[bigheadface]

I've just started organic at my university and we're in review from gen chem. The book lists formamide as an example of electrons being delocalized by resonance.
My question is, why is formamide resonant at all? It seems to me that one of the possible structures is completely stable - all atoms have proper valence and all have an octet. So why is it resonant with an unstable version of the molecule?

[rolnor]

This question can be applied to many molecules and is very basic, here is some reading:
https://en.m.wikipedia.org/wiki/Resonance_(chemistry)

[bigheadface]

That wiki article is a basic primer on resonance. I already understand the basics of resonance. What I don't understand is why a perfectly stable lewis structure would be resonant with an unstable lewis structure and be resonant.

[Babcock_Hall]

Do both structures that one can draw for formamide have full octets for C, O, and N?  I agree that the first time one encounters this example, it bends one of the guidelines that is taught about drawing resonance structures.  This is an important functional group in organic chemistry and biochemistry BTW.

[bigheadface]

So is it simply because a structure that is not stable is possible? I was under the impression (again, from just the single semester of gen chem) that if a stable structure is possible, then that is the correct structure if all other possibilities are unstable. Is this not the case, or is this molecular pattern an exception to that?

[rolnor]

Again, is this not the basic thing with all resonance? Just because a structure is stable it does not mean that there are no resonance structures. The resonance structures are also stable and contributes to the overall structure of the molecule.

[bigheadface]

Okay so I wrote the wrong concept for my line of reasoning.
My line of reasoning is that a structure with the least number of atoms with a formal charge is the preferred structure, and that a structure with no atoms with a formal charge is 'correct' if the other possible structures require charged atoms.
Is that not a correct understanding? I wouldn't be surprised if it is not. My prof glossed over this entire section and did not really go into explaining it at all.

[rolnor]

Its not a question of "correct" structure. If you can draw a resonance structure this structure will more or less contribute to the real structure. Its a matter of probability where you find electrons in a molecule and the structures we draw is a simplification of what the molecule really looks like.

[rolnor]

One example that can be nice is Dewar benzene, it contributes very little to the structure if benzene but it is not zero:
https://www.researchgate.net/figure/Kekule-and-Dewar-contributions-to-the-resonance-valence-bond-energy-in-the-benzene_fig4_257308383

So even benzene wich is "stable" can have this almost crazy resonance structure to some degree.

[bigheadface]

So if I'm understanding this correctly, every single possible legit Lewis structure that can be made for a molecule should be considered a contributor to a resonant structure of that molecule? If that's the case, I really wish my gen chem prof had given a little more attention to teaching us that.

[Babcock_Hall]

The planarity of the atoms in the amide bond suggest that the contribution made by the structure with two formal charges makes a significant contribution to the overall structure.  It is probably not the the single structure that contributes most to the hybrid; the structure with no formal charges is probably as or more important.  Yet it does sit uneasily with the rule about least formal charges that is often taught.

[bigheadface]

I think I'm getting it. And I think I either understood my prof incorrectly, or they just didn't teach it as 'correctly' as they maybe could have.

Thank you both for the help.  I'm also meeting my orgo prof in the morning to get further clarification. This should help make that conversation go a bit more quickly though.

[rolnor]

Yes, but its important to understand that the contribution can be so small that a resonance structure is practically unimportant. But theoretically its very important to understand that these structures contribute.

[rolnor]

Its also interesting that the electrons in orbitals can be far away from the nucleus but the probability for this is very small. I wonder about the math, how low probability is it that a electron from an atom in my body is in the Andromeda galaxy?
Its not zero. Or am I wrong?

https://physics.stackexchange.com/questions/92565/is-it-that-electron-of-an-atom-can-be-found-anywhere-in-the-space

[Borek]

Its also interesting that the electrons in orbitals can be far away from the nucleus but the probability for this is very small. I wonder about the math, how low probability is it that a electron from an atom in my body is in the Andromeda galaxy?
Its not zero. Or am I wrong?

You can easily do some estimate of the probability for a hydrogen atom and 1s orbital from the wavefunction - it is typically listed in any introductory QM textbook, and for s it is particularly simple.

While you obviously differ from the hydrogen atom that should give some idea about the magnitude.