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I'm new here and I'm still learning, I understand that there four rules for aromaticity:

  1. cyclic
  2. conjugated double bonding (every atom in the ring should be sp2)
  3. huckle number should be 2, 6, 10, 14, 18, ..18
  4. planar ring

The 2nd question should be 4 electrons and is antiaromatic, but how about the first question?

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Cytosine is aromatic, there are 6 electrons in the ring. Two pairs of pi bonds(thus 4 electrons) and a lone pair on Nitrogen(the one which has H written below it). The other Nitrogen (which is forming a pi bond) in the ring, does have a lone pair, however it is "localised". That lone pair cannot participate in resonance, and hence it is not counted when considering aromaticity.

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  • $\begingroup$ I put 6, but I got wrong answer, that is why I'm asking here, is 6 or 10? @Kaiser $\endgroup$ – user99277 Nov 1 '20 at 7:31
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My answer would be to refuse to answer this case.

The 4n+2 rule applies only if the electrons are conjugated in a cycle -- nothing more, nothing less. If you have pendant pi bonds in the conjugated system, like the carbonyl group in cytosine and many similarly structured compounds, you don't have the right conditions to apply the 4n+2 rule at all.

Interior conjugated atoms, as in pyrene, also mean you don't have specifically conjugated cycle.

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  • $\begingroup$ Cytosine is proven to be aromatic, how do they calculate the electrons inside the ring, it seem they add 2 if there is HN: present as hetero, but not the case in NH2, I want to know exactly how many pi bonds and electrons inside the cytosine ring. @OscarLanzi $\endgroup$ – user99277 Nov 1 '20 at 16:45
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    $\begingroup$ The issue, of course is not whether it's aromatic. No doubt it is. Rather the issue is whether we can really apply the 4n+2 rule that they rendering in many a textbook. No doubt we can't; IUPAC recommends the rule only for single, fully symmetric rings with no additional conjugated atoms. $\endgroup$ – Oscar Lanzi Nov 1 '20 at 16:53
  • $\begingroup$ hmm..that's interesting...thank you for pointing this out!, would the answer be 10 electrons in resonance? @OscarLanzi $\endgroup$ – user99277 Nov 1 '20 at 17:16
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    $\begingroup$ Yes there would be ten electrons in resonance. But without them being in a ring, the figure being 4n+2 is not really a predictor of aromaticity. Cyclopropenone has a pendant carbonyl group and shows aromatic character too -- with four conjugated electrons. $\endgroup$ – Oscar Lanzi Nov 1 '20 at 17:23
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    $\begingroup$ Thank you so much for these tips, I know now that Huckel rule has its limitation like Lewis acid base concept, or octet rule in carbon, so when we have pendant carbonyl group or methylene such in fulvene, do we consider the conjucated electron to be zero because there is a missing conjugation fulvene en.wikipedia.org/wiki/Fulvene @OscarLanzi $\endgroup$ – user99277 Nov 1 '20 at 17:45

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