Understanding Organic Chemistry Resonance

Hello organic chemistry students and in this video we are going to cover resonance and line angle formula. Now I'm actually going to cover this in reverse order. We're going to cover the line angle formula first because it's really going to help us figure out or help us with this. speed in which we can draw resonance structures. Now the first thing I want to show you is two compounds that I've already drawn. This first one has four carbons in it with a whole bunch of hydrogens. Now, just a quick question. How many bonds does carbon have to have? It has to have four bonds, doesn't it? Because with four bonds, two electrons per bond, that gives us eight electrons. Octet is satisfied. So carbon must have four bonds all the time. in these standard molecules. Now, let me go ahead and take that now. We're going to go back a step and go back into the line angle formula. For line angle formula, what we do is, let me go ahead and switch colors here. Oops, there it is. I'm going to show a line just like that. Now, with a line, you have the beginning and the end of a line, don't you? We have a starting point that I started with, the ending point right here. This starting point represents a carbon. So the two end points of a line represent carbons. So in this one line, here's a carbon, here's a carbon. That's great. But what if we want to show more than just two carbons? Let me go ahead and just finish it right here. So now we have the beginning of a line, the end of a line. So we know these are carbons right here. What about these angles? The angles also represent a carbon. So if I go ahead and superimpose carbons at each one of these points, here are our four carbons for the molecule that we have on the left-hand side. This molecule on the left-hand side is called butane, and so is the same one on the right side. Now notice with that line angle formula that I just drew, I'm going to draw it again without anything on top of it. I do not show the element carbon, do I? No. Every beginning part of a point... every angle represents a carbon. So we do not actually show C whatsoever. So we don't show it in this way at all. I was just overlaying the carbon so you can see where they are. This is the correct way. What other element is also not shown? You got it. Hydrogen. While this carbon is connected to this carbon, we see one bond. So that means that this carbon, we have to have four bonds on carbon. There has to be three bonds going to other hydrogens. on that carbon, which we see right here. So the line angle formula does not show carbon. It uses the beginning, the end point of a line, and angles to represent them, and we always exclude hydrogens on carbons. Let me say that again. We don't show carbons. We represent them by the beginning of point of a line, or an angle, or the end part of a line, and we never show the hydrogens on carbons whatsoever. That's going to help us with this. speed in which we can draw molecules. Let's try the one down below. We have three carbons. One, two, three. There's a double bond in it and here's a chlorine. I have just taken this molecule and drawn it in line angle formula just that quick. It is a very useful tool in drawing organic molecules. Okay, believe it or not, that's line angle formula. But let's go ahead and practice one more just so we can look at it. So I'm just going to go ahead and draw it first in line angle formula. And I'm going to ask a couple questions. How many carbons do we have? How many hydrogens and how many oxygens? So when we're looking at this, this is a carbon right here. And I'll go ahead and highlight this in blue. So here's a carbon. Here's a carbon. Here's a carbon. But the question is, is there a carbon right here? No. This carbon is, we have a line going to oxygen. So this is showing the bond between carbon and oxygen right there. So we have a grand total of three carbons in this molecule. What about the hydrogens? I'm going to say one, right? I just see one right there. Nope. Remember, we're showing two bonds on carbon. So there's two hydrogens here, two hydrogens here, three hydrogens here. So three, five, seven. Eight hydrogens in this molecule and one oxygen. That's the line angle formula. Very, very powerful technique. And we're going to be utilizing that here in this video when we talk about resonance. Let's go ahead and get right into that. So what is resonance? Resonance is the movement of electrons. That's the simple term that most books like to use. Movement of electrons. Now, I'm going to go ahead and put another caveat on this in a moment. Actually, I'm just going to jump into it right now. It is the movement of electrons without the destruction or formation of sigma bonds. Remember, sigma bonds are the... basic bonding framework that keeps a molecule intact. We cannot form or destroy a sigma bond. But nowhere in that statement does it say pi bond, right? And that's an important thing. We're going to use pi bonds in a wonderful way for resonance. Now, keep in mind, we cannot form a sigma bond. So if I was to show you this reaction or this idea right here, here's good old water or dihydrogen monoxide. And I'm going to write something called an electrophile, E+. This is something that wants electrons. We're going to define this more later on. I'll go ahead and write it right now. So electrophile. And an electrophile is a species that wants electrons. It's electron deficient. So I'm going to have this lone pair attack the electrophile. And I'm going to form this molecule right here. And we have a positive charge on oxygen. We'll go ahead and practice why that's a positive charge in a moment, but the first question I'd like to ask is, is this a resonance process? In this reaction, did we not take this lone pair and move them to the electrophile? We did, so that's the movement of electrons. But did we not form a new unique bond with this electrophile? That's a sigma bond right there, isn't it? It sure is. but we can't destroy or form any new sigma bonds. So this is not resonance whatsoever. So this is not resonance, but rather this is a reaction. So resonance can only be the movement of electrons without the formation or destruction of any sigma bonds. Now, how did we know that this oxygen is a plus one charge? We know that there are six electrons in oxygen in its periodic state. We are going to subtract the sum of the bonding electron, or the lone pairs on this, we have two electrons in the lone pair, plus one half of all the bonding electrons, six. So two plus three is five, so six minus five is plus one. And that's how we know the formal charge for that oxygen is a positive one. Now, that is a chemical reaction. Let's go ahead and see what resonance actually is. So, resonance is, I'm going to go ahead and show this right here, put a negative charge on this carbon, and actually we're going to, I'm going to take a step back for a moment, please forgive me, let me erase that for one second. And I'm going to draw this one, something that we've kind of seen before. We've seen carboxylic acids in the past, in general chemistry. Now, we have an oxygen here that has a full-out negative charge on it. If that oxygen holds the negative charge only, that makes it very unstable. So if we can somehow distribute the negative charge over some other atoms, that delocalizes the electron density. And putting it into another way, think about the U.S. deficit. If the whole U.S. deficit was on one person, that's horrible, isn't it? Trillions of dollars on one person. But if you spread it across all the people in the U.S., each person's share of it, I think, is somewhere around $100,000 to $200,000, which is still pretty horrible, but it's better than a trillion. So resonance is the movement of those electrons across other atoms. Where can we move these electrons to? And I'm going to go ahead and show the resonance arrows, and I'll talk about how this happens. Electrons are going to go down between the oxygen and carbon. I'm going to go ahead and put a hydrogen here so we know we can't have any more bonds on it. Four bonds on carbon are already being shown. So when those electrons go down, this pi bond can break and we can give electrons to oxygen. Oxygen is electronegative. It can handle the electron density. Now, I'm going to show the proper residence arrow, and we'll talk more about that in a moment. And we've just drawn this structure right here. Now, this negative charge could donate down again, and we can break open this newly formed pi bond. Forming and breaking pi bonds are perfectly fine. Not sigma bonds, though. So here, we're taking the negative charge and distributing it over. three atoms. That allows for some stabilization in the molecule. Now what we can draw at the end of this is what we call the resonance hybrid or the conical resonance structure. We put it into brackets. We show all the sigma bonds in set standard lines and then the movement of electrons in dashed lines. We have a partial negative on this carbon, partial negative on this carbon. Not full because we're moving the electrons back and forth. And the non-chemistry example is your arm. Take your right arm and put it right out in front of you. You're still you, right? But if you move it straight up above your head, you're still the same person. It has rotation. We're not breaking our arm. We're not moving it in our body. We're just allowing it to move around. The same thing with these electrons. We have free movement of electrons. How does this happen, though? So let me go back here. And I'm going to try to show this molecule right here. with the molecular or the atomic orbitals on each one of these atoms. So here's this oxygen right here, and it's in an orbital. Which orbital it is, we don't know just yet. So here's this carbon connected to oxygen right here, and here is the p orbital, a p orbital right there. And we have electrons in that p orbital. And here's two electrons in that orbital right there. So... We have a pi bond formed between that oxygen and carbon. What's going to happen is that the electrons in this oxygen, this looks like it's sp3 hybridized because it has two lone pairs of electrons as well, in addition to the third and the one sigma bond. These electrons, one of those electrons is going to move into this orbital. But if we do that, that puts too much electron density on the carbon. And that allows this carbon's electron in the p orbital to go over to the other p orbital. In this process, we've moved the electrons between different atoms, and we're also changing the hybridization states of the atoms. So moving this up a little bit. Once this is done, this carbon remains having one p orbital on it. But we've now connected it to this oxygen, and here's the new p orbital. So here are those two electrons. What about the oxygen next door? It's no longer part of the double bond, so now it becomes an sp3 hybridized oxygen with its two electrons in that sp3 hybridized orbital. But let's not forget, resonance is the fact that it can move back and forth, and this is a resonance arrow. Notice the double-edged arrow on this thing, back and forth, back and forth. The type of arrows are very important. So with resonance, we can move electrons and we change the hybridization state of the atom. Now, both of these structures are the full structure for that molecule I just showed up above, isn't it? It sure is. Let's see if I can move this down, shrink it down a little bit. There we go. So both of these are true. and this is the overall resonance hybrid where it shows all the possibilities at once. So the big question I have to ask you is right now, and we're jumping right into the deep end is, what's the hybridization state of that oxygen? We set up above sp3, now down here it's sp2. So which one is it in the canonical resonance structure? Think about those dashed lines. We have partial double bond character moving all about this, don't we? We do, and we have to keep that in mind. So because of resonance, both of these oxygens are sp2 hybridized because the conical resonance structure, the resonance hybrid, is showing that there is double bond character at all times because of the resonance movement. So overall, what's the basic trend that we can see or make if we want to talk about resonance? If we have atom z and it has a negative charge, a lone pair. If it's connected to, I'm going to call this y and x, and if it's attached to something that has a double bond, we can have resonance. We are moving electrons in this way, giving us a resonance structure. And that is the movement of electrons forming resonance. Now, there's a couple examples I also want to go over that says, that shows when resonance cannot happen. And let's go ahead and open up another page and go over that. So, I'm going to go ahead and go right into line angle formula. Here it is. There's a lone pair of electrons on this oxygen. Can one of those lone pairs just donate down in between that carbon and form this species right here? that oxygen would become a positive charge. So what I'm proposing here is that this is a resonance process. And the answer is no, this does not happen whatsoever. That structure doesn't form. Because if we go to the starting material, let's not forget that there's two hydrogens on that carbon. We already have four bonds on carbon. So if this lone pair donated down, it would form a fifth bond on carbon. So there's hydrogen. One, there's hydrogen too. We can't have five bonds on carbon. That's violating its octet rule for sure. So this is where the p orbitals comes into play. A lone pair or a negative charge can only donate into the next neighboring atom if it has a p orbital. Here, there's no p orbital on that carbon because this carbon is sp3 hybridized. And we have... four sp3 hybridized orbitals no p orbital is present in this molecule whatsoever now let's go ahead and look at another example we have lone pairs of electrons on this oxygen lone pairs on this one now this lone pair right here nothing can happen because we can't resonate down because We can't push electrons off to this carbon or this oxygen because there's no double bonds on it. So once again, this lone pair or this one cannot donate down, because if we did, we'd put five bonds on carbon, and we can't push electrons elsewhere. But what about one of these lone pairs? So, if this lone pair was to donate down, that would put five bonds on this carbon. That can't happen. But luckily, there's a pi bond that we can break, and we can give electrons. up to the oxygen up above. And this would give us this structure right here. Keep in mind, we just donated a lone pair down, so we're losing electrons. That makes this oxygen positive. We're breaking open the pi bond and giving it to that top oxygen, making it negative. And this is a resonance structure for this molecule. Now, the million dollar question is, does resonance stabilize a molecule? Hmm, electrons, they don't like each other, do they? They don't, they repel. So if we have the ability to move electrons back and forth and spread them out over a larger area and get them delocalized, as a whole, that lowers the energy of a molecule. So resonance helps stabilize a molecule. So the more resonance present in a molecule, the more stabilized it is. So if you have large amounts of resonance, you have a very, very, very stable molecule. Okay, let's stop for a second. Now, I'm going to show another crazy looking structure. We're going to talk about this class of functional groups later on. And I'm going to put two double bonds here. And let me go ahead and put another one where I put three double bonds. Looking at these two molecules, they're very similar, aren't they? They are. One has resonance. One does not. If we look at the one on the left, a double bond can move just like a lone pair of electrons can. So let me say that again. A double bond can move just like a lone pair of electrons. So we're taking the double bond, moving it down in between these two carbons because this carbon has a double bond on it. This is going to violate the octet rule of one of those carbons, so we will break it and put a charge down below. What is the structure that we just drew? We just pushed the resonance structure to form this compound right here. Now, that is a resonance structure. It is. But is it a stabilizing resonance structure? Does it actually do anything to help this molecule? And the answer is no. Carbon does not like being negative. It is not electronegative. It's electropositive. So this resonance structure, while it is still resonance, is what we call deconstructive resonance. deconstructive residence, it is not a favorable residence structure, and it's not actually considered. exist. Deconstructive residence does not happen because it raises the energy of the molecule, not lowering the energy of the molecule. Let's look at the one over on the other side. If this double bond was to move here, we could put a negative charge on this carbon, couldn't we? But that negative charge is on a carbon directly attached to a carbon of a double bond. So it might move electrons down. And then, uh-oh, this double bond is going to put a negative charge on this carbon. charge right here. Let's go back to the beginning. When this double bond moves, these electrons move over here to this carbon. This carbon becomes positive. So we put the electrons here, we have a negative charge, donates down. We put the electrons here, negative charge, and that could donate back down to ultimately stabilize the positive charge that we formed as this started. So when we are all said and done, all we've done is moved the double bonds. is that a resonant structure? It is. And most importantly, it is a very constructive resonant structure. We're not making any carbon full-out positive, full-out negative. We're moving electrons without building up charge. This is a highly stabilizing form of resonance. And it's so stabilizing, it has its own unique name, and it's called aromaticity. And we're going to talk about this topic later on. Aromaticity is so stable, it's amazing. a million folds more stable than non-aromatic compounds. And luckily for us, our DNA, the nitrogenous bases within it, is made of aromatic rings for this reason. It prevents change. Helps us preventing us from getting mutations and causing random things to happen to our bodies. Wonderful. Let's go ahead and practice resonance one more time. So here's a carbon-carbon double bond and an NH2. Can this molecule have any resonance in it whatsoever? That's what I'd like you to think about. So here, we have a double bond. Could maybe the double bond move down here to this nitrogen? No, this nitrogen has no p orbitals on it, right? We can't move electrons there because there's no p orbitals whatsoever. But if you look at this nitrogen, there's three bonds on it. So what does it have one pair of? And you got it, one lone pair of electrons. Could this lone pair donate down between the nitrogen and carbon, the carbon with the p orbital, and break the carbon-carbon double bond? And it sure can, and we've now just formed this resonance structure as shown, positive and negative. If we draw the resonance hybrid of this, we show... the sigma bonds intact and we show where the electrons are moving in this molecule. This is a partial positive nitrogen, this is a partial negative carbon. Now I know I said before carbon with a negative charge is really, really bad. It's electropositive, not electronegative. But this nitrogen or this carbon right here is only partial negative. The electrons move back to nitrogen, back and forth, back and forth. That's why we can form these systems right here. This is a control. constructive resonance form. Unlike if we're just moving electrons between carbons, like this. If we had this system right here, it looks very similar to the one on the previous page. If we move electrons in this fashion, it kind of looks very similar to what we just have up above. We put a double bond here. Here's negative, here's positive. This is highly unstable right here, this carbon with the negative charge. This is a deconstructive resonance form, and it does not exist. exist whatsoever. Let me just do it this way. That does not exist. We have no resonance. This molecule just stays as is with no resonance in it whatsoever. So the concept of resonance is important and we're going to see this play out throughout the entire semester. So from this point on, I always want you to keep the idea of resonance in the back of your head. We're going to come back to it in other videos for sure and see what does it do for chemical reactivity. and how does it help bias chemical reactivity in really amazing ways. So there's the video over resonance and the line angle formula. I highly recommend watching this video again, especially about the line angle formula to make sure you understand it and to hear about resonance. If you have questions on this, please do not feel... Please do not feel... Please feel free... Wow, gosh, I'm just tripping over my words. Please feel free to email me... and let me know what questions you might have or talk to me in office hours and things like that. I hope each of you are doing well and I look forward to seeing you in the next lecture video.

speed in which we can draw resonance structures. Now the first thing I want to show you is two compounds that I've already drawn. This first one has four carbons in it with a whole bunch of hydrogens.

Now, just a quick question. How many bonds does carbon have to have? It has to have four bonds, doesn't it? Because with four bonds, two electrons per bond, that gives us eight electrons. Octet is satisfied.

So carbon must have four bonds all the time. in these standard molecules. Now, let me go ahead and take that now.

We're going to go back a step and go back into the line angle formula. For line angle formula, what we do is, let me go ahead and switch colors here. Oops, there it is. I'm going to show a line just like that. Now, with a line, you have the beginning and the end of a line, don't you?

We have a starting point that I started with, the ending point right here. This starting point represents a carbon. So the two end points of a line represent carbons.

So in this one line, here's a carbon, here's a carbon. That's great. But what if we want to show more than just two carbons?

Let me go ahead and just finish it right here. So now we have the beginning of a line, the end of a line. So we know these are carbons right here.

What about these angles? The angles also represent a carbon. So if I go ahead and superimpose carbons at each one of these points, here are our four carbons for the molecule that we have on the left-hand side.

This molecule on the left-hand side is called butane, and so is the same one on the right side. Now notice with that line angle formula that I just drew, I'm going to draw it again without anything on top of it. I do not show the element carbon, do I? No. Every beginning part of a point...

every angle represents a carbon. So we do not actually show C whatsoever. So we don't show it in this way at all.

I was just overlaying the carbon so you can see where they are. This is the correct way. What other element is also not shown? You got it.

Hydrogen. While this carbon is connected to this carbon, we see one bond. So that means that this carbon, we have to have four bonds on carbon.

There has to be three bonds going to other hydrogens. on that carbon, which we see right here. So the line angle formula does not show carbon. It uses the beginning, the end point of a line, and angles to represent them, and we always exclude hydrogens on carbons.

Let me say that again. We don't show carbons. We represent them by the beginning of point of a line, or an angle, or the end part of a line, and we never show the hydrogens on carbons whatsoever.

That's going to help us with this. speed in which we can draw molecules. Let's try the one down below.

We have three carbons. One, two, three. There's a double bond in it and here's a chlorine.

I have just taken this molecule and drawn it in line angle formula just that quick. It is a very useful tool in drawing organic molecules. Okay, believe it or not, that's line angle formula.

But let's go ahead and practice one more just so we can look at it. So I'm just going to go ahead and draw it first in line angle formula. And I'm going to ask a couple questions.

How many carbons do we have? How many hydrogens and how many oxygens? So when we're looking at this, this is a carbon right here.

And I'll go ahead and highlight this in blue. So here's a carbon. Here's a carbon.

Here's a carbon. But the question is, is there a carbon right here? No. This carbon is, we have a line going to oxygen. So this is showing the bond between carbon and oxygen right there.

So we have a grand total of three carbons in this molecule. What about the hydrogens? I'm going to say one, right?

I just see one right there. Nope. Remember, we're showing two bonds on carbon. So there's two hydrogens here, two hydrogens here, three hydrogens here. So three, five, seven.

Eight hydrogens in this molecule and one oxygen. That's the line angle formula. Very, very powerful technique. And we're going to be utilizing that here in this video when we talk about resonance. Let's go ahead and get right into that.

So what is resonance? Resonance is the movement of electrons. That's the simple term that most books like to use. Movement of electrons.

Now, I'm going to go ahead and put another caveat on this in a moment. Actually, I'm just going to jump into it right now. It is the movement of electrons without the destruction or formation of sigma bonds. Remember, sigma bonds are the... basic bonding framework that keeps a molecule intact.

We cannot form or destroy a sigma bond. But nowhere in that statement does it say pi bond, right? And that's an important thing. We're going to use pi bonds in a wonderful way for resonance.

Now, keep in mind, we cannot form a sigma bond. So if I was to show you this reaction or this idea right here, here's good old water or dihydrogen monoxide. And I'm going to write something called an electrophile, E+. This is something that wants electrons. We're going to define this more later on.

I'll go ahead and write it right now. So electrophile. And an electrophile is a species that wants electrons. It's electron deficient.

So I'm going to have this lone pair attack the electrophile. And I'm going to form this molecule right here. And we have a positive charge on oxygen.

We'll go ahead and practice why that's a positive charge in a moment, but the first question I'd like to ask is, is this a resonance process? In this reaction, did we not take this lone pair and move them to the electrophile? We did, so that's the movement of electrons. But did we not form a new unique bond with this electrophile? That's a sigma bond right there, isn't it?

It sure is. but we can't destroy or form any new sigma bonds. So this is not resonance whatsoever. So this is not resonance, but rather this is a reaction.

So resonance can only be the movement of electrons without the formation or destruction of any sigma bonds. Now, how did we know that this oxygen is a plus one charge? We know that there are six electrons in oxygen in its periodic state.

We are going to subtract the sum of the bonding electron, or the lone pairs on this, we have two electrons in the lone pair, plus one half of all the bonding electrons, six. So two plus three is five, so six minus five is plus one. And that's how we know the formal charge for that oxygen is a positive one.

Now, that is a chemical reaction. Let's go ahead and see what resonance actually is. So, resonance is, I'm going to go ahead and show this right here, put a negative charge on this carbon, and actually we're going to, I'm going to take a step back for a moment, please forgive me, let me erase that for one second. And I'm going to draw this one, something that we've kind of seen before. We've seen carboxylic acids in the past, in general chemistry.

Now, we have an oxygen here that has a full-out negative charge on it. If that oxygen holds the negative charge only, that makes it very unstable. So if we can somehow distribute the negative charge over some other atoms, that delocalizes the electron density. And putting it into another way, think about the U.S. deficit.

If the whole U.S. deficit was on one person, that's horrible, isn't it? Trillions of dollars on one person. But if you spread it across all the people in the U.S., each person's share of it, I think, is somewhere around $100,000 to $200,000, which is still pretty horrible, but it's better than a trillion. So resonance is the movement of those electrons across other atoms. Where can we move these electrons to?

And I'm going to go ahead and show the resonance arrows, and I'll talk about how this happens. Electrons are going to go down between the oxygen and carbon. I'm going to go ahead and put a hydrogen here so we know we can't have any more bonds on it.

Four bonds on carbon are already being shown. So when those electrons go down, this pi bond can break and we can give electrons to oxygen. Oxygen is electronegative. It can handle the electron density. Now, I'm going to show the proper residence arrow, and we'll talk more about that in a moment.

And we've just drawn this structure right here. Now, this negative charge could donate down again, and we can break open this newly formed pi bond. Forming and breaking pi bonds are perfectly fine. Not sigma bonds, though.

So here, we're taking the negative charge and distributing it over. three atoms. That allows for some stabilization in the molecule. Now what we can draw at the end of this is what we call the resonance hybrid or the conical resonance structure.

We put it into brackets. We show all the sigma bonds in set standard lines and then the movement of electrons in dashed lines. We have a partial negative on this carbon, partial negative on this carbon. Not full because we're moving the electrons back and forth.

And the non-chemistry example is your arm. Take your right arm and put it right out in front of you. You're still you, right?

But if you move it straight up above your head, you're still the same person. It has rotation. We're not breaking our arm.

We're not moving it in our body. We're just allowing it to move around. The same thing with these electrons.

We have free movement of electrons. How does this happen, though? So let me go back here.

And I'm going to try to show this molecule right here. with the molecular or the atomic orbitals on each one of these atoms. So here's this oxygen right here, and it's in an orbital. Which orbital it is, we don't know just yet. So here's this carbon connected to oxygen right here, and here is the p orbital, a p orbital right there.

And we have electrons in that p orbital. And here's two electrons in that orbital right there. So... We have a pi bond formed between that oxygen and carbon. What's going to happen is that the electrons in this oxygen, this looks like it's sp3 hybridized because it has two lone pairs of electrons as well, in addition to the third and the one sigma bond.

These electrons, one of those electrons is going to move into this orbital. But if we do that, that puts too much electron density on the carbon. And that allows this carbon's electron in the p orbital to go over to the other p orbital.

In this process, we've moved the electrons between different atoms, and we're also changing the hybridization states of the atoms. So moving this up a little bit. Once this is done, this carbon remains having one p orbital on it.

But we've now connected it to this oxygen, and here's the new p orbital. So here are those two electrons. What about the oxygen next door?

It's no longer part of the double bond, so now it becomes an sp3 hybridized oxygen with its two electrons in that sp3 hybridized orbital. But let's not forget, resonance is the fact that it can move back and forth, and this is a resonance arrow. Notice the double-edged arrow on this thing, back and forth, back and forth. The type of arrows are very important. So with resonance, we can move electrons and we change the hybridization state of the atom.

Now, both of these structures are the full structure for that molecule I just showed up above, isn't it? It sure is. Let's see if I can move this down, shrink it down a little bit.

There we go. So both of these are true. and this is the overall resonance hybrid where it shows all the possibilities at once. So the big question I have to ask you is right now, and we're jumping right into the deep end is, what's the hybridization state of that oxygen?

We set up above sp3, now down here it's sp2. So which one is it in the canonical resonance structure? Think about those dashed lines.

We have partial double bond character moving all about this, don't we? We do, and we have to keep that in mind. So because of resonance, both of these oxygens are sp2 hybridized because the conical resonance structure, the resonance hybrid, is showing that there is double bond character at all times because of the resonance movement.

So overall, what's the basic trend that we can see or make if we want to talk about resonance? If we have atom z and it has a negative charge, a lone pair. If it's connected to, I'm going to call this y and x, and if it's attached to something that has a double bond, we can have resonance. We are moving electrons in this way, giving us a resonance structure. And that is the movement of electrons forming resonance.

Now, there's a couple examples I also want to go over that says, that shows when resonance cannot happen. And let's go ahead and open up another page and go over that. So, I'm going to go ahead and go right into line angle formula.

Here it is. There's a lone pair of electrons on this oxygen. Can one of those lone pairs just donate down in between that carbon and form this species right here?

that oxygen would become a positive charge. So what I'm proposing here is that this is a resonance process. And the answer is no, this does not happen whatsoever. That structure doesn't form.

Because if we go to the starting material, let's not forget that there's two hydrogens on that carbon. We already have four bonds on carbon. So if this lone pair donated down, it would form a fifth bond on carbon. So there's hydrogen. One, there's hydrogen too.

We can't have five bonds on carbon. That's violating its octet rule for sure. So this is where the p orbitals comes into play. A lone pair or a negative charge can only donate into the next neighboring atom if it has a p orbital. Here, there's no p orbital on that carbon because this carbon is sp3 hybridized.

And we have... four sp3 hybridized orbitals no p orbital is present in this molecule whatsoever now let's go ahead and look at another example we have lone pairs of electrons on this oxygen lone pairs on this one now this lone pair right here nothing can happen because we can't resonate down because We can't push electrons off to this carbon or this oxygen because there's no double bonds on it. So once again, this lone pair or this one cannot donate down, because if we did, we'd put five bonds on carbon, and we can't push electrons elsewhere.

But what about one of these lone pairs? So, if this lone pair was to donate down, that would put five bonds on this carbon. That can't happen. But luckily, there's a pi bond that we can break, and we can give electrons. up to the oxygen up above.

And this would give us this structure right here. Keep in mind, we just donated a lone pair down, so we're losing electrons. That makes this oxygen positive.

We're breaking open the pi bond and giving it to that top oxygen, making it negative. And this is a resonance structure for this molecule. Now, the million dollar question is, does resonance stabilize a molecule? Hmm, electrons, they don't like each other, do they? They don't, they repel.

So if we have the ability to move electrons back and forth and spread them out over a larger area and get them delocalized, as a whole, that lowers the energy of a molecule. So resonance helps stabilize a molecule. So the more resonance present in a molecule, the more stabilized it is.

So if you have large amounts of resonance, you have a very, very, very stable molecule. Okay, let's stop for a second. Now, I'm going to show another crazy looking structure. We're going to talk about this class of functional groups later on.

And I'm going to put two double bonds here. And let me go ahead and put another one where I put three double bonds. Looking at these two molecules, they're very similar, aren't they? They are. One has resonance.

One does not. If we look at the one on the left, a double bond can move just like a lone pair of electrons can. So let me say that again.

A double bond can move just like a lone pair of electrons. So we're taking the double bond, moving it down in between these two carbons because this carbon has a double bond on it. This is going to violate the octet rule of one of those carbons, so we will break it and put a charge down below.

What is the structure that we just drew? We just pushed the resonance structure to form this compound right here. Now, that is a resonance structure. It is. But is it a stabilizing resonance structure?

Does it actually do anything to help this molecule? And the answer is no. Carbon does not like being negative.

It is not electronegative. It's electropositive. So this resonance structure, while it is still resonance, is what we call deconstructive resonance. deconstructive residence, it is not a favorable residence structure, and it's not actually considered.

exist. Deconstructive residence does not happen because it raises the energy of the molecule, not lowering the energy of the molecule. Let's look at the one over on the other side.

If this double bond was to move here, we could put a negative charge on this carbon, couldn't we? But that negative charge is on a carbon directly attached to a carbon of a double bond. So it might move electrons down. And then, uh-oh, this double bond is going to put a negative charge on this carbon.

charge right here. Let's go back to the beginning. When this double bond moves, these electrons move over here to this carbon. This carbon becomes positive.

So we put the electrons here, we have a negative charge, donates down. We put the electrons here, negative charge, and that could donate back down to ultimately stabilize the positive charge that we formed as this started. So when we are all said and done, all we've done is moved the double bonds. is that a resonant structure? It is.

And most importantly, it is a very constructive resonant structure. We're not making any carbon full-out positive, full-out negative. We're moving electrons without building up charge.

This is a highly stabilizing form of resonance. And it's so stabilizing, it has its own unique name, and it's called aromaticity. And we're going to talk about this topic later on. Aromaticity is so stable, it's amazing.

a million folds more stable than non-aromatic compounds. And luckily for us, our DNA, the nitrogenous bases within it, is made of aromatic rings for this reason. It prevents change.

Helps us preventing us from getting mutations and causing random things to happen to our bodies. Wonderful. Let's go ahead and practice resonance one more time. So here's a carbon-carbon double bond and an NH2.

Can this molecule have any resonance in it whatsoever? That's what I'd like you to think about. So here, we have a double bond. Could maybe the double bond move down here to this nitrogen?

No, this nitrogen has no p orbitals on it, right? We can't move electrons there because there's no p orbitals whatsoever. But if you look at this nitrogen, there's three bonds on it.

So what does it have one pair of? And you got it, one lone pair of electrons. Could this lone pair donate down between the nitrogen and carbon, the carbon with the p orbital, and break the carbon-carbon double bond?

And it sure can, and we've now just formed this resonance structure as shown, positive and negative. If we draw the resonance hybrid of this, we show... the sigma bonds intact and we show where the electrons are moving in this molecule. This is a partial positive nitrogen, this is a partial negative carbon.

Now I know I said before carbon with a negative charge is really, really bad. It's electropositive, not electronegative. But this nitrogen or this carbon right here is only partial negative.

The electrons move back to nitrogen, back and forth, back and forth. That's why we can form these systems right here. This is a control.

constructive resonance form. Unlike if we're just moving electrons between carbons, like this. If we had this system right here, it looks very similar to the one on the previous page.

If we move electrons in this fashion, it kind of looks very similar to what we just have up above. We put a double bond here. Here's negative, here's positive.

This is highly unstable right here, this carbon with the negative charge. This is a deconstructive resonance form, and it does not exist. exist whatsoever.

Let me just do it this way. That does not exist. We have no resonance. This molecule just stays as is with no resonance in it whatsoever. So the concept of resonance is important and we're going to see this play out throughout the entire semester.

So from this point on, I always want you to keep the idea of resonance in the back of your head. We're going to come back to it in other videos for sure and see what does it do for chemical reactivity. and how does it help bias chemical reactivity in really amazing ways.

So there's the video over resonance and the line angle formula. I highly recommend watching this video again, especially about the line angle formula to make sure you understand it and to hear about resonance. If you have questions on this, please do not feel...

Please do not feel... Please feel free... Wow, gosh, I'm just tripping over my words. Please feel free to email me... and let me know what questions you might have or talk to me in office hours and things like that.

I hope each of you are doing well and I look forward to seeing you in the next lecture video.

Transcript for:Understanding Organic Chemistry Resonance

Transcript for:
Understanding Organic Chemistry Resonance