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Nitrogen is similar to carbon in terms of electronegativity and size. MO theory suggests that the more electronegative N would result in a lowering of the of the π-bonding energy, E(πCN), below the E(πCC) of ethylene. As I understand it, the MO energy trend should be:

  • Eπ(CO) > Eπ(CN) > Eπ(CC) > Eπ*(CO) > Eπ*(CN) > Eπ*(CC)

Meaning the trend in bond strength should be:

  • formaldehyde (H2C=O) > azomethine (H2C=NH) > ethylene (H2C=CH2)

However, bond dissociation energy implies that the C=N bond is the weakest compared to C=C and C=O. https://labs.chem.ucsb.edu/zakarian/armen/11---bonddissociationenergy.pdf

  • formaldehyde (H2C=O) = 732 kJ/mol
  • ethylene (H2C=CH2) = 682 kJ/mol
  • azomethine (H2C=NH) = 644 kJ/mol
JAClarke
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  • https://chemistry.stackexchange.com/questions/164270/why-does-a-carbon-double-bond-break-readily-but-this-is-not-so-for-other-eleme – Mithoron Nov 26 '22 at 19:06
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    To me the argument you make appears to be only taking the pi bond into consideration, which cannot be explained by dissociation energies (or enthalpies). For your argument you might want to focus on something like hydration energies to come to an understanding about the pi bond. For examples ethene to ethane or formaldehyde to methanol. When you look at bonding enthalpies you need to consider the stability of the fragments, too. (FWIW you can use MathJax in combination with mhchem to make you post look beautiful.) – Martin - マーチン Nov 26 '22 at 19:24
  • Thanks for the comments, in reading more on the subject I can see why using bond energies as a measure of bond strength can be somewhat misleading. "Carbon–Nitrogen Double Bonds (1970) by C. Sandorfy pg 6-7" has a great discussion of this exact question. Essentially, bond energies are highly dependent on molecular composition and structure and "transferability to bond strength can only be approximate". In fact the bond energies referenced match my expected values of C=O > C=N > C=C.

    Edit: Using an alternative bond strength observable like bond length or IR frequency stretch is preferable.

     – JAClarke Nov 26 '22 at 23:53

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