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Aromaticity of Conjugated Ring - Counting Pi Electrons

  1. Mar 27, 2017 #1
    My professor drew the following molecule on the board and asked us how many pi electrons this aromatic molecule has.

    Screen Shot 2017-03-27 at 11.26.29 AM.png

    Everyone in the class said 14, as there are 7 double bonds, with two pi electrons from each bond.

    He told us that there are only 10 pi electrons in this molecule, refused to explain why there were 10 instead of 14, and berated us for not understanding why. My study group and I have spent the last week pouring over textbooks and websites and none of us, nor anyone in the class, can understand why there are only 10 pi electrons. So I'm turning to y'all.

    What are we missing here? Can anyone give me a clue?

    (And sorry for the weird drawing - the two bonds in the center of the molecule to the two hydrogens should be the same length but I was having trouble with the online drawing tool. He also didn't specify the stereochemistry of anything, so I'm not sure if they are cis or trans to each other. Would that affect it?)
     
  2. jcsd
  3. Mar 28, 2017 #2

    TeethWhitener

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    I'll be honest, I don't see how there could be anything other than 14 pi electrons in this molecule. The only thing I could maybe believe is that the central hydrogens distort the structure enough to make the outer ring nonplanar and therefore non-aromatic. But I wouldn't expect someone to be able to glean that from a skeletal structure like the one you were given.
     
  4. Mar 28, 2017 #3
    I don't know. I think it's 14 too. There are 14 sp2 carbons. Although due to the hydrogen atoms in the middle, the orbitals would be deformed a little, but that shouldn't be significant enough to cut off the pi orbitals.
     
  5. Mar 28, 2017 #4

    DrDu

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    In the following article, they analyze the dimethyl analog, which is certainly more stable than the original compound. They also state explicitly that it is a 14 pi electronic compound.
    http://www.sdsc.edu/~kimb/mscat/mitchell.html
     
  6. Mar 28, 2017 #5

    TeethWhitener

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    A structure search in SciFinder turns up only papers referring to this as a [14]annulene. Maybe your professor is using base 14? :woot:

    Edit: Just saw @DrDu got to this right before me.
     
  7. Mar 28, 2017 #6
    Thank you for the link!

    I now suspect the professor misspoke and was unwilling to admit he made an error. He has also claimed that in Huckel's rule, n = the number of pi electrons, so I take all of his scoldings with several grams of NaCl. :olduhh:
     
  8. Mar 28, 2017 #7
    Ha! Perhaps. Thanks for the help!
     
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