Benzene Stability: Bond Dissociation vs. Aromaticity

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    Benzene Stability
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Discussion Overview

The discussion centers on the stability of benzene, particularly examining the relationship between bond dissociation energy, bond lengths, and aromaticity. Participants explore how these factors contribute to the overall stability of benzene compared to alkenes and other molecular structures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether stability is reflected in bond dissociation energy, noting that benzene's stability is higher than expected due to aromaticity and electron delocalization.
  • Another participant requests clarification on the bond dissociation energy and the specifics of the IR absorption mentioned.
  • It is suggested that breaking the bonds in benzene also breaks its aromaticity, which may influence its stability.
  • A participant clarifies that their initial reference was to bond lengths rather than bond dissociation energies, noting that benzene's bond lengths are longer than those of alkenes.
  • One participant argues that bond length alone does not indicate further stabilization, while another claims that shorter bonds typically indicate stronger bonds and thus less reactivity.
  • There is a discussion about the role of delocalized pi electrons in contributing to benzene's stability, with uncertainty about how bond length factors into this stability.
  • A comparison is suggested between benzene and other similar structures lacking aromatic stabilization, such as anti-aromatic molecules, to better understand stability representations.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between bond length, bond dissociation energy, and molecular stability. There is no consensus on how to best represent or measure the stability of benzene.

Contextual Notes

Participants indicate that bond length may not fully capture the stability of benzene, and there is mention of potential limitations in comparing benzene to other molecular structures without aromatic stabilization.

mayer
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Might be a stupid question but got to 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
 
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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?
 
That's probably because when you break the one and half bond you also break the aromaticity of the benzene ring.
 
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.
 
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.
 
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.
 
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.
 
mayer said:
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.

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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