Draw the important resonance forms of the following cations an...

Question

Draw the important resonance forms of the following cations and anions:

(c)

(d)

Accepted Answer

Hey everyone. Let's solve this problem. It says give the resonance structures for the ions mentions below a couple of things to remember about resonance is that it occurs when we have pi bonds in conjugation with a lone pair or charge. Okay, and when the electrons are moving they always like to move from negative to positive. Okay, so an electron dense area to an electron deficient area. Alright, so let's see what we're working with here. I'm first going to redraw structure a we have NH two ch double bonds ch bonded to C. H double bond into O. H with a positive charge on that oxygen. Okay, so we said that resonance occurs when we have a pi bond and congregation with a lone pair or a charge. Well I see a lone pair on the nitrogen but there's not a pi bond right next to it. But over here we see a pi bond right next to that charge on the oxygen. So the electrons are going to move from negative to positive from that pi bond onto that positively charged oxygen, giving us a resonant structure that looks like this H two N with the lone pair bonded to carbon double bonded to carbon bonded to carbon. So now we have all single bonds, 1, 2, 3 carbons and now R O. H will have no more charge. Right because now it has two lone pairs on it but our third carbon only has 123 sigma bonds now so that carbon is going to have a positive charge because it likes to have four electrons touching it and now it has 1231 from each of those three sigma bonds. Okay, any other significant resonance structures? Well I see this pi bond next to that charge over here. So these electrons will want to share this carbon is sharing the electrons with that first carbon. But now it can go the other way and share it with that other carbon. So let's see what that looks like. We'll have each two N single bond into C. H single bond to ch double bonded to C. H bonded to O H. So now our charge, like I said, the second carbon was sharing its electrons with the first carbon. But now we took those away so that first carbon will have the positive charge now because it only has three sigma bonds. And lastly now we have that lone pair next to a positive charge. So now that lone pair on the nitrogen can oops it will donate to that carbon by creating a double bond right there. NH two double bonded to C. H single bond to ch still double bonded to the last carbon. And now do we have any formal charges here? Yes, the nitrogen has 1234 sigma bonds which means it has four electrons touching it. And we know nitrogen likes to have five electrons touching it. So that nitrogen will have a positive charge. And that is all the residents structures that we can have for compound A Let's go to compound B. We have nitrogen triple bonded to carbon which is a night trial group bonded to a negatively charged carbon double bonded to an oxygen. Or that next carbon double bonded to an oxygen which is a carbon eel group. Single bond into carbon and double bond into carbon. Okay, well we're looking for a charge in a pi bond. Well, I see this negative charge sticks out to me. So that means this carbon, does it have a lone pair? Yes, because it has 123 sigma bonds. If there was no lone pair, that would be a plus charge. Right? But since it's a negative charge, we know there has to be a lone pair on that carbon. Okay. And it's next to a pi bond in the carbondale group. So those electrons can share with that third carbon and the carbon l electrons will go up onto that electro negative oxygen. Which will look like this. So now oxygen will have three lone pairs which means it will have a negative formal charge. Right? 1234567 electrons touching it. It usually likes to have six and the left, the right side of the structure stays the same. Okay. Any more structures here? Yes. So these electrons we can the electrons on the oxygen are again next to this double bond here that we just formed. But the carbon nitrogen bond has some electrons that can also be moved because there are two pi bonds. So the electrons on the oxygen can swing down here. These electrons can move between carbons one and two and the night trial, electrons can go onto the nitrogen Giving us one more resonant structure. That looks like this nitrogen double bonded to carbon double bonded to carbon single bonded to carbon, which is now again double bonded to that oxygen. So, this is a good example because it shows you that there can be resonance even if the charge is not right next to. So we're moving the charge all the way over to this nitrogen. Right? So there can be a few arrows that can move, whereas in most of our examples we see one electron arrow moving or two, but this one has three arrows. Okay, so just be aware that they can move pretty far as long as there are pi systems that the electrons are conjugated with. Alright, so that is it for this problem. I hope this video is helpful and thanks for watching

Draw the important resonance forms of the following cations and anions:
(c)
(d)

Key Concepts

Resonance Structures

Cation and Anion Stability

Electron Delocalization

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