Stereochemistry of Monosaccharides - Online Tutor, Practice Problems & Exam Prep

Monosaccharides exhibit various isomers, including constitutional isomers, stereoisomers, enantiomers, diastereomers, and epimers. Stereoisomers differ in three-dimensional arrangement, while enantiomers are non-superimposable mirror images. Epimers are a specific type of diastereomer differing at one chiral carbon. The number of stereoisomers can be calculated using the equation $2^n$, where n is the number of chiral carbons. Understanding these concepts is crucial for grasping carbohydrate chemistry.

Stereochemistry of Monosaccharides

In this video, we're going to begin our lesson on monosaccharide stereochemistry. Before we actually get started, I want to point out that most of the information in this topic is actually going to be a review from your old organic chemistry courses. I want you guys to note that, way back in some of our previous lesson videos, we did an organic chemistry review for your biochemistry course. We covered a lot of organic chemistry topics in more detail. If you find a lot of the terminology and vocabulary on this page completely unfamiliar, then be sure to check out those older lesson videos on organic chemistry review before you move on with this video here. Alright, so that being said, let's go on and talk about what we have on this page.

It's important to note that the 3D structures of linear monosaccharides are commonly displayed on pieces of paper using Fischer projections. Throughout our lesson on carbohydrates, we're going to continue to see lots of Fischer projections. Notice over here on the far left, we've got this really cool image to help remind you of an important feature of Fischer projections. All of the horizontal bonds that go from side to side in every Fischer projection are popping out of the page as wedges. Notice we have this three-dimensional sugar molecule and we're shining this flashlight down on the 3D sugar molecule to create this shadow of the sugar molecule on this yellow plane. If we were to draw that shadow onto a piece of paper, we would get the Fischer projection. Notice that the horizontal bonds here in the Fischer projection correspond to bonds that are popping out of the page as wedges. So that's important to keep in mind when we're considering the stereochemistry of Fischer projections.

Now, we can move on to the terms that we have in this table. The first term that we have is 'constitutional isomers.' Recall that isomers are different molecules with the same chemical formula. Constitutional isomers are only going to differ in their constitution, or the connectivity of the atoms. A classic example is glyceraldehyde and dihydroxyacetone (DHA). Notice they have the same chemical formula but different connectivity of atoms. The next term that we have is 'stereo isomers.' These are isomers, so they have the same exact chemical formula. But this time, they don't differ in the connectivity of the atoms. Instead, they differ in their three-dimensional space, which is what the prefix 'stereo' is referring to. Notice that these two molecules only differ in the three-dimensional space of this hydroxyl molecule that we see here.

The next term is 'enantiomers.' Enantiomers are just a specific type of stereo isomer. Recall that they are non-superimposable mirror images. A classic example is our hands. Our left hand is a mirror image of the right hand, and they are non-superimposable since we can't perfectly overlap them in space when facing the same direction. This applies to the molecules here, D-Glyceraldehyde and L-Glyceraldehyde, which are mirror images of one another and cannot be superimposed or overlapped in space.

Lastly, we have 'diastereomers,' which are another type of stereo isomers. Diastereomers are not mirror images. We can see below that these two structures are not mirror images of one another because they differ in their 3D space, the configuration here. That's why we've got this broken mirror to represent the diastereomer.

This concludes our introduction to the stereochemistry of monosaccharides. If you need a review, make sure to go back and check out those older lesson videos. I'll see you in our next video.

Stereochemistry of Monosaccharides

In this video, we're going to remind you how to calculate the number of stereo isomers that a molecule has. And so the number of stereo isomers that a molecule has is going to be equal to 2n, where n is equal to the number of chiral carbons that the molecule has. And so recall from your previous organic chemistry courses that a chiral carbon is literally a carbon atom that is covalently bound to 4 distinct chemical groups or 4 different chemical groups and so in our example down below, we're going to circle all of the chiral centers or chiral carbons for that matter. These are synonyms of one another, and we're going to determine how many stereo isomers each of the following molecules has. And so we'll start with this molecule over here on the left-hand side. And looking at its first carbon on the left, what we need to realize is that there are 2 hydrogen atoms branching off that aren't being shown. And that means that there are 2 bonds leading to the same chemical group of a hydrogen atom. And so that means that this carbon atom does not have 4 distinct chemical groups since it has 2 of the same chemical groups and the same logic applies for this carbon atom up here. It's also bound to 2 hydrogen atoms that aren't being shown which means that it is not a chiral carbon. Now, this carbon atom right here is also not a chiral carbon because it has a double bond, meaning that it has 2 bonds leading to the same chemical group, which means that it won't have 4 distinct chemical groups. And so the only chiral carbon in this entire molecule is this carbon right there that's circled. And so notice that the hydroxyl group going down is different than this group going to the right, which is different than this group going to the left, which is different than the invisible hydrogen atom that isn't being shown. And so this circled carbon is a chiral carbon because it has 4 distinct chemical groups. And so now that we've circled all of the chiral centers in this molecule, let's go ahead and calculate the number of stereo isomers. And then as we mentioned above, the number of stereo isomers is 2n. So 2, and then n is the number of chiral carbons, which is 1 for this molecule, so 21, and that is of course equal to 2. So there are 2 stereo isomers that this molecule has. Now, looking at this molecule over here, we can do the same-looking at each chiral, looking for the chiral carbons and if we look at this carbon atom right here on the far left, left, it has a double bond which we already know means that it's not going to be a chiral carbon. And then when we look at this carbon over here, notice that it has 2 hydrogen atoms branching off, which means that it won't have 4 distinct chemical groups. And of course, that means that it's not going to be a chiral carbon. And so the only chiral carbons are right here in these positions and notice that all 4 of these carbon atoms are chiral because they all have 4 distinct chemical groups attached. And so now that we've identified the 4 Chiral Centers in this molecule, let's calculate the number of Stereoisomers which again is 2n And so we have 2 and this time it's going to be 4 since there are 4 Chiral Carbons. And if you take your calculators and type in 24, you'll get an answer of 16. So this molecule has 16 stereo isomers. And so, this here concludes our lesson on how to calculate the number of stereo isomers and we'll be able to apply this, moving forward in our practice problem. So I'll see you guys in our next video.

Stereochemistry of Monosaccharides

Alright. So in this video, we're going to introduce monosaccharide epimers. So what in the world are epimers? Well, epimers are really just a specific type of diastereomer, and recall from our previous lesson videos that diastereomers are just stereo isomers that are not mirror images, which means that epimers are also not mirror images. And so, to remind you that the epimers are not mirror images, we have this big broken mirror here to, again, remind you that they are not mirror images. But more specifically, an epimer is going to be diastereomers that differ only in the configuration of just any one single chiral carbon. And so, the only difference between two epimers is again going to be the configuration of just one single chiral carbon. So if we take a look at our image down below, you can see that we have an example of a pair of epimers. And so, on the left-hand side, we have the structure for D-glucose, and on the right-hand side, we have the structure of D-mannose. And D-glucose and D-mannose differ from each other in only the configuration of one single chiral carbon. And that is the chiral carbon that is highlighted in this pink box. And so D-mannose, instead of having the hydroxyl group pointing to the right of the Fischer projection, like what D-glucose has, D-mannose has its hydroxyl group pointing to the left. And instead, its hydrogen is going to be pointing to the right. And so because D-mannose and D-glucose differ from each other, again, only in the configuration of just one single chiral carbon that makes them epimers. And, again, all epimers are not going to be mirror images. And so that concludes our introduction to monosaccharide epimers and we'll be able to get some practice utilizing this concept as we move forward in our course. So, I'll see you guys in our next video.

Which sugar is an enantiomer of sugar A? How many stereoisomers does sugar A have?

• A. None of them.
• B. Sugar B only.
• C. Sugar C only.
• D. Sugar D only.
• E. Both Sugars B and D.

Use the Fischer projections of the four monosaccharides below to answer the next two problems (A & B).

A) The term that best describes the relationship of all four sugars to each other is:

a) Diastereomers. 

b) Enantiomers. 

c) Epimers.

• A. Diastereomers
• B. Enantiomers
• C. Epimers

Stereochemistry of Monosaccharides

In this video, we're going to review the different types of monosaccharide isomers. None of the information in this video is new; it's all review from our previous lesson videos. If you already feel comfortable with all of the terms on this page, then feel free to skip this video. However, if you're struggling even just a little bit with any of these terms, then stick around because this is an excellent way to visualize how all of these different terms relate to one another. Alright. So let's get started with this flowchart, and notice at the very top here we have the term isomers, which recall are just different molecules with the same exact molecular formula.

There are two branches of isomers. We have the constitutional isomers and the stereo isomers. Constitutional isomers have the same chemical formula but different connectivity of the atoms. A classic example is glyceraldehyde and dihydroxyacetone, where you can see just by focusing on the carbonyl group that they have different connectivity of the atoms. The other branch of isomers is the stereo isomers, which have the same connectivity but different spatial three-dimensional arrangements, and that's why we've got these 3D glasses here.

Stereo isomers can also be broken up into two categories: enantiomers and diastereomers. Enantiomers are non-superimposable mirror images. A classic example here is D-glyceraldehyde and L-glyceraldehyde, which are mirror images of each other. Now, diastereomers, on the other hand, are stereo isomers that are not mirror images. For instance, glucose and ultras have two chiral carbons that are not in opposite configurations, making them diastereomers.

A very specific type of diastereomer is the epimer. Epimers are diastereomers that have a different configuration at any one chiral carbon. If two molecules differ only in the configuration of one chiral carbon, that makes them an epimer. A classic example is D-glucose and D-mannose, which differ only in the configuration of the hydroxyl group at the C₂ carbon.

This concludes our flowchart, and we'll be able to get some practice applying all of these concepts in our next couple of videos. So, I'll see you guys there.

Two sugars which differ from one another only in configuration of one of many chiral carbon atoms are termed:

• A. Epimers.
• B. Enantiomers.
• C. Optical isomers.
• D. Stereoisomers.
• E. Conformers.
• F. None of these are correct.

Amongst the aldopentoses shown below, identify the pairs that are enantiomers.

____ and ____

____ and ____

____ and ____

____ and ____

So this practice problem says, among the aldopentoses shown below right here, identify the pairs that are enantiomers. Recall that enantiomers are non-superimposable mirror images. And so if we start with sugar number 1 over here, if we want to find the enantiomer, all we have to do is find the mirror image. And so, one way to help us find the mirror image is to note that for all of these chiral carbons right here, the OH groups are pointing to the right. And so if we want to find the mirror image, we need to find the structure where all of the OH groups are pointing in the opposite direction. So all of the OH groups pointing to the left instead of pointing to the right. Notice that matches with sugar 6. Notice that all of the hydroxyl groups are pointing to the left instead of pointing to the right, which means that sugar 1 and sugar 6 are pairs of enantiomers. So over here on the left-hand side, we can say that sugar 1 and sugar 6 are indeed pairs of enantiomers. Now recall that enantiomers is a word that describes the relationship only between 2 sugars. And so what that means is that sugar 1 and sugar 6 are not going to be possible answers, are not going to be pairs of enantiomers with anything except each other. And so, as we move on to sugar 2, we know that sugar 2 is not going to be a pair of enantiomers with sugar 1 or sugar 6. And so, taking a look at sugar 2, we can apply this same strategy where we look at the chiral carbons and the hydroxyl groups coming off. And so you can see that the C2 hydroxyl is going to the right, and the C3 and C4 hydroxyls are going to the left. And so if we want to find the enantiomer of sugar 2, then all we need to do is find these hydroxyls that are going in the opposite directions. Notice that matches with sugar 4. So notice that its C2 hydroxyl is going left in the opposite direction and its C3 and C4 hydroxyls are going to the right in the opposite directions. And so sugar 2 and sugar 4 are indeed pairs of enantiomers and for that reason, we can go ahead and list them over here. Sugar 2 and sugar 4 are pairs of enantiomers and again, enantiomers is a word that describes the relationship between just 2 sugars. So what that means is that sugar 2 and sugar 4 are enantiomers with each other, but they are not going to be enantiomers with any of the other sugars on here. So we can go and cross those off. And so, of course, what we are left with is just sugar 3 and sugar 5. So that means that they are going to be enantiomers with each other, right? Sugar 3 and sugar 5? No, not quite. So if we take a closer look and apply the same strategy to sugar 3, notice that its C2 and C3 hydroxyls are pointing to the right, and then the C4 hydroxyl is pointing to the left. And so if we wanted to find the enantiomer of sugar 3 then these hydroxyls need to be pointing in the opposite directions. However, when we take a look at sugar 5 over here, notice that its first hydroxyl is pointing in the same direction. So it is not pointing in the opposite direction as it should. And the OH here is pointing in the opposite direction, and so is this one down below, but the first one is not and that means that sugar 3 and sugar 5 are not enantiomers of each other and so really this last blank here is just a trick and we can cross it off. And so, the answers to our practice problem are sugar 1 and sugar 6, and sugar 2 and sugar 4 are enantiomers of each other. And that concludes this practice problem, so I'll see you guys in our next video.

Which term best describes the relationship between D-Mannose & L-Mannose? 

• A. Enantiomers.
• B. Anomers.
• C. Epimers.
• D. Diastereomers.
• E. Same molecules

According to the Fischer projections of the following monosaccharides, circle the C-5 epimer of L-Talose?

So this practice problem says, according to the Fisher projections of the following monosaccharides below, circle the C5 epimer of L-talose. Notice that we're actually given the D configurations for all 8 of these monosaccharides, including D-talose. But again, it's asking us about the C5 epimer of L-talose. So what we need to do is use the structure of D-talose to help us determine the structure of L-talose. Once we have the structure of L-talose, then we can determine which of these other molecules are going to be the C5 epimer. The way that we can determine the structure of L-talose is to realize that L-talose is going to be the enantiomer of D-talose, which means that all we need to do is draw the mirror image of D-talose, and we'll get the structure of L-talose. And so the structure of L-talose is as follows. Basically, all we need to do is take all of the hydroxyl groups on the chiral carbons and change the positions of those hydroxyl groups. If they're on the left side, change them to the right, and if they're on the right side, change them to the left. Notice the first three hydroxyls are now pointing to the right and the last hydroxyl here is pointing to the left, and so this is going to be the structure of L-talose right here. Essentially, what we can do is number these carbon atoms, and of course, when we number the carbon atoms of linear monosaccharides, we number them so that the carbonyl group carbon atom gets the lowest possible number. So this is going to be 1 followed by 2, 3, 4, 5, and 6. If we want the C5 epimer, that means that we're going to be focusing on the 5th carbon and changing the configuration of just this carbon. Recall that epimers differ in the configuration of only one chiral carbon. So, of course, if we were to redraw this structure of L-talose, except we change the C5 epimer here so that its OH group is now pointing in the opposite direction, then we would get the C5 epimer. This is the C5 epimer of L-talose. When we compare all of these other structures above to this structure right here, what we'll realize is the correct answer is D-allose. Notice that the structure of D-allose matches the structure of the C5 epimer. So the correct answer to this practice problem is D-allose. That concludes this practice problem, and I'll see you guys in our next video.

Here’s what students ask on this topic:

What are the different types of isomers in monosaccharides?

Monosaccharides exhibit several types of isomers, including constitutional isomers, stereoisomers, enantiomers, diastereomers, and epimers. Constitutional isomers have the same chemical formula but differ in the connectivity of atoms. Stereoisomers have the same connectivity but differ in their three-dimensional arrangement. Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. Diastereomers are stereoisomers that are not mirror images. Epimers are a specific type of diastereomer that differ at only one chiral carbon. Understanding these isomers is crucial for grasping the complexity of carbohydrate chemistry.

How do you calculate the number of stereoisomers for a molecule?

The number of stereoisomers for a molecule can be calculated using the formula $2^n$, where $n$ is the number of chiral carbons in the molecule. A chiral carbon is a carbon atom that is covalently bonded to four distinct chemical groups. For example, if a molecule has 3 chiral carbons, the number of stereoisomers would be $2^3$, which equals 8. This formula helps in understanding the diversity of stereoisomers in carbohydrate chemistry.

What are enantiomers and how do they differ from diastereomers?

Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. A classic example is the pair of D-glyceraldehyde and L-glyceraldehyde. Diastereomers, on the other hand, are stereoisomers that are not mirror images. They differ in the spatial arrangement of atoms but are not related as mirror images. For instance, D-glucose and D-mannose are diastereomers because they differ in the configuration at more than one chiral carbon. Understanding these differences is essential for studying the stereochemistry of monosaccharides.

What are epimers in the context of monosaccharides?

Epimers are a specific type of diastereomer that differ in the configuration at only one chiral carbon. For example, D-glucose and D-mannose are epimers because they differ only at the C2 carbon. In D-glucose, the hydroxyl group at C2 is on the right in the Fischer projection, while in D-mannose, it is on the left. Epimers are not mirror images of each other and are crucial for understanding the structural variations in monosaccharides.

What is the significance of Fischer projections in studying monosaccharides?

Fischer projections are a two-dimensional representation of three-dimensional molecules, commonly used to depict the stereochemistry of monosaccharides. In Fischer projections, horizontal lines represent bonds that come out of the plane of the paper (wedges), and vertical lines represent bonds going into the plane of the paper (dashes). This method simplifies the visualization of complex molecules and helps in understanding the spatial arrangement of atoms, which is crucial for studying the stereochemistry of carbohydrates.