14
$\begingroup$

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?

$\endgroup$
  • 12
    $\begingroup$ 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 Feb 10 '16 at 10:10
  • $\begingroup$ @bon so you are saying that practically anti-aromatic systems do not exist. $\endgroup$ – Abhirikshma Feb 10 '16 at 12:54
  • 5
    $\begingroup$ 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 Feb 10 '16 at 20:33
  • $\begingroup$ @Jan Apparently some cyclobutadiene derivatives are non-planar. onlinelibrary.wiley.com/doi/10.1002/anie.198002111/pdf $\endgroup$ – bon Feb 10 '16 at 21:28
  • 5
    $\begingroup$ @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 Dec 13 '16 at 2:44
6
$\begingroup$

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

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.