Resonance structures you don't see

nobahar

Hello!

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!
Thankyou!

ptolema

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.