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Why can't just all anti-aromatic systems lose planarity (and hence conjugation) and be non-aromatic like cyclooctatetraene ?

Would they not be more stable that way? Because anti-aromatic systems tend to be highly destabilized.

What can be a good working rule to predict whether an anti-aromatic system will remain anti-aromatic or whether it will somehow manage to lose planarity and become non-aromatic?

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    $\begingroup$ All the anti-aromatic systems I can think of undergo distortions to avoid it. Can you provide some examples of the systems you are thinking of? $\endgroup$
    – bon
    Commented Feb 10, 2016 at 10:10
  • $\begingroup$ @bon so you are saying that practically anti-aromatic systems do not exist. $\endgroup$
    – Abhirikshma
    Commented Feb 10, 2016 at 12:54
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    $\begingroup$ I think that there is exactly one antiaromatic system (and its derivatives) known to mankind and that is cyclobutadiene — which is too strained to deplanarise in any way so it has to be planar. All other systems distort themselves as far as I know to avoid antiaromaticity — or in some cases such as the cyclopentadienyl cation, don’t even allow formation. $\endgroup$
    – Jan
    Commented Feb 10, 2016 at 20:33
  • $\begingroup$ @Jan Apparently some cyclobutadiene derivatives are non-planar. onlinelibrary.wiley.com/doi/10.1002/anie.198002111/pdf $\endgroup$
    – bon
    Commented Feb 10, 2016 at 21:28
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    $\begingroup$ @Jan Doesn't cyclobutadiene avoid being antiaromatic by distorting from a square structure to a rectangular structure with alternating single and double bonds? $\endgroup$
    – ron
    Commented Dec 13, 2016 at 2:44

1 Answer 1

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They do try to escape planarity if possible. As a simple example, consider cyclooctatetraene. It has 8 electrons, which is a non-Huckel number, considering it's a multiple of four, and would correspond to an antiaromatic system, if the ring was planar, and allowed delocalization. However, cyclooctatetraene isn't planar. It's rather "tub-shaped", as seen in this structure here:

enter image description here

Click the image for interactive 3-D model

The electrons aren't delocalised either. Cyclooctatetraene is known to add bromine, and generally behave like a cyclic polyalkene in many aspects.

The only cases where molecules don't escape planarity is when they cannot. This happens in the case of cyclobutadiene. Such a molecule is so small, it cannot be distorted to such extents. As a result, it remains planar, antiaromatic, and immensely unstable:

enter image description here

Click the image for interactive 3-D model

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