Resonance structures you don't see

In summary, The speaker had a question about resonance in a molecule with an O=CR-CH=CH2 bonding. They were wondering why the negative charge is on the oxygen instead of the other carbon atoms and why the electrons from the pi-bond can't transfer onto the other atoms. The reason for this is likely due to the electronegativity and formal charges of the atoms.
  • #1
nobahar
497
2
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 that's 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!
 
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  • #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.
 

1. What are resonance structures you don't see?

Resonance structures are different representations of a molecule or ion that have the same arrangement of atoms but differ in the distribution of electrons. Resonance structures you don't see are those that are not typically shown in Lewis structures because they are not the most stable or contribute significantly to the overall structure.

2. Why are some resonance structures not shown?

Some resonance structures are not shown because they are considered less stable or have a minimal contribution to the overall structure. These structures may have a large formal charge or violate the octet rule, making them less favorable.

3. How do you determine which resonance structures to show?

The most stable resonance structures and those that contribute significantly to the overall structure are typically shown. This is determined by evaluating formal charges, bond lengths, and the octet rule.

4. Can resonance structures you don't see affect the molecule's properties?

Yes, resonance structures can greatly influence a molecule's properties. Although some structures may not be shown, they can still have an impact on the overall structure and properties of the molecule.

5. Are resonance structures you don't see less important?

Not necessarily. While they may not be shown in a Lewis structure, these resonance structures still play a role in the overall structure and properties of a molecule. Some may have a minor contribution, but others may have a significant impact on the molecule's properties.

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