Chair Conformations in Organic Chemistry

Hello organic chemistry students. In this video we're going to talk about chair conformations. Chair conformations, what is that? I know it sounds absolutely crazy. If we look at one methylcyclohexane, and that basic name is shown here in black, in red I'm showing some hydrogens. Now those hydrogens are on this molecule which we normally don't show in line angle formula, but I'm showing them here for some questions. If we look at these two hydrogens right here, the question is, are they both equal in energy? Now, when we draw a structure like this, we assume, yes, one's coming out, one's coming in, pretty much equal in energy. But there's far more to the story. Now, more appropriately, this molecule doesn't even adopt this configuration that we're showing, where these black bonds are all in the same plane. It's going to adopt, ah, the chair conformation, and we're going to get into that. Now, if you remember about a tetrahedral carbon, we used to draw a tetrahedral carbon. like this, bonds sticking straight out. And then we realize, wait, this is 90 degrees, this is 180 degrees, and that can't be. And then all of a sudden we started talking about the 109.5 degree bond angle of tetrahedral carbons. And that's how we normalize the energy of this molecule. We're going to have the same exact thing here in the chair of conformation. And most importantly, A chair conformation is going to allow this 109.5 degree bond angle to remain true. As shown right here, we can't adopt the standard 109.5, so we're violating the tetrahedral bond angles. Let's go ahead and show what I mean by a chair conformation. It sounds crazy. There it is. That is a... chair confirmation. I've noticed some of the lines don't connect up exactly. I apologize for that. That's just sloppy drawing on my behalf. I want to go up here and just show carbon one, two, three, four, five, and six. I can call this carbon one, two, three, four, five, and six. Or I can call this carbon one, this carbon one, and so on. Now when we show it in this chair confirmation, And this chair conformation is kind of funny because if you imagine it, it's kind of like a lazy boy recliner. Where if we had a little person in here, you can imagine them sitting down. Here are the arms right here. Someone reclining in this chair. I am not an artist. Please feel free to laugh at that absolutely horrible picture. But it is like a lazy boy chair. If you think of it that way, you always remember that this is a chair, like a recliner. And the code for today's video is actually brought to you by Lazy Boy. So Lazy Boy is the code for the video here today. Now, we have the basic carbon structure, and that's the Lazy Boy recliner. But we have to talk about the positions on the Lazy Boy recliner. So I'm going to go ahead and redraw the chair. And the textbook does a nice example of talking about which bonds have to be parallel to which bonds. So here, this bond should be parallel to this bond. This one should be parallel to this one and this one roughly parallel to that one. Now we're going to go ahead and look at this carbon right here and place some groups on it. And the first group I'm going to place in is with my pen on that carbon is I'm going to go straight down. I'm going to go to the carbon next door to it and go straight up, straight down, straight up, straight down, straight up. Now if you notice these bonds are parallel to the side of this projection right here, isn't it? Yes it is. All of them are parallel to the side of this video. We call those bonds that we're showing here in black going straight up and down the axial bonds. Every carbon has one axial bond. Now this one's going down, this one's going up. If it was going straight up, wouldn't it be in the same environment as this carbon bond right here? Yes. That's why it goes down, up, Down, up, down, up. Now, we have to talk about the other bond on this carbon that I'm going to show in red. It's kind of going up and off to the side. The one up above is going to kind of go down and off to the side, down and off to the side, up and off to the side. The ones in the middle are a little bit trickier. It's going to go up and off to the side, down and off to the side. Those ones in red are what we call the equatorial bonds. Now when we look at the equatorial bonds, they're not going straight up or down relative to the side of this projection right here. It's going up slightly to the left, down slightly to the left. Up slightly to the left, down slightly to the right. Up slightly to the right, down slightly to the right. So it's not fully alternating. It's not going up, right, lower, left. It's going up, down, up, down, up, down, left, left, left, right, right, right. Now we're showing this axial equatorial. What is this doing? It's allowing us to put all these bonds in exactly 109.5 degree bond angles on each one of the carbons. The carbon framework was within the plane of this piece of paper right here. This is going slightly up, slightly down. Now it's going to beg the question, which position is more stable? And please forgive me as I'm just writing this down. Which position is more stable? I'm going to go ahead and put some hydrogens or a methyl group on this carbon here, a methyl group here. A methyl group here and a methyl group here. I'm going to call this methyl group 1, 2, 3, 4. If this methyl group is going straight down, as is this one, they're parallel to one another, which means this bond and this bond are all in the same space. So let me go ahead and put the carbon in it and the hydrogen. Carbon, hydrogen. Steric clash is occurring there. So when oxygen is in the same space, In axial positions we can have what we call 1,3 axial strain. Carbon 1 to 3. So here is this axial 1,3 strain. What about the equatorial positions? This methyl group is going up, this methyl group is going down, pointed away from one another. But you might be saying, wait Professor Rafferty, what about this one right here? That one's pointing on the same side as well. But this one is pointing up. This one is pointing down. It would be in two different chemical environments. So if I call this carbon 1 and this carbon 2 right here, we would have carbon 1 connected to carbon 2, the methyl group going down, this methyl group going up. And they're not going to be in the same space. So equatorial positions are always more stable than axial positions, and that's going to be the biggest thing to take home right now. Equatorial positions are more stable than axial positions. So what I'd like to do now is go ahead and we're going to draw that molecule that we just had at the top of the page in a chair conformation. Here it is. I'm going to put the methyl group in. There it is right there. Now notice, I could have chosen any carbon that I wanted. Now we have a couple things we have to consider. This methyl group is pointing up. So I have to have this methyl group pointing slightly up or directly up. Here, by putting it into this slightly up position, this is the equatorial position. That's favorable. If I would have put it on the carbon directly above it, I would have had to have shown the methyl group right here for going up, which is an axial position. Down here, this is equatorial. Which one is more favorable? the equatorial position. And this is the structure for methyl cyclohexane. Now, just like a lazy boy recliner, we can recline and retract this position. And we're going to do something called a ring flip. In the ring flip, the chair does just that. It flips. Now this methyl group with this carbon right here, carbon 1, is now up here as carbon 1. And the methyl group is now going straight up in an axial position. So you might be saying, wait a second, you just said this right here wasn't the best one. You're right. This is the most stable conformation of one methyl cyclohexane. This is the least stable conformation. So if the question just said, draw a chair conformation of one methyl cyclohexane, either one is right. But if it says draw the most stable conformation, this right here is the most stable. This is the least stable. Now this chair can go back and forth, back and forth, but we're going to reside in this conformation 95% of the time, this one 5% of the time. Why? This is an axial conformation, but both of them are possible. Just like we saw the free rotation in a Newman projection, we can have chair flips going in these chair conformations pretty nicely. They can always do chair flips for the most part. We'll see an exception in a couple of minutes. So what I'd like to do now is go ahead and show this molecule right here in a chair structure. Now to do that, we first have to draw our chair. Now, we've drawn the chair, which means, I'm just going to go ahead and color this in red, we have named this part of the molecule. Great. We still need the methyl group and the ethyl group. Now, great, methyl, ethyl, we want to put them both equatorial, but we can't. We can't just assume that. The first thing we have to do is identify the largest substituent out of these two. two carbons is bigger than one methyl. So the ethyl group is the biggest group. I need to put the largest group into an equatorial position first. So here I put the ethyl group in, abbreviated as ET, and that is in an equatorial position, pointing slightly down, pointing down. So we're matching. I'm going to go to this carbon now, and if this one's pointing down, the methyl group has to be pointing. up. And that also is in an equatorial position. So, good for us. Both positions are in equatorial positions. But what if we were to draw this molecule here? The chair conformation is going to look very similar. We put the ethyl group in, but now on the other carbon, we have to show that methyl group going down. This is pointed down, pointed down, pointing down. pointing down. So once you place the largest group on the chair, every other substituent you add to the chair has to match the pattern of the molecule shown. We can't just put everything equatorial and call it good. It has to follow the molecule of interest. Now, could both of these molecules undergo chair flips? Absolutely. And let's go ahead and show those right here. So we're taking this molecule doing the chair flip. Here's the ethyl group right there. And then there's the methyl, both in axial positions. Down below, I'm not drawing the best chairs right now, so I apologize. We have the axial ethyl group and then the equatorial methyl group. So here, this is the most favored, this is the least favored for these conformations. And then up above, this is the most favored and the least favored. What's the percentages? We don't care. I'm not going to ask you to give me percentages up above. I was just showing you how it favors one side over the other. So now, we can place multiple groups inside chair conformations. So let's go ahead and just have fun with this. I'm going to put a methyl group here. I'm going to put an ethyl group here. Down below, I'm going to put an isopropyl and a fluorine. The first thing you have to do, if I'm asking you to draw the most stable... Chair confirmation is draw the chair. Now we've just drawn the chair. I'm going to go ahead and highlight what we've just drawn now we have to worry about the ethyl methyl fluorine and Isopropyl which one's the biggest group the isopropyl so it has to go into an equatorial position So I'm going to start at this carbon right here. Oops. I'm gonna start this carbon and put it into an equatorial position pointing up Uh-oh, that's axial, isn't it? I chose this carbon to go with, and I have to point up to match this coming out of the plane. That doesn't work. So I can come in here, erase that error, and I'm just going to go one carbon down, and now show the isopropyl group. Perfect. The isopropyl group and the fluorine are on opposite sides, so if this one's pointing up, the fluorine has to be pointing down. Great. Happens to be in an equatorial position. We're doing great. The fluorine is pointed down. The next one, the methyl group, has to point slightly up, which is another, you said it, equatorial position. But now the methyl and ethyl group are pointed towards the same side, so this top carbon here has to have the ethyl group pointing straight up in an axial conformation. All the other ones are equatorial. So notice, once we place the biggest, bulkiest group, everything has to match the pattern of this molecule right here. Now that's not a solid line. That's just supposed to be one single line. Let me see if I can go ahead and just fix that really quickly. I don't want you to think it has to be that. So let's take that out. Redraw the bond here and there is our equatorial position. That's drawing chair structures. Now you could draw the chair flip of this. Feel free. If you'd like me to take a look at it, I'd be happy to do so. We're going to do one more. And we're going to put in tert-butyl, what I call the chicken foot, and you're going to see why. Let's go ahead and draw the chair structure for this tert-butyl-cyclohexane. Have to have it pointing up, so here is the equatorial position. There's the tert-butyl group, a.k.a. the chicken foot. Let's draw the chair flip for this. There isn't one. There's no chip, or no chair flip for a tert-butyl group. Why? If I was to show all the hydrogens in here, if we invoked a chair flip, would it automatically start having steric clashes with neighboring atoms? Yes. It is so repulsive, we say that tert-butyl groups do not undergo chair flips. Now, we've never detected a chair flip of a tert-butyl. Could that change? Maybe one day. But as of today, this summer, 2019, it's not going to change. It's constant right there. No chair flip. is allowed when you're dealing with a Turk-Butyl group. You put it in an equatorial position, and you're good to go. Don't forget the code for this video is LAZYBOY, L-A-Z-Y-B-O-Y, because we're dealing with chair confirmations. Best thing to do here is practice, practice, practice. I hope everyone's doing well, and I look forward to seeing you all in lecture.

Hello organic chemistry students. In this video we're going to talk about chair conformations. Chair conformations, what is that? I know it sounds absolutely crazy.

If we look at one methylcyclohexane, and that basic name is shown here in black, in red I'm showing some hydrogens. Now those hydrogens are on this molecule which we normally don't show in line angle formula, but I'm showing them here for some questions. If we look at these two hydrogens right here, the question is, are they both equal in energy?

Now, when we draw a structure like this, we assume, yes, one's coming out, one's coming in, pretty much equal in energy. But there's far more to the story. Now, more appropriately, this molecule doesn't even adopt this configuration that we're showing, where these black bonds are all in the same plane.

It's going to adopt, ah, the chair conformation, and we're going to get into that. Now, if you remember about a tetrahedral carbon, we used to draw a tetrahedral carbon. like this, bonds sticking straight out.

And then we realize, wait, this is 90 degrees, this is 180 degrees, and that can't be. And then all of a sudden we started talking about the 109.5 degree bond angle of tetrahedral carbons. And that's how we normalize the energy of this molecule. We're going to have the same exact thing here in the chair of conformation.

And most importantly, A chair conformation is going to allow this 109.5 degree bond angle to remain true. As shown right here, we can't adopt the standard 109.5, so we're violating the tetrahedral bond angles. Let's go ahead and show what I mean by a chair conformation.

It sounds crazy. There it is. That is a... chair confirmation. I've noticed some of the lines don't connect up exactly.

I apologize for that. That's just sloppy drawing on my behalf. I want to go up here and just show carbon one, two, three, four, five, and six.

I can call this carbon one, two, three, four, five, and six. Or I can call this carbon one, this carbon one, and so on. Now when we show it in this chair confirmation, And this chair conformation is kind of funny because if you imagine it, it's kind of like a lazy boy recliner. Where if we had a little person in here, you can imagine them sitting down. Here are the arms right here.

Someone reclining in this chair. I am not an artist. Please feel free to laugh at that absolutely horrible picture. But it is like a lazy boy chair. If you think of it that way, you always remember that this is a chair, like a recliner.

And the code for today's video is actually brought to you by Lazy Boy. So Lazy Boy is the code for the video here today. Now, we have the basic carbon structure, and that's the Lazy Boy recliner.

But we have to talk about the positions on the Lazy Boy recliner. So I'm going to go ahead and redraw the chair. And the textbook does a nice example of talking about which bonds have to be parallel to which bonds. So here, this bond should be parallel to this bond.

This one should be parallel to this one and this one roughly parallel to that one. Now we're going to go ahead and look at this carbon right here and place some groups on it. And the first group I'm going to place in is with my pen on that carbon is I'm going to go straight down. I'm going to go to the carbon next door to it and go straight up, straight down, straight up, straight down, straight up. Now if you notice these bonds are parallel to the side of this projection right here, isn't it?

Yes it is. All of them are parallel to the side of this video. We call those bonds that we're showing here in black going straight up and down the axial bonds.

Every carbon has one axial bond. Now this one's going down, this one's going up. If it was going straight up, wouldn't it be in the same environment as this carbon bond right here? Yes.

That's why it goes down, up, Down, up, down, up. Now, we have to talk about the other bond on this carbon that I'm going to show in red. It's kind of going up and off to the side.

The one up above is going to kind of go down and off to the side, down and off to the side, up and off to the side. The ones in the middle are a little bit trickier. It's going to go up and off to the side, down and off to the side. Those ones in red are what we call the equatorial bonds.

Now when we look at the equatorial bonds, they're not going straight up or down relative to the side of this projection right here. It's going up slightly to the left, down slightly to the left. Up slightly to the left, down slightly to the right.

Up slightly to the right, down slightly to the right. So it's not fully alternating. It's not going up, right, lower, left.

It's going up, down, up, down, up, down, left, left, left, right, right, right. Now we're showing this axial equatorial. What is this doing? It's allowing us to put all these bonds in exactly 109.5 degree bond angles on each one of the carbons. The carbon framework was within the plane of this piece of paper right here.

This is going slightly up, slightly down. Now it's going to beg the question, which position is more stable? And please forgive me as I'm just writing this down. Which position is more stable? I'm going to go ahead and put some hydrogens or a methyl group on this carbon here, a methyl group here.

A methyl group here and a methyl group here. I'm going to call this methyl group 1, 2, 3, 4. If this methyl group is going straight down, as is this one, they're parallel to one another, which means this bond and this bond are all in the same space. So let me go ahead and put the carbon in it and the hydrogen.

Carbon, hydrogen. Steric clash is occurring there. So when oxygen is in the same space, In axial positions we can have what we call 1,3 axial strain.

Carbon 1 to 3. So here is this axial 1,3 strain. What about the equatorial positions? This methyl group is going up, this methyl group is going down, pointed away from one another.

But you might be saying, wait Professor Rafferty, what about this one right here? That one's pointing on the same side as well. But this one is pointing up. This one is pointing down. It would be in two different chemical environments.

So if I call this carbon 1 and this carbon 2 right here, we would have carbon 1 connected to carbon 2, the methyl group going down, this methyl group going up. And they're not going to be in the same space. So equatorial positions are always more stable than axial positions, and that's going to be the biggest thing to take home right now.

Equatorial positions are more stable than axial positions. So what I'd like to do now is go ahead and we're going to draw that molecule that we just had at the top of the page in a chair conformation. Here it is. I'm going to put the methyl group in. There it is right there.

Now notice, I could have chosen any carbon that I wanted. Now we have a couple things we have to consider. This methyl group is pointing up. So I have to have this methyl group pointing slightly up or directly up. Here, by putting it into this slightly up position, this is the equatorial position.

That's favorable. If I would have put it on the carbon directly above it, I would have had to have shown the methyl group right here for going up, which is an axial position. Down here, this is equatorial. Which one is more favorable?

the equatorial position. And this is the structure for methyl cyclohexane. Now, just like a lazy boy recliner, we can recline and retract this position.

And we're going to do something called a ring flip. In the ring flip, the chair does just that. It flips.

Now this methyl group with this carbon right here, carbon 1, is now up here as carbon 1. And the methyl group is now going straight up in an axial position. So you might be saying, wait a second, you just said this right here wasn't the best one. You're right. This is the most stable conformation of one methyl cyclohexane. This is the least stable conformation.

So if the question just said, draw a chair conformation of one methyl cyclohexane, either one is right. But if it says draw the most stable conformation, this right here is the most stable. This is the least stable.

Now this chair can go back and forth, back and forth, but we're going to reside in this conformation 95% of the time, this one 5% of the time. Why? This is an axial conformation, but both of them are possible.

Just like we saw the free rotation in a Newman projection, we can have chair flips going in these chair conformations pretty nicely. They can always do chair flips for the most part. We'll see an exception in a couple of minutes.

So what I'd like to do now is go ahead and show this molecule right here in a chair structure. Now to do that, we first have to draw our chair. Now, we've drawn the chair, which means, I'm just going to go ahead and color this in red, we have named this part of the molecule.

Great. We still need the methyl group and the ethyl group. Now, great, methyl, ethyl, we want to put them both equatorial, but we can't. We can't just assume that.

The first thing we have to do is identify the largest substituent out of these two. two carbons is bigger than one methyl. So the ethyl group is the biggest group.

I need to put the largest group into an equatorial position first. So here I put the ethyl group in, abbreviated as ET, and that is in an equatorial position, pointing slightly down, pointing down. So we're matching.

I'm going to go to this carbon now, and if this one's pointing down, the methyl group has to be pointing. up. And that also is in an equatorial position. So, good for us.

Both positions are in equatorial positions. But what if we were to draw this molecule here? The chair conformation is going to look very similar.

We put the ethyl group in, but now on the other carbon, we have to show that methyl group going down. This is pointed down, pointed down, pointing down. pointing down. So once you place the largest group on the chair, every other substituent you add to the chair has to match the pattern of the molecule shown. We can't just put everything equatorial and call it good.

It has to follow the molecule of interest. Now, could both of these molecules undergo chair flips? Absolutely. And let's go ahead and show those right here. So we're taking this molecule doing the chair flip.

Here's the ethyl group right there. And then there's the methyl, both in axial positions. Down below, I'm not drawing the best chairs right now, so I apologize. We have the axial ethyl group and then the equatorial methyl group. So here, this is the most favored, this is the least favored for these conformations.

And then up above, this is the most favored and the least favored. What's the percentages? We don't care. I'm not going to ask you to give me percentages up above. I was just showing you how it favors one side over the other.

So now, we can place multiple groups inside chair conformations. So let's go ahead and just have fun with this. I'm going to put a methyl group here.

I'm going to put an ethyl group here. Down below, I'm going to put an isopropyl and a fluorine. The first thing you have to do, if I'm asking you to draw the most stable... Chair confirmation is draw the chair. Now we've just drawn the chair.

I'm going to go ahead and highlight what we've just drawn now we have to worry about the ethyl methyl fluorine and Isopropyl which one's the biggest group the isopropyl so it has to go into an equatorial position So I'm going to start at this carbon right here. Oops. I'm gonna start this carbon and put it into an equatorial position pointing up Uh-oh, that's axial, isn't it? I chose this carbon to go with, and I have to point up to match this coming out of the plane.

That doesn't work. So I can come in here, erase that error, and I'm just going to go one carbon down, and now show the isopropyl group. Perfect. The isopropyl group and the fluorine are on opposite sides, so if this one's pointing up, the fluorine has to be pointing down.

Great. Happens to be in an equatorial position. We're doing great.

The fluorine is pointed down. The next one, the methyl group, has to point slightly up, which is another, you said it, equatorial position. But now the methyl and ethyl group are pointed towards the same side, so this top carbon here has to have the ethyl group pointing straight up in an axial conformation. All the other ones are equatorial. So notice, once we place the biggest, bulkiest group, everything has to match the pattern of this molecule right here.

Now that's not a solid line. That's just supposed to be one single line. Let me see if I can go ahead and just fix that really quickly. I don't want you to think it has to be that.

So let's take that out. Redraw the bond here and there is our equatorial position. That's drawing chair structures. Now you could draw the chair flip of this.

Feel free. If you'd like me to take a look at it, I'd be happy to do so. We're going to do one more. And we're going to put in tert-butyl, what I call the chicken foot, and you're going to see why. Let's go ahead and draw the chair structure for this tert-butyl-cyclohexane.

Have to have it pointing up, so here is the equatorial position. There's the tert-butyl group, a.k.a. the chicken foot. Let's draw the chair flip for this.

There isn't one. There's no chip, or no chair flip for a tert-butyl group. Why? If I was to show all the hydrogens in here, if we invoked a chair flip, would it automatically start having steric clashes with neighboring atoms? Yes.

It is so repulsive, we say that tert-butyl groups do not undergo chair flips. Now, we've never detected a chair flip of a tert-butyl. Could that change? Maybe one day. But as of today, this summer, 2019, it's not going to change.

It's constant right there. No chair flip. is allowed when you're dealing with a Turk-Butyl group.

You put it in an equatorial position, and you're good to go. Don't forget the code for this video is LAZYBOY, L-A-Z-Y-B-O-Y, because we're dealing with chair confirmations. Best thing to do here is practice, practice, practice.

I hope everyone's doing well, and I look forward to seeing you all in lecture.

Transcript for:Chair Conformations in Organic Chemistry

Transcript for:
Chair Conformations in Organic Chemistry