6
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I am trying to figure out which rings in callistephin are aromatic:

Structure of callistephin with pyrylium ring highlighted

I know that the benzene rings definitely are aromatic, but I'm not sure about the ring with the positively charged oxygen (highlighted in red).

How does the Hückel $4n+2$ rule apply to this ring?

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    $\begingroup$ If you take both the rings together, you get a hetero-naphthalene which is aromatic. Other than that you should have asked your former question in a more general way that would have allowed answering both in the same way. $\endgroup$ – Jan Oct 8 '15 at 20:31
  • $\begingroup$ @Jan can I ask how many pi-electrons are in this hetero-napthalene? $\endgroup$ – justbehappy Oct 8 '15 at 20:47
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    $\begingroup$ 10, just like in normal naphthalene. (Substitute the $\ce{O+}$ with $\ce{CH}$ and you get naphthalene; that was what I was playing at.) $\endgroup$ – Jan Oct 8 '15 at 20:49
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Pyrylium is aromatic. Following Hückel's rule, pyrylium is uninterrupted (no $\mathrm{sp^3}$ hybridization), all atoms have p-orbitals and there is 6 π electrons. the 6 π electrons indicate that Hückel's rule $(4n+2)$ is agreeable for pyrylium, $n = 1$.

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