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Example compound

Here, the compound itself appears to be non-aromatic, but if the double bond between the ring and the oxygen is broken into a single bond, with oxygen pulling the electrons towards itself and acquiring a negative charge, and the ring acquiring a positive charge, it appears to be aromatic. Does that make the overall compound aromatic?

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2 Answers 2

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Not always.

Consider, for example, 4-pyrone. It is possible to draw canonical structures containing the pyrylium heterocycle, which is aromatic. However, the contribution of this aromatic resonance form is rather small, as it requires creating separate opposing charges which could otherwise cancel out, and in particular places a formal positive charge on an electronegative oxygen atom. But more than just argumentation or application of simplistic rules of thumb (e.g. Hückel's rule), certain physical measurements can give an indication for or against aromaticity. The response of compounds to magnetic fields, in particular, is often used. In the case of 4-pyrone, there is indication it may not be aromatic.

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In the case of cyclopropenone, however, it seems that aromaticity is important. Even though its reactivity is completely different to most standard aromatic compounds (due to its ring strain), the heavily deshielded $\mathrm{^1H}$ NMR signal at a chemical shift of 9.11 ppm in $\ce{CDCl3}$ (see note 7) is suggestive of an aromatic ring current (another magnetic field effect), with the particularly high value being attributable to the positive charge in an aromatic cyclopropenium ring.

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    $\begingroup$ Hückel's simplistic rule arrives at the same result in a fraction of the time. $\endgroup$ Dec 21, 2022 at 4:20
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    $\begingroup$ @KarlsMaranjs ..... but as pointed out in the answer you just replied to, Hückel's rule is hardly a foolproof method. It has arrived at the right conclusion purely by chance. If I came up with a "rule" that said "all molecules are aromatic", it would also arrive at the same (correct) conclusion in an even shorter time, but you surely wouldn't think it sensible to use such a rule. $\endgroup$ Dec 21, 2022 at 18:02
  • $\begingroup$ @orthocresol by chance? Hückel’s rule is derived from the MOT. A huge portion of theoretical chemistry was developed based on this theory, that you just called chance. Still, it’s not the only criteria to meet to determine if a molecule is aromatic. Of course every rule comes with exceptions, but in most cases if all 4 criteria are met then the molecule is almost certainly aromatic. Even for this one which is a very particular case, with the right analysis you have the right answer. And yes, exp methods are always the way to go, but without extra information Hückel is your best ally here. $\endgroup$ Dec 21, 2022 at 18:27
  • $\begingroup$ @KarlsMaranjs Huckel's 4n+2 rule is a single, highly simplified, rule-of-thumb obtained from the generalisation of MO theory to monocyclic compounds. It does not have the same fundamental importance that MO theory itself has, so the wider application of MO theory within chemistry that you cite is not relevant. For the rest of your comment, I have nothing to say except what I already said, so I won't repeat myself - e.g. "with the right analysis you get the right answer" - I can claim the same about my obviously-false rule. I am not saying that Huckel is false, btw, just too simplified. $\endgroup$ Dec 21, 2022 at 19:32
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It is cyclic, planar and conjugated, so the last criteria to meet is the Huckel rule. In this case you have 2 $\pi$ electrons, because you can consider the O takes one out of the ring so the O - C bond will be polarized as $\ce{O-}-{C}+$:

$$\ 4n + 2 = 2 $$ $$\ 4n = 0 $$ $$\ n = 0 $$

So I would say the molecule is aromatic

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