Pyranose Conformations: Study with Video Lessons, Practice Problems & Examples

Pyranose sugars, cyclic monosaccharides with a six-membered ring, can adopt various conformations, primarily chair and boat forms. The chair conformation is more stable due to lower steric hindrance, with bulky groups in equatorial positions. The chair flip interconverts these forms without changing the configuration. In glucose, the beta anomer predominates (64%) due to its stability from equatorial preference, while the alpha anomer constitutes about 35%. Understanding these concepts is crucial for grasping carbohydrate chemistry and molecular stability.

Pyranose Conformations

In this video, we're going to begin our lesson on pyranose conformations. So recall from our previous lesson videos that a pyranose is literally just a cyclic sugar or a cyclic monosaccharide with a 6-membered ring. And so, these cyclic monosaccharides can actually exist in a variety of conformations, which recall conformations are just potentially flexible three-dimensional arrangements. Now, also recall that way back in some of our previous lesson videos, we had distinguished between conformations and configurations. And so recall that unlike configurations, which are more permanent and more fixed, conformations on the other hand, because they are flexible, can actually change without breaking or reforming bonds. Whereas configurations, on the other hand, the only way that they can change is by breaking and reforming bonds. And so this is important to keep in mind about conformations as we move forward and talk about the pyranose conformations in our next lesson video. So, I'll see you guys there.

Pyranose Conformations

So really the most common pyranose conformations are the chair and the boat conformations, which might actually sound familiar to you guys because these are the same exact conformations that cyclohexane has. And you might recall from way back in your previous organic chemistry courses that you guys covered the chair and the boat conformations of cyclohexane. And so also recall from your previous organic chemistry courses that the substituents on the chair and boat conformations can either occupy a more crowded axial position that will encounter more steric hindrance or the substituents could occupy a less crowded equatorial position that are going to branch away from the ring and encounter less steric hindrance. Now, you might also recall from your previous organic chemistry courses, that the chair conformation is actually more stable than the boat conformation. And so for that reason, the chair conformation predominates and is more common than the boat conformation.

And so if we take a look down below at our image over here, notice on the left we're showing you guys a Haworth projection of a pyranose that has a 6 membered ring. And then over here, we're showing you the chair conformation of that same exact pyranose. And over here we're showing you the boat conformation of that same exact pyranose. And so if you were wondering, the chair conformation is called the chair conformation because it literally looks like a chair. And so you can see this guy chilling, watching TV in his chair, and you can see this lady over here is just sunbathing in this really nice reclining chair. And, of course, the boat conformation is called the boat conformation because it literally looks like a boat. And so you can see this guy rowing his boat down the river, and you can see this guy rowing his boat as well. Now also recall that again, the substituents of both the chair and the boat conformation can either take the axial or equatorial position, and so down here in this corner, we're reminding you of that concept.

And so notice that each of these carbons will have substituents that take on either an axial position, that is going straight up and down. So the a represents the axial. And, again, they're going straight up and down. And these are going to encounter more steric hindrance and they're going to be more crowded just because they're going straight up and down above and below the ring. So, they're not going away from the ring. Whereas, notice that each of these e's that you see here are the equatorial positions that are going slanted and diagonally, and so they're going away from the ring so they're gonna be less crowded and they're going to encounter less steric hindrance. So, the equatorial positions are going to be more stable positions.

Now, over here on the far right, notice that we have an energy diagram with the free energy on the y-axis and the reaction coordinate on the x-axis and what you'll notice is that the chair conformation is lower in energy, lower in free energy in comparison to the boat conformation which is much higher in free energy. And so again what this means is that the chair conformation because it's lower in energy, it's more stable and, the boat conformation because it's higher in energy, it's going to be less stable and so, of course, the more stable chair is going to predominate. Now, you'll also notice that there are actually 2 chair conformations in this diagram and so in our next lesson video, we're going to talk about how we can interconvert between the 2 different chairs with a chair flip. So, I'll see you guys in our next video.

Pyranose Conformations

In this video, we're going to refresh your guys' memories of the chair flip. And so you might recall from our last lesson video that pyranose rings can actually assume 2 different chair conformations. And so the chair flip is literally just the process of converting one chair conformation into the other chair conformation. And so if we take a look at our image down below over here on the left-hand side, notice that we have the same exact lady sunbathing in our pyranose chair conformation. And so, imagine that our pyranose chair conformation in this image is actually a reversible reclining chair. And so imagine our lady right here standing up and grabbing this end of the chair and pulling it downwards, and then grabbing this end of the chair over here and pulling it upwards. And then, when she does that, she can go and sit right back down in the chair. And really, this is the process of the chair flip. And so what we see occurring here in this image is really the same thing that occurs with our pyranose chair flip. Now, what's important to note is that during the process of a chair flip, all of the substituents of the chair are going to change their axial and their equatorial positions. However, the upwards and the downwards positions of the substituents are going to remain unchanged during a chair flip, and we'll be able to visualize this down below in this part of our image. And the best way to analyze this is by looking at one carbon atom at a time. And so we're gonna focus in on carbon number 4, but really everything that applies to carbon number 4 is also going to apply to the other ring carbons. And so notice that carbon number 4 has both an axial position and an equatorial position. And notice that in this chair, the axial position is going upwards. So we have an upwards axial position. And notice that the equatorial position is going downwards for this chair. And so if we take a look at an actual pyranose right here, which corresponds with this one up above, again, focusing in on carbon number 4, it has an axial position going straight up and down with a hydrogen atom and then it has an equatorial position going downwards here with an alcohol group. And, of course, during the process of a chair flip, as we already noted, it's carbon number 4 that's going to be pulled downwards and carbon number 1 that's going to be pulled upwards. Just like this lady flipped the chair over here previously. And so notice that carbon number 4 is now in this position over here after the chair flip. And what you can see is that going upwards this time, we have an equatorial position, whereas previously going upward, we had an axial position. And going downwards this time, notice we have an axial position whereas previously we had an equatorial position. And so if we take a look at the actual pyranose, notice that going above the chain, we still have the same hydrogen atom. However, the hydrogen atom is now in an equatorial position whereas before it was in an axial position. And then notice going below the ring we still have the same hydroxyl group as before but the hydroxyl group is now in an axial position over here whereas previously it was in an equatorial position. And so what we're seeing here is that the substituents are changing their axial equatorial positions but again, the upwards downwards positions remain unchanged. Now, the last thing that's important to note here is that equatorial preference can sometimes help us determine the most stable chair conformation. And so equatorial preference tells us that the most stable chair conformation is going to have its bulky groups in a less crowded equatorial position. And so when we take a look at the actual pyranose conformations on the bottom, notice that this structure over here on the right, if we highlight all of the equatorial positions, if we circle all of the equatorial positions here, notice that they are all bulky groups. So all of the bulky groups are in equatorial position over here. However, when we take a look at this over here, notice that all of its bulky groups and its hydroxyl groups are in axial positions and axial is not going to be the preference. The preference is going to be equatorial. And so equatorial preference tells us that this equilibrium is going to favor the one that forms more equatorial, positions with bulky groups. And so, we can have a larger equilibrium arrow going towards the one that's favored and a smaller equilibrium arrow going to the one that's not as favored. And so we'll be able to touch up again on this idea of equatorial preference in our next video. But for now, this concludes our lesson on the chair flip, and I'll see you guys in our next video.

Circle the TWO chair conformations that could apply upon cyclization of the following linear monosaccharide:

So this practice problem wants us to circle the 2 chair confirmations that could apply upon cyclization of the following linear monosaccharide shown right here. And notice that we're given these 6 chair confirmations and again, we only need to choose 2 of these 6. And so, of course, whenever we want to convert the Fischer projection of a linear monosaccharide into its cyclic form, then, the two words, 'up lefting' and 'down right' can help us do that. Now, previously, we had used these two words to help us convert Fischer projections into Haworth projections. However, these words can also help us with converting Fischer projections into these chair confirmations. And so of course, left pointing groups in the Fischer projection are going to end up pointing upwards in the cyclic version and so we can go ahead and focus only on the hydroxyl group. So, this hydroxyl group here, we would expect to be pointing upwards. And then, of course, right pointing groups on the Fischer projection, we would expect to be pointing downwards in the cyclic version. So, that would be these hydroxyl groups here. We would expect to be pointing downwards and then, of course, numbering these carbons can help us keep track of the right one. So, the way that we number the carbon atoms of linear monosaccharides, we need to make sure that the carbonyl group, carbon atom gets the lowest possible number. So, here we have the aldehyde group, we want this carbon to have the lowest possible number, so we can give this a one and then number these sequentially down carbon 4, carbon 5, and carbon 6. And then of course, when taking a look at these carbon atoms over here, we can number these as well to help us better understand exactly how to keep track of these hydroxyl groups. So, notice that for this sugar A, the anomeric carbon is going to be the one that's bonded to 2 oxygen atoms. So, the anomeric carbon is here and it's bonded to this oxygen and this oxygen and so this is going to be the first carbon. So, carbon 1 and then we can number the others sequentially. 2, 3, 4, and 5 and where's the 6th carbon? Notice that this has a total of 6 carbons and sugar A only has a total of 5 carbons. So, there's no way that option A is going to be the correct answer. Now, notice looking at sugar D down here, there actually is not an anomeric carbon whatsoever. Notice that none of these carbons that we see are bonded to 2 oxygens. And also notice that there's a total of, let's see. There are a total of 7 carbon atoms. So 3, 4, 5, 6, 7 carbon atoms, which again, our structure only has 6 carbon atoms. So again, there's no way option D is going to be correct. So now, when we look at all of these other ones, we can see that they do have anomeric carbons and, they're gonna be numbered as follows. 4, 5, and 6, and they all have 6 carbons as well. Here's the anomeric carbon for this one. 2, 3, 4, 5, and carbon 6. Here's the anomeric carbon for this sugar. So 1, 2, 3, 4, 5, and carbon 6. And then here's the anomeric carbon for this sugar. 1, 2, 3, 4, 5, and 6. And so of course, we want the alcohol group of carbon number 2 which is right pointing to be pointing downwards. And so notice that in sugar C, carbon number 2 has its alcohol group not pointing downwards, but it's pointing upwards. And so that is not going to be correct. I mean, we can go ahead and eliminate option C for that reason. And then also notice that moving on to carbon 3 that it is left pointing and so because it's left pointing, it should be pointing upwards in the cyclic version. And so looking at carbon number 3 here on sugar B, notice that it's not pointing upwards. Instead, the alcohol group is pointing downwards which is incorrect and so sugar B is also not going to be correct. And so just by using this process, we've already limited our answer options down to sugar E and sugar F. And so it turns out that these are actually the correct answers. And so we could also confirm this by looking at sugar 2 and seeing that it's going downwards, looking at sugar 3 and seeing that it's going upwards, and again looking at sugar 4 and it's going, should be going downwards. And, the same goes for, these over here and I meant to say alcohol groups not sugars. These alcohol groups are going down, up, and down respectively and the same with, these alcohol groups over here. And so, what you'll note is that the only difference between sugar E and sugar F is the configuration of the anomeric carbon. And so notice that the anomeric carbon over here is pointing upwards in the same direction as the highest number carbon. And so this is going to be a beta configuration here for sugar E. And then notice for this sugar over here, the, alcohol group of the anomeric carbon is pointing downwards in the opposite direction of the highest numbered carbon. So, it's reaching down for the ants, so it's gonna be alpha configuration. But again, these are the correct answers to our problems and so, that concludes this practice problem and I'll see you guys in our next video.

Pyranose Conformations

In this video, we're going to talk about how the beta anomer of glucose predominates. And so glucose exists predominantly in its cyclic beta D-glucopyranose antimer. And if we were to analyze glucose's composition in a biological solution, we would find that about 64% or about two-thirds of all of the glucose molecules would be in the beta anomer form, then about 35% or about one third of all of the glucose molecules would be in the alpha anomer form, and then, of course, that leaves about less than 1% of all of the glucose molecules in other forms such as the linear chain. But why is it that the beta anomer form of glucose is so predominant? Well, it turns out it's because it's the most stable due to its equatorial preference, which you might recall from our previous lesson video just means that it has its bulky groups in equatorial positions, allowing them to minimize steric hindrance and be more stable.

Now, the last thing that's important to note here is that the chair flip is not to be confused with mutarotation. Recall that the chair flip only changes the confirmation, whereas mutarotation on the other hand changes configuration. And so we'll be sure to distinguish between these two down below in our image. Now focusing over here in this image with the black box, notice that we're showing you one of the chair confirmations of glucose. And if we take a look at its anomeric carbon here, notice that its alcohol group is going downwards in the opposite direction of the highest numbered carbon. And so the alcohol group here is reaching down for the ants, and so that reminds us that this is the alpha anomer. So this is alpha D-glucopyranose, which we already said up above makes up about 35% of glucose's composition at equilibrium.

Now notice that most of the bulky groups here in this chair confirmation are actually taking the equatorial position, and we do have one bulky group in this alcohol group here that is taking the axial position. And so through the chair flip, we know that all of the axial equatorial positions change and, the upwards and downwards positions do not change. And so if we take a look at this anomeric carbon over here, notice that its alcohol group is still going downwards in the opposite direction of the highest numbered carbon. So it's still reaching down for the ants and still going to be the alpha anomer. And so we haven't changed the configuration, so it's alpha and alpha on both sides. And since there's no configuration, this is not mutarotation. This is going to be a chair flip.

And, again, notice that over here in this chair confirmation that there's only one bulky group in the equatorial position, which is this one right here. All of the other ones are in the axial position, which are going to be less stable. And so equatorial preference tells us that this chair confirmation over here on the left-hand side which has more bulky groups and equatorial positions is going to be favored over the other chair confirmation. So, we can draw a larger equilibrium arrow going to the left.

Now, if we move on to the bottom box that we have down below, notice that we're showing you another chair confirmation of glucose. And so if we take a look at its anomeric carbon, notice that the alcohol group is going upwards in the same direction as the highest numbered carbon. And so the bumps of the beta being on the same side reminds us that this is going to be the beta anomer. And so this is beta D-glucopyranose, which, again, we set up above makes up about 64%, the vast majority of glucose's composition and equilibrium. Now, notice that this chair confirmation of beta D-glucopyranose actually has all of its bulky groups in the equatorial position. And the equatorial preference through the equatorial position tells us that this is going to be the most stable chair confirmation. And so if we were to do a chair flip and get this other chair confirmation over here, again, looking at its anomeric carbon, the alcohol group is going upwards still in the same direction as the highest numbered carbon. So the bumps of the beta being on the same side tells us that this is also going to be a beta anomer. And so we have beta and beta. So the configuration hasn't changed. So, this is not going to be muted rotation. Instead, this is going to be a chair flip. And notice that again that all of the axial equatorial positions change, but upwards, downwards positions don't change. And so this time, we have all of the bulky groups in axial positions, which are going to be less stable. And again, equatorial preference tells us that it's this chair confirmation over here on the left-hand side, that's going to be more stable and so, we can draw equilibrium arrows, larger equilibrium arrow going to the left showing that this will be the most stable form. And then, of course, what we have here in the middle is going to show the change in configuration from the alpha configuration that we have up at the top. Down to the beta configuration, what we have in the bottom box. And so, this is going to be representing mutarotation. A change in the configuration of the anomeric carbon. And, of course, because this one here has all of its bulky groups in equatorial preferred positions, Equatorial preference tells us that it's this bulky group down below that's going to be more stable, and so we can draw a smaller equilibrium arrow going backward. And so, really this is the conclusion to our lesson on how the beta anomer of glucose predominates and, that's important to keep in mind as we move along and so that concludes this video and I'll see you guys in our next one.

Here’s what students ask on this topic:

What are pyranose conformations?

Pyranose conformations refer to the different three-dimensional shapes that pyranose sugars, which are cyclic monosaccharides with a six-membered ring, can adopt. The most common conformations are the chair and boat forms. These conformations are flexible and can change without breaking or reforming bonds, unlike configurations. The chair conformation is more stable due to lower steric hindrance, with bulky groups typically occupying equatorial positions. Understanding these conformations is crucial for studying carbohydrate chemistry and molecular stability.

Why is the chair conformation more stable than the boat conformation in pyranose sugars?

The chair conformation is more stable than the boat conformation in pyranose sugars because it minimizes steric hindrance. In the chair form, bulky substituents can occupy equatorial positions, which are less crowded and encounter less steric hindrance compared to axial positions. This lower steric hindrance results in lower free energy, making the chair conformation more stable. In contrast, the boat conformation has higher steric hindrance and free energy, making it less stable.

What is a chair flip in pyranose conformations?

A chair flip in pyranose conformations is the process of converting one chair conformation into another. During a chair flip, all axial substituents become equatorial and vice versa, but the up and down positions of the substituents remain unchanged. This interconversion allows the molecule to adopt the most stable conformation, typically with bulky groups in equatorial positions to minimize steric hindrance.

Why does the beta anomer of glucose predominate?

The beta anomer of glucose predominates because it is the most stable form due to equatorial preference. In the beta anomer, the bulky groups, including the hydroxyl group on the anomeric carbon, are in equatorial positions, minimizing steric hindrance. This stability results in about 64% of glucose molecules being in the beta anomer form in a biological solution, compared to about 35% in the alpha anomer form and less than 1% in other forms.

What is the difference between a chair flip and mutarotation in pyranose sugars?

A chair flip in pyranose sugars involves the interconversion between two chair conformations, changing the axial and equatorial positions of substituents without altering the configuration. Mutarotation, on the other hand, involves a change in the configuration at the anomeric carbon, converting between alpha and beta anomers. While a chair flip only affects the conformation, mutarotation changes the actual structure of the molecule, impacting its chemical properties.