Is 5,6,11,12-tetradehydrodibenzo[a,e][8]annulene aromatic?


If I use Hückel's rule for the whole molecule, I get the answer as anti-aromatic. But the compound has two benzene rings which are individually aromatic.

  • 8
    $\begingroup$ I'd go as far as to say there is no such thing as aromatic compounds. Sure, people say it all the time, and we even have a tag to that effect; so what? This is just a shorthand for "compounds with aromatic rings". Aromaticity is a property of one ring, not of the whole compound. A molecule, as you know, may be arbitrarily complicated. It may involve some anti-aromatic rings together with some aromatic ones. Then you'll be compelled to consider such compound both aromatic and anti-aromatic at the same time, which does not feel well. $\endgroup$ Commented Aug 20, 2016 at 7:01

1 Answer 1


The molecule actually optimizes to a planar, delocalized $\pi$ system

Using B3LYP/6-31G(d) optimizations, we see that the molecule optimizes to become planar. (The initial configuration was intentionally bent.)

enter image description here

Moreover, if I plot the HOMO, it's highly delocalized:

enter image description here

Similarly, the LUMO is also delocalized across the entire molecule:

enter image description here

Consequently, I don't see any indication that the "side" benzene rings are localized from the cycloocta-1,5-diyne ring. The bond lengths are in fairly good agreement with the crystal structure:

enter image description here

Here's the crystal structure from J. Am. Chem. Soc., 1975, 97 (3), 658–659. Note there is a profound triple bond character, but the bridging bonds (1.44 Å) are somewhere between a typical aromatic bond length (1.40 Å) and a full single bond (~1.54 Å).

enter image description here

  • 2
    $\begingroup$ I will admit my initial intuition was that the central ring was non-planar, but figured "why not run a calculation and see?" $\endgroup$ Commented Aug 23, 2016 at 18:12
  • 1
    $\begingroup$ That is bloody weird. Glad you ran the calc. Would you add HOMO/LUMO +/- 3 energies to your post? $\endgroup$
    – Lighthart
    Commented Aug 23, 2016 at 18:25
  • 4
    $\begingroup$ Guys, guys, that's how stabilisation via anellation with aromatic rings works. Benzene rings are destabilised, but central more stable. Cyclooctadienediyn most probably doesn't exist. $\endgroup$
    – Mithoron
    Commented Aug 23, 2016 at 18:46
  • 1
    $\begingroup$ @Mithoron - yes, exactly. That's why I wanted to add a better answer for this one. $\endgroup$ Commented Aug 23, 2016 at 18:50
  • 1
    $\begingroup$ @Geoff it turns out that breaking the nonconjugated pi bond by adding those hydrogens does destroy the planarity of the central ring. See here. $\endgroup$ Commented Oct 27, 2019 at 21:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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