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I'm trying to work out which are the aromatic ring systems in the drug aklavinone:

Structure of aklavinone with quinone ring highlighted

I am trying to follow Hückel's rules that the compound must be cyclic, be planar, possess a p-orbital on every atom in the ring, and have a π-system with $4n+2$ electrons.

I see that the benzene rings are obviously aromatic. But does the quinone ring (in red) also count as part of the 'aromatic ring system'?

orthocresol
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justbehappy
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  • related http://chemistry.stackexchange.com/questions/22100/is-the-aromaticity-broken-in-some-resonance-structures-of-para-nitro-aminobenzen – Mithoron Oct 08 '15 at 13:35

3 Answers3

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Strictly speaking, once we have anything more than a single cycle without conjugated side chains, the Hückel's rule does not really apply, and the very definition of aromaticity becomes somewhat muddy. But that's not an answer they expect from you, anyway. So just point to the first and third rings and call them aromatic, and that would be it.

Why not the second? Well, because the quinone fragment is not aromatic. Its bond length are different from each other. These $\ce{C=O}$ bonds are always double, and the $\ce{C-C}$ bonds next to them are always single; you can't draw a resonance structure where that would be otherwise.

Ivan Neretin
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    But I can ;) Sorry but they are still conjugated. – Mithoron Oct 08 '15 at 13:46
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    And more importantly that quinone ring only has $4\ \pi$ electrons. – Ben Norris Oct 08 '15 at 13:47
  • This is too much of a simplification imho. Either the entire system is aromatic through all of its conjugation, or the conjugation is broken somewhere (cyclooctatetraene). It’s not like the bond lengths of the two phenolic rings be equal whether single or double. – Jan Oct 08 '15 at 13:50
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    @Mithoron, I'd like to take a look at these. The quinone system is conjugated, I won't deny the obvious, but aromatic it is not. – Ivan Neretin Oct 08 '15 at 13:58
  • @Jan, of course my answer contains a good deal of simplification, but I don't agree with your concept either. Surely this whole system is conjugated to some extent, but still we may point out some rings that are "more aromatic" and others that are less so. – Ivan Neretin Oct 08 '15 at 13:58
  • @IvanNeretin It's not aromatic but still your last sentence is wrong 'cause it suggests no conjugation. – Mithoron Oct 08 '15 at 14:10
  • @Mithoron So you are judging conjugation based on resonance structures? What kind of structures should those be, say, for butadiene? Something with the double bond in the middle and loose radicals dangling on both ends? Well, something like that you can have here, too. – Ivan Neretin Oct 08 '15 at 14:16
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Whether the ring is "aromatic" or not is of no importance. You can't ever fit a structure with pendant pi bonds into the "4n+2" rule. But you can draw molecular orbital structures. In para-quinone you find an alternant system with four bonding pi orbitals and four antibonding ones. So the important number is eight pi electrons being the optimal number for the system, not any number of electrons in the ring or whether the ring fits some rule for aromaticity. And we have that both here and in the parent quinone.

Oscar Lanzi
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1,4-benzoquinone is NOT aromatic even if it could satisfy Huckel's (4n+2) pi electron rule. The carbonyl pi electrons do not resonate within the ring = no proper conjugation, just cross-conjugation.

Indrajit Chakraborty
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