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I am wondering which of the inner two rings of this compound are aromatic? Compound 153

I know the rules for aromaticity. But this case is difficult for me.

Regarding the smaller ring, I guess the sulfur contributes one lone pair of its valence electrons into the system. This means from the nitrogen (in the 5-ring) only one electron is needed for the π-system (with 6 electrons). But this nitrogen atom has only one lone electron pair, thus one of the electrons would not be part of the π-system of the smaller ring. However, this nitrogen atom in the 5 membered ring is also part of the 6-ring and there it could share both of its free electrons to this larger ring. Then the second nitrogen which is only in the 6-membered ring could contribute one electron only to the aromatic system, and thus having a happy free lone pair left.

What do you think of that? Which rules, logic or reasoning can confirm this? And isn't it a problem that the first nitrogen shares one electron with the 5-ring and the two with the 6-ring?

Also, is there software which can predict this reliably? I haven't found anything so far.

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    $\begingroup$ That is kinda interesting, your reasoning is rather muddled though. If not taking into account other mesomeric structures, both heterocyclic rings would be non aromatic, but those structures could be so big contributors that they could be aromatic nevertheless. $\endgroup$
    – Mithoron
    Commented Jun 13, 2017 at 0:16

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I think that all the four rings are aromatic.

If the top nitrogen donates it's pair of electrons to the ring, and oxygen takes it's pair, the three fused rings would have 14 electrons in the rings' peripheral, causing it to be aromatic:

enter image description here

Even the 3D drawings of this molecule confirm it's planarity of the 3 fused rings:

enter image description here

enter image description here

I think we can at least suspect all the rings in the molecule to be aromatic.

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    $\begingroup$ Do I conclude correctly from your post that we don't have to consider each individual ring as aromatic (as I have done), but the entire peripheral ring (as you have done), to get the aromaticity of polycyclic compounds? And that peripheral ring has to satisfy Hückels rule I guess. $\endgroup$
    – Jadzia
    Commented Jun 13, 2017 at 11:22
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    $\begingroup$ @Jadzia That's right, just the periphery. For the 3D structure I used this website: chemapps.stolaf.edu/jmol/jmol.php?model=[INSERT SMILES HERE] In the place of [INSERT SMILES HERE] input the SMILES of the compound. For your molecule the SMILES is: Oc3nc2sc1cc(Cl)ccc1[n+]2c([O-])c3Cc4ccccc4 $\endgroup$
    – Pritt says Reinstate Monica
    Commented Jun 13, 2017 at 11:26
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    $\begingroup$ Thanks for your answer. I just found out that for polycyclic aromatic compounds the Hückel rule does not need to be satisfied. en.wikipedia.org/wiki/H%C3%BCckel%27s_rule $\endgroup$
    – Jadzia
    Commented Jun 13, 2017 at 13:05
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    $\begingroup$ @Jadzia Not in the usual way, but the peripheral does count as one ring, and should satisfy Huckel rule. $\endgroup$
    – Pritt says Reinstate Monica
    Commented Jun 13, 2017 at 13:24
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – Jadzia
    Commented Jun 13, 2017 at 17:11

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