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Which of the following planar structures would be aromatic?

molecules

Could someone help me determining the aromaticity of the compounds?

My Attempt:

(i) Not an aromatic compound since it only has 4 pi electrons that does not satisfy Hückel's (4n+2) rule.

(ii) Not aromatic because it only has 4 pi electrons, however I'm not sure if the oxygen contributes a p-orbital, but it shouldn't matter since 5 pi electrons still does not satisfy Hückel's rule.

(iii) I'm very confused because I'm not sure if the ketones are considered "part of the ring" therefore do they also contribute p orbitals? However if we ignore the ketones, then I believe the Nitrogens contribute 2 e- since they are sp2 hybridized? Therefore the 2 N atoms and 2 C atoms contribute a total of 4-not aromatic either?

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closed as too broad by orthocresol Jan 8 '18 at 15:35

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(i) that's correct

(ii) is aromatic, it fit's the 4n+2 rule with n=1. Here is a drawing of furan showing 2 lone pair electrons in a p-orbital that completes the cyclic aromatic ring. Furan's other lone pair of electrons is perpendicular to the pi system and does not play a role in furan's aromaticity.

enter image description here

(image source)

(iii) Edit to incorporate @jerepierre's comment:

Just like phenol can exist in equilibrium with its non-aromatic tautomeric keto form

enter image description here

so can derivatives of pyrimidinedione (or uracil). However, unlike phenol were the equilibrium lies far to the aromatic phenol side, these compounds usually (it depends on the substituents) exist in the keto form. The strength of the keto and amide bonds outweigh the weaker aromaticity of the pyrimidine skeleton and shift the equilibrium to the keto side.

enter image description here

(image source)

The question remains, is the keto form aromatic? As jerepierre points out, you can draw the following resonance structure for the keto form. In this double-dipolar resonance structure there are 6 pi electrons 2 in the double bond and 2 [a lone pair] on each nitrogen) suggesting that this resonance structure would be aromatic. Since this resonance structure contributes to the overall description of the molecule, it would be fair to say that this molecule has some aromatic character; certainly not as much as benzene or furan - but some.

enter image description here

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    $\begingroup$ I'm not sure about the uracil. Cyclohexadienone is clearly non-aromatic because of the sp3 center. Uracil is fully conjugated, and you can draw an aromatic resonance structure of uracil. I suspect that makes it aromatic, but one could argue either way. $\endgroup$ – jerepierre Apr 17 '15 at 15:34
  • $\begingroup$ @jerepierre I agree, it could be argued either way, not a good question. $\endgroup$ – ron Apr 17 '15 at 15:37
  • $\begingroup$ i also thought that the ketone substituents didn't influence aromaticity either $\endgroup$ – Nerdingout Apr 17 '15 at 15:40
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    $\begingroup$ It does influence aromaticity. It is only because we can draw the dipolar resonance form of the ketone that the molecule can be considered aromatic $\endgroup$ – ron Apr 17 '15 at 16:14
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    $\begingroup$ I would again like to point out, that aromaticity is a feature, that correctly can only be obtained from experiments. Lewis structures, even resonance structures can only explain, why there is a ring current, not predict if there is. Hückel's rule is a nice guideline for an ad hoc chemical assumption, but it can hardly be treated as a law. For what it's worth, aromaticity is simply defined, as "having a chemistry typified by benzene". goldbook.iupac.org/A00441.html $\endgroup$ – Martin - マーチン May 13 '15 at 12:01

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