136 <---> [H]+ [F]-
    This trend is often rationalized in terms of increasing ionic character (or with no-bond resonance). However, the H-NH2 BDE should have a higher ionic contribution than the H-CH3 BDE. Yet the H-NH2 bond is weaker. Can anyone explain this apparent contradiction?
     
  2. jcsd
  3. Oct 3, 2016 #2

    TeethWhitener

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    Bond dissociation energies are usually reported for homolytic bond dissociation: HF would dissociate to neutral H and F atoms, rather than H+ and F-. Where are you getting these values from? Here's one comparable source:

    https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf

    (with values in kJ/mol), where we note two things: First, the trend in the link provided is different (BDE of C-H is less than N-H), and second (and more importantly), the error in the measurements of CH and NH bond scission (8 kJ/mol) is larger than the difference between their BDE's (4 kJ/mol). So I think the best way to think about this is that H-CH3 and H-NH2 BDE's are comparable, meaning that the effects which stabilize the bonds in H2O and HF are of far less importance in CH4 and NH3.
     
  4. Oct 3, 2016 #3
    the reported values are homolytic. I'll have to check with my prof. to see where he got them. but yes, I think you're right, the two bde's are comparable. but then why is the difference between N and O larger than that between C and O?

    ps. sorry, <---> these are resonance arrows.
     
  5. Oct 4, 2016 #4

    TeethWhitener

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    Again, referring back to my original post, whether the N-H BDE is higher or lower than the C-H BDE is a matter of who you ask (compare the numbers from the pdf I posted to the ones you posted). The point is that the measurement error is larger than the difference between them, so that you can't really say for certain which one is lower. The only thing you can conclude is that they are comparable. The takeaway from this is that the processes that drive the BDE's higher for HF and H2O are clearly not very important in CH4 and NH3 (or at least, they don't lead to substantial differences between CH4 and NH3).
     
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