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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?

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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.

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    $\begingroup$ This is correct, but I believe @terra2322 was asking how aromaticity is explained in the context of MO theory. $\endgroup$
    – Tyberius
    Mar 28 '17 at 3:28
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    $\begingroup$ {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.) $\endgroup$ Mar 28 '17 at 14:07
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    $\begingroup$ @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. $\endgroup$
    – Tyberius
    Mar 28 '17 at 16:17
  • $\begingroup$ 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. $\endgroup$ Mar 28 '17 at 21:46
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    $\begingroup$ @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. $\endgroup$ Apr 27 '17 at 4:03

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