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Which have unusually large dipole moments?

  1. Jul 18, 2012 #1
    There are three compounds:

    1.) fulvene

    2.) calicene

    3.) I don't know the name of the third compound. It looks like calicene except that there is not a double bond inside of the three-membered ring.

    Anyways, it turns out that fulvene and calicene have unusually large dipole moments, but not the third compound.

    One answer choice was only calicene, another was both calicene and fulvene. There was no answer choice for all three.

    I knew that calicene definitely has a very large dipole moment, because the resonance structure formed when the pi electrons joining the two rings moves to the carbon of the pentadiene acheives two aromatic compounds. As for fulvene and the third compound, only one ring acheives aromaticity. The resonance structure for fulvene has a primary carbocation substituent, and the one for the third compound has a tertiary carbocation substituent, and angle strain. I thought that if fulvene had an unusually large dipole moment, so must the third compound. The resonance structure for compound 3 has angle strain, but I thought the overall stability would be similar to the stability of a primary carbocation.
    I could not choose all three, and I believed the third compound would have an equal or greater dipole moment than fulvene, so I went with just calicene. But I'm wrong. Could someone help me understand why fulvene would have a larger dipole moment than the third compound?
     
  2. jcsd
  3. Jul 21, 2012 #2

    chemisttree

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    And you say this third compound doesn't have the double bond in the three-member ring that calicene has?

    Hmmmm.

     
  4. Jul 22, 2012 #3
    Thanks for the response.

    As I said, the original answer was both fulvene and calicene. The resonance structure for fulvene only has one aromatic ring yet still apparently has a large dipole moment, so we can't dismiss the third compound on the basis that its resonance structure only has one aromatic ring instead of two.

    Anyways, as it turns out, the third compound was drawn incorrectly on the test (there should not have been a double bond between the 5-membered ring and 3-membered ring). If it has been drawn correctly, there would be no ambiguity and the answer would be just calicene and fulvene. As it was drawn on the test, the third compound did have a large dipole moment. It was just an error. I wanted to edit my post to say this but I couldn't find the edit option.
     
  5. Jul 22, 2012 #4

    chemisttree

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    No, but if the generation of two aromatic structures stabilizes the charge separation in the zwitterion it should have a larger dipole moment than if it resonated between a structure containing a relatively unstabilized cyclopropyl cation and the neutral structure, the equilibrium favoring the neutral structure rather than the zwitterion.
     
  6. Jul 22, 2012 #5
    So you're saying that despite the fact that the resonance structure for fulvene has a primary carbocation as opposed to the resonance structure for the third compound (really wish I knew the name) which has a tertiary carbocation, the fact that it is also a cyclopropyl cation far outweighs the relative stability gained by having a tertiary carbocation as opposed to a primary carbocation? So much so that it does not have a large dipole moment, whereas fulvene does?
     
  7. Jul 22, 2012 #6

    chemisttree

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    Sorry, I got a little fixated on calicene/'not-calicene' comparison and forgot you actually asked about fulvene. You're right of course, if fulvene has an unusually high dipole moment then 'not-calicene' should as well.
    BTW, I would call 'not-calicene' 1-(cycloprop-1-ylidene)-2,4-dicyclopentadiene.
     
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