Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: bigheadface on September 05, 2021, 10:28:55 PM
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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?
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This question can be applied to many molecules and is very basic, here is some reading:
https://en.m.wikipedia.org/wiki/Resonance_(chemistry)
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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.
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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.
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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
Strictly speaking this is true but even beyond the exponential falloff of the probability distribution, from a practical standpoint this assumes that there are no other electrostatic potential surfaces between here and, as you put it, the Andromeda galaxy. I.e., it assumes an empty universe. Obviously other atoms exist, and the potential energy function of one atom in your body would be influenced by that of the next atom over. Moreover it's worth reminding you that electrons don't have precise locations. Using language like "probability...that an electron from an atom in my body is in the Andromeda galaxy", which is predicated on macroscopic ideas like instantaneous observations at defined points in time and space, isn't really up to the task of disentangling the wave/particle duality and observable uncertainties of the quantum world.
Quantum mechanics is weird.
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Yes, but an aproximation is possible. And, for simplification, lets assume its all empty space.
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@OP,
Thinking in terms of bond lengths and bond angles, what would you consider evidence that the resonance structure with two charges does or does not make a significant contribution to the overall structure of form amide?
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@Rolnor
All you need is the radial distribution function. (Keep in mind that analytical solutions exist only for hydrogenic atoms.). For an electron in the lowest energy orbital of the hydrogen atom, the probability drops off as a function of e-2r/a, where r is the separation from the nucleus and a is the Bohr radius (about 5 picometers). You can estimate the probability that the electron exists in a region defined by r1 to r2 by integrating the radial distribution function (see: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/hydrng.html#c1). Let's forget the Andromeda galaxy and just estimate it for an distance more than 1 nanometer away from the nucleus - that's about 20x the Bohr radius . Using the approach above, the probability the electron is further than 1 nm away from the nucleus is about ~4E-13 to 1. The probability that it is more than 1 micron away from the nucleus is too small to be calculated by online worksheet, or even by Excel. So, maybe that gives some sense of scale.
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By the way, this kind of thing is true for any potential field that drops off exponentially with distance. A simple bar magnet in your living room technically will attract another bar magnet in the andromeda galaxy.... same thing with your personal gravitational pull. But does it really mean anything?
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Thanx,
I am not surpriced in anyway, electrons are to 90% found in the orbitals.
I just wanted to further explain resonance, that the molecules we draw are simplifications and that almost anything is possible, but often very improbable.
I guess that all particles in the universe can be found anywhere in the universe if we observe it for very long time. This is theoretically important but has no practical use for us. You mention forces and that is different from particles, forces are something else.
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Well forces act on particles and determine their properties. Position is one property. In the case of nuclei and electrons, the forces are electrostatic, which constrain position, momentum, and so on. In the case of planets, the forces are gravitational. I (a particle) am within the gravitational field of the Andromeda galaxy, and so those stars affect my position, velocity, and so on. As it happens, I'm sitting in a much stronger local field, imposed by our earth, which sits within an even stronger field (that of our sun), so Andromeda's grav fields are inconsequential to me. But if I and Andromeda were the only things that existed, I would be pulled toward it, despite how far away it is, because the field does extend to infinity. This is only if you ignore all the other stuff in the universe, of course ;)
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I still think its different with particles or mass, if the universe life-span was infinite, my whole body could be in the Andromeda and this would happen an infinite number of times.