Which of the following pairs of structures represent resonance forms, and which do not?
(a)

Question

Accepted Answer

welcome back everyone. We need to identify the pair that are not residents forms. So we want to recognize that residents forms are results of pi bonds and lone pair electrons moving So to get to this second structure here, What occurred was that one of our pi bonds on the left hand side of the molecule, shifted to this oxygen atom to now give it a total of three electrons. And we also would have observed that one of our loan electrons shifted here to this position to form a pi bond between our oxygen atoms on the right hand side of the molecule. And so now we see that with three lone pairs of electrons on itself. This oxygen now has a formal charge of -1 because it has seven directly attached electrons, which is one more than I would prefer to have being six since oxygen is located in group six a. And this oxygen we see as a formal charge of zero because it has 123456 electron theatrically attached to itself. So we would confirm that choice A. Is a pair of resonance structures. So we would rule out choice A. Since we want the pair that are not residents forms of one another. Moving onto choice B. We have in or to go from choice from the first structure and choice B to the second structure. We would likely observe that we actually won't be able to go from this structure to this one because we should recognize that this carbon atom only originally had two bonds to hydrogen where now all of a sudden in this structure, it has three bonds to hydrogen that cannot occur from resonance. So what this means is that we have a sigma bond, which we recall is also representative of a single bond. So a sigma bond here changes in the molecule and so therefore these are not resonant structures. So we would likely consider choice B as a choice, but let's move on to choice C. To see if we have any other correct choices. Moving on to choice, see comparing these two structures to go from this structure to the structure, we would likely observe that one of these lone electrons on this nitrogen atom will have shifted to form a pi bond between carbon and nitrogen, resulting in a total of four bonds now around this nitrogen atom, meaning that it now only has 1234 directly attached valence electrons to itself, which is one less than nitrogen would prefer being in group five A. On the periodic table, contributing to this plus one formal charge. We also would observe the change where one of our pi bonds would shift as a lone pair onto this oxygen, creating a total of three sets of lone pairs on this oxygen atom. Now giving this oxygen atom a formal charge of minus one since it has seven directly attached fans electrons, which is one more than it would prefer to have being in group six A on the periodic table. And so we do have the shift of a pi bond and loan electrons in this example. And so we would confirm that choice C does represent resonance structures. And so we would rule Choice C out because we want to go with the pair that are not resident structures. Lastly, we have choice D. To consider so to go from this first structure to the second structure, we would observe that we likely shifted one of the lone pair electrons from sulfur on the right hand side of the molecule as a sigma bond between carbon and sulfur, where one of our sigma bonds on nitrogen, which has shifted to nitrogen as a lone pair, which is why we have a formal charge of minus one on this nitrogen in the structure on the right, and a formal charge of zero for the silver because it has 123456 directly attached electrons, which is what sulfur would want being in group six A on the periodic table to be stable. And so because we have the shifting of pi bonds and sigma bonds here in choice D. We do have a set of resonant structures, meaning we can rule out choice D. And the only correct choice to complete this example as a pair of nonresident structures would be choice B. Because Choice B was where we observed a a change in the single bond being our sigma bonds here, where we have a third sigma bond additives carbon atom, which does not ever happen in resonance. So B would be our final answer. I hope everything I explained was clear. If you have any questions, please leave them down below and I'll see everyone in the next practice video.

Which of the following pairs of structures represent resonance forms, and which do not? (a)

Verified Solution

Key Concepts

Resonance Structures

Lewis Structures

Delocalization of Electrons