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Homework Help: Resonance structures you don't see

  1. May 20, 2012 #1

    I have silly question on resonance, it's so silly, I couldn't find an answer:
    Lets say there is a O=CR-CH=CH2 bonding in a molecule, R = some other group. You can get (I have put the charges in brackets and Superposition (I think thats what its called):
    (1-)O-C=C-C(1+) (ignoring hydrogens for simplicity)
    But why not (1+)O-C=C-C(1-)? Why not the carbanion, and the oxygen with the positive charge?
    And if the 2 electrons from the pi-bond can transfer on to the oxygenm, why not nay of the other carbons, e.g. (1+)O-C(1-)-C=C and O=C-C(1-)-C(1+)?
    Does that make sense? Is it because they are just so unfavourable they are not worth including?

    Any help appreciated!
  2. jcsd
  3. May 21, 2012 #2
    I'm not an expert, but this is how I see it:
    in the (-1)O-C=C-C(1+), O is more electronegative than C, so it would have a higher tendency to gain electrons. Also, if you look at formal charges, negative charges are usually associated with more electronegative atoms and positive charges with less electronegative. I think if you apply this reasoning of formal charges to the other structures you've shown, (-1)O-C=C-C(1+) ends up being the most reasonable one.
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