I studied cyclobutadiene as an anti-aromatic compound. But I am unable to draw the resonance structures for cyclobutadiene. Can anyone help me?

What is the driving force for the resonance to start?

Is it possible that a compound is having conjugate double bond system but not satisfying resonance structures?

Is a compound called as anti-aromatic if it is not undergoing resonance?

  • $\begingroup$ possible duplicate of Is cyclobutadiene anti-aromatic? $\endgroup$ – Klaus-Dieter Warzecha Jan 30 '15 at 18:52
  • $\begingroup$ Not quite a duplicate $\endgroup$ – Lighthart Jan 30 '15 at 19:05
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    $\begingroup$ @KlausWarzecha, it's not a duplicate. The problem was the poorly-stated title of this question. $\endgroup$ – M.A.R. Jan 30 '15 at 19:24
  • $\begingroup$ @KlausWarzecha I think some of the question are answered in that question but still is can't be duplicate. $\endgroup$ – Freddy Jan 31 '15 at 6:54

Antiaromaticity is a concept describing an explicitly fictional situation, no molecule is antiaromatic, because all molecules will react to avoid the situation.

So the answer to the implied question is that cyclobutadiene has very little resonance because the molecule is prevented from being antiaromatic (by the nature of the universe and its physics).

Elaboration: To avoid this situation, butadiene is very reactive; it will undergo an intermolecular Diels-Alder reaction avoid this antiaromatic state. When reaction is prevented something much more interesting happens. When frozen in an argon matrix, x-ray crystallography of cyclobutadiene indicates the molecule is highly rectangular, not square. For dienyl resonance, there is some changing of bond lengths. In butadiene, the 2,3 bond shortens. If this were to occur in cyclobutadiene, then there would be degeneracy in the orbitals leading to a Frost diagram that would describe an antiaromatic state. So if reaction is prevented, the molecule distorts its own geometry.

The fictional resonance structures you would draw are double bonds on the top and bottom, and double bonds on the sides. However, generally, one might assume there is no resonance for this molecule.

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    $\begingroup$ I disagree with the first statement of this answer. Antiaromaticity must be a real concept if molecules go out of their way to avoid it. Perhaps there aren't any molecules that exhibit antiaromaticity, but that's not what this statement implies. $\endgroup$ – jerepierre Jan 31 '15 at 2:18
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    $\begingroup$ Molecules " will go to great lengths to avoid ..."? Avoid an explicitely fictional concept? $\endgroup$ – Georg Jan 31 '15 at 14:11
  • $\begingroup$ People used to go to great lengths to avoid dragons as well. Although I must concede, as jerepierre indicates, there may be a better way to say it. $\endgroup$ – Lighthart Jan 31 '15 at 17:17
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    $\begingroup$ What you mean is that there are no compounds that exhibit antiaromaticity. Molecules will avoid antiaromaticity in a number of ways, such as skewing out of plane (eg cyclooctatetraene) or acting as two separate pi systems instead of a conjugated system (eg cyclobutadiene). The only time that there's trouble is when an exam, let's say, has a question to pick out antiaromatic compounds. In that case, the correct response would be that none exist for the reasons that you describe. But we use the concept of antiaromaticity to explain why cyclobutadiene doesn't behave the way we expect (conjugated) $\endgroup$ – jerepierre Feb 10 '15 at 1:01

You mix up resonance and (anti-)aromaticity the latter being
interpreted today by molecular orbital symmetry. This is unhistoric.

Resonance is a concept predating Woodward-Hoffman and Fukui. Resonance meant that delocalizing the electrons of bond(s) lowers the energy irrespective of the kind of orbitals. This works for conjugated double bonds, but for aromatic compounds it is not wrong, but insufficient. The amount of stabilisation of eg benzene is more than conjugation can explain.

For antiaromatic compounds oldfashioned resonance is simply wrong, and drawing "resonance stuctures" is misleading, because the resonance destabilisizes the molecule.

If we had a friendly Laplacian demon clamping the 4 carbon atoms of cyclobutadiene on the corners of a square with a bond length similar to benzene, we would see that cyclobutadiene is a triplett diradical.

Maybe somebody will synthesize a molecule where a cyclobutadiene core is clamped in in such a manner by some planar ring structures "around" some day?


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