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Why is styrene aromatic? Doesn't it have 8 pi electrons, thereby violating Hückel's rule? Can someone offer an alternate, more through explanation?


marked as duplicate by Jan, M.A.R. ಠ_ಠ, Philipp, Wildcat, Loong Oct 27 '15 at 11:27

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    $\begingroup$ A compound is aromatic if it has 6 carbon atoms in a ring with 3 "double bonds", e.g. a benzene type ring. The rings can be joined. It doesn't matter what else is attached to the rings. Typically the hexagonal ring is drawn with one line between the hexagon vertices and a circle inside the hexagon to indicate the aromatic character of the bonding. $\endgroup$ – MaxW Oct 27 '15 at 6:11
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    $\begingroup$ @MaxW That is a very limited definition of aromaticity which isn't really useful. There are many aromatic compounds which do not have six membered rings (cyclopentadienide anion for example) and have rings containing elements other than carbon (pyridine, furan etc.). Also most people still draw benzene with alternating double and single bonds because it is more useful for drawing mechanisms. $\endgroup$ – bon Oct 27 '15 at 12:19

As Wikpedia Article says ,the criteria for a molecule to be aromatic are --

  • the molecule must follow Hückel's rule, having 4n+2 electrons in the delocalized, conjugated p-orbital cloud;

  • the molecule must be able to be planar;

  • the molecule must be cyclic; and,

  • every atom in the ring must be able to participate in delocalizing the electrons by having a p-orbital or an unshared pair of electrons.

When we are talking about aromaticity , we consider the electrons inside the ring. You are right that the styrene has 8 pi electrons but when checking for Huckel's rule, we only consider the electrons inside the ring. The alkenyl group is just an attachment to the stable aromatic compound which might take part in resonance but wouldn't affect the compound's aromaticity.

  • $\begingroup$ It's not quite that simple. Consider benzoquinone; isn't it a planar cyclic molecule with 6 electrons in a delocalized, conjugated p-orbital cloud and some side attachments (oxygens)? True, it is. But is it aromatic? $\endgroup$ – Ivan Neretin Oct 27 '15 at 7:12
  • $\begingroup$ @Ivan Neretin Benzoquinone is a case where double bonded oxygen atom is directly attached to the molecule which can undergo resonance to produce a resonance stabilised compound. And therefore it exists in aromatic state. A compound which has an option to enter either aromatic or antiaromatic state, will always go for aromatic state because it is more stable. And that is why we call it to be aromatic. $\endgroup$ – Quark Oct 27 '15 at 14:27
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    $\begingroup$ That's where we disagree. I would never call quinone aromatic. $\endgroup$ – Ivan Neretin Oct 27 '15 at 14:33
  • $\begingroup$ @IvanNeretin You confused me. Why is Quinone aromatic or anti aromatic? $\endgroup$ – Quark Oct 27 '15 at 16:37
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    $\begingroup$ As to me, quinone is not aromatic because the C-C bond lengths in its cycle are distinctly different, and it does not manifest chemical reactions typical of all aromatic compounds. The number of electrons is of secondary importance here. As for the Hückel's rule, it is not really applicable to the compounds with conjugated side chains. $\endgroup$ – Ivan Neretin Oct 27 '15 at 20:03

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