Benzene Stability

  1. Might be a stupid question but gotta ask. The bonds of benzene have a bond dissociation energy in between that of a single C-C bond and an alkene yet its stability is much higher that expected due to aromaticity/e- delocalization. Is stability not necessarily reflected in bond dissociation energy. This fact had slipped my mind till I found out that the IR absorption is lower for a aromatic C-C bond then it is for a Alkene C-C bond.

    Thanks
     
  2. jcsd
  3. DrDu

    DrDu 4,189
    Science Advisor

    Sorry, but I don't get your question.
    Do you have some numbers to make your point on dissociation energy clearer?
    And what is lower? IR absorption intensity, frequency?
     
  4. That's probably because when you break the one and half bond you also break the aromaticity of the benzene ring.
     
  5. Ah i probably shouldn't have said bond dissociation. The values I had were bond length values. Alkene ( 1.34 A) and Aromatic C=C (1.38 A). I don't recall the single bond bond length though it was longer than those aforementioned. The point is that the benzene bond lengths were measured longer than that of an alkene. The stability does not seem to be apparent in the bond length when it comes to Benzene. That is my question, hopefully that clears things up. My apologies for the confusion.
     
  6. DrDu

    DrDu 4,189
    Science Advisor

    In benzene, you would expect the bond length to be between that of a single and a double bond, which is what you observe. I don't see how you could deduce further stabilization from the bond length alone.
     
  7. Well bond length is a partial indicator of molecular stability because shorter bonds equal stronger bonds equals less reactive. My question is where is the stability represented within the molecule if not in the bonds.
     
  8. I'm not sure if it's a good way to simply determine the stability by bond length but a part of the stability of benzene comes from the sort of delocalized pi electrons. I don't know the role of bond length plays here; the stability of benzene might be due to the fact that once you break one bond you break the aromaticity like I mentioned.
     
  9. DrDu

    DrDu 4,189
    Science Advisor

    It probably is, but to see it, you would have to compare the bond length in benzene with that in another molecule having similar structure and bond order but no aromatic stabilization. Anti-aromatic molecules come to mind, but these show usually bond length alternation or deformation.
    However, this comparison may be a fun project using some ab initio quantum chemistry program where geometry optimization can easily be constrained to allow only for equal bond length and planar molecules .
     
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