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For example, benzene has 6 $\pi$ electrons. Which means the p orbitals will be occupied, and only the bonding molecular orbitals. But 1,3-cyclopentadiene has 4 $\pi$ electrons, which means 2 of the 3 molecular bonding orbitals will be occupied. Why does this make it non-aromatic?

Melanie Shebel
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user42041
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There's 4 criteria for aromaticity: 1. The compound must be cyclic. As in a ring structure of some kind. 2. The compound must be planar. All the atoms making up the ring must be on the same plane. The hybridization of bonds causes this which is why benzene is planar and cyclohexane is not. 3. The compound must be conjugated. Therefore it must alternate between single bonds, double bonds, lone pairs, or empty orbitals. For example, two double bonds in a row mean it is not conjugated. But a single bond connected to an atom with an empty orbital still satisfies the rules of conjugation. Another example, benzene alternates single bonds and double bonds, so it is conjugated. 4. Must satisfy Huckel's rule. So the number of pi electrons must fit in the equation 4n+2, with n being a whole number or 0. So 6=4n+2 works but 4=4n+2 would mean n=1/2 so that does not work.

Nick
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    This is correct, but I believe @terra2322 was asking how aromaticity is explained in the context of MO theory. – Tyberius Mar 28 '17 at 03:28
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    {4n+2} is such an obsolete way to describe the Huckel number. A compound satisfies Huckel's Rule if it contains an odd number of pi-electron pairs. Done. No more need to confuse the new students. (Not directed at Nick specifically, just trying to get the word out.) – electronpusher Mar 28 '17 at 14:07
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    @electronpusher I'm a little embarrassed that I never made that connection, but looking at the formula after reading your comment its fairly obvious now that is all the formula is saying. – Tyberius Mar 28 '17 at 16:17
  • It's ok, I felt that way too. It's still a little known simplification. An excellent example of how cumbersome formalities may persist for historical reasons, even in the face of more efficient and clear ways of presenting information. – electronpusher Mar 28 '17 at 21:46
  • Yeah, this Huckel number was very odd to me. I know it works, but i don't feel like it would be right to justify aromaticity using only this... – user42041 Mar 29 '17 at 01:06
  • Many times i rather see if the carbons are all conjugated, if there is sp3 in the middle of them... – user42041 Mar 29 '17 at 01:06
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    @electronpusher finally someone agrees with me! I also used to insist on using "odd numbers of electron pairs". More often than not, my peers would count individual electrons, try to use 4n+2; the really long way of finding. – Pritt says Reinstate Monica Apr 27 '17 at 04:03
  • @electronpusher Hückel's rules are derived from a very narrow set of compounds, it does not make sense to use that rule at all in the broader concept of aromaticity, as there is still no consensus of what actually constitutes aromaticity. Just drop the concept completely. – Martin - マーチン Apr 27 '17 at 05:18
  • My statement was not about the validity or scope of Huckel's Rule, I just suggested improving the pedagogical presentation whenever it does happen to be invoked. – electronpusher Apr 27 '17 at 23:34