|May20-12, 07:30 PM||#1|
Resonance structures you don't see
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!
|May21-12, 10:57 AM||#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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