Is 5,6,11,12-tetradehydrodibenzo[a,e][8]annulene aromatic?
![5,6,11,12-tetradehydrodibenzo[a,e][8]annulene](https://i.sstatic.net/yJZjM.png)
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.
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asked Aug 19, 2016 at 21:33
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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. $\endgroup$– Ivan NeretinCommented Aug 20, 2016 at 7:01
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1 Answer
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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.)
Moreover, if I plot the HOMO, it's highly delocalized:
Similarly, the LUMO is also delocalized across the entire molecule:
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:
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).
answered Aug 23, 2016 at 18:01
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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
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1$\begingroup$ That is bloody weird. Glad you ran the calc. Would you add HOMO/LUMO +/- 3 energies to your post? $\endgroup$ Commented Aug 23, 2016 at 18:25
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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. $\endgroup$– MithoronCommented Aug 23, 2016 at 18:46
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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
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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