Monosaccharide - Online Tutor, Practice Problems & Exam Prep

Carbohydrates, or sugars, are organic molecules that consist of carbon, hydrogen, and oxygen, typically represented by the formula C_n(H₂O)_n. The simplest form, monosaccharides, must have at least three carbons and can exist as straight chains or rings. They begin as either aldehydes (aldoses) or ketones (ketoses). The total number of stereoisomers for a sugar is given by 2ⁿ, where n is the number of chiral centers. An epimer is a specific type of diastereomer differing at only one chiral carbon, highlighting the complexity of sugar stereochemistry.

Monosaccharide

Hey everyone. I'm about to take you on a journey through one of the oldest and most elegant fields of organic chemistry and that's the field of sugar chemistry, also known as carbohydrate chemistry. So let's go ahead and get started. Alright guys, so before we even really get started I need to tell you one thing which is that sugars, saccharides, and carbohydrates are all different words for the same types of molecules. So I'm going to be using those words interchangeably throughout this course and you should just think whenever you hear sugar, saccharide, or carbohydrate, those are all describing the same types of molecules.

Now one of those words that you might say often but not really think about too much is carbohydrate. We talk about carbohydrates and nutrition all the time, but when you break the word down to its roots, what you see is that it's actually a hydrate of carbon. That's literally what the word breaks down to, what it's symbolizing. And in chemistry, we know that a hydrate is any atom or molecule that's combined with water. So when we say that a sugar is a carbohydrate, what we're saying is that a sugar is a carbon that's been combined with water.

Okay. Now there are many sugars out there, but the most basic unit of sugar is called a monosaccharide. So we're going to be spending a lot of time in this course, just talking about the properties of monosaccharides before we expand that further into larger sugars. All unmodified monosaccharides have the same general formula. No matter what their shape, what their size, they always have the same formula, which is carbon, H₂, and oxygen all to the n, where n is a number greater than or equal to 3.

C H 2 O n

Okay? So let's break that down for a second, a few words at a time. So why did I say unmodified? Unmodified because I'm saying this is the way the sugar begins its life, okay? You could always change a sugar to have other atoms on it later, but when it starts off, it's always going to start off as CH₂O to the n. Now when you look at this general formula, it describes exactly what I just said about a carbohydrate, which is that a carbohydrate is literally going to be one carbon that's been combined with one water molecule. So it's cool how it actually breaks down definitionally like that.

Now the last thing is that where n is equal to 3 or more, all that's saying is that your smallest possible sugar is a 3 carbon sugar. It can be as big as you want, but it has to be at least 3 carbons. If it's less than 3, if it's 2, it doesn't really count as a sugar anymore because it just has a very different reactivity. It doesn't behave like the other sugars do. So we wouldn't call a 2 carbon molecule usually a sugar.

Monosaccharides can be represented as either straight chains or as rings. So you're going to see that throughout this section I'm going to be showing them to you as straight chains and then other times you're going to see them as rings, and we're going to explain more why they cyclize.

Now something you should know is that by definition a carbohydrate needs to have 1 oxygen attached to every carbon to be a carbohydrate. You can't have 2 oxygens on 1 carbon and then 0 on the other; that would not be a carbohydrate. Now remember guys, what does IHD mean? It meant that you're missing hydrogens in order to be a saturated molecule. And remember that an IHD could either come from a double bond or a ring, right? Not necessarily a 6 member, but a ring. So that makes sense according to my fact about that they could either be straight chains or rings. That means when it's a ring, it has the 1 IHD from the ring, right? And then when it's a straight chain, it must have the 1 IHD from a double bond.

Monosaccharides always begin as either aldehydes or ketones. Once again, I'm using this word begin because you could always modify that functional group later. If you wanted to maybe could change the aldehyde into something else, but what I'm saying is that when they begin their life, they always start off as either an aldehyde or a ketone. Those are your only two possibilities. We call an aldehyde sugar an aldose and a ketone sugar a ketose. Let's look at two examples of monosaccharides.

Provide the generic name for the following monosaccharide.

• A. aldotetrose
• B. ketopentose
• C. ketotetraose
• D. oxopentose

How many possible stereoisomers AND epimers exist for the following aldopentose? Draw all of the possible epimers. 

• A. 4
• B. 8
• C. 16
• D. 32

Identifying Types of Stereoisomers

Identify and label all the possible stereoisomers of aldotetraose, as enantiomers, diastereomers, or epimers of each other. Cool. So let's go ahead and start off with the new definition which is epimer's, and then we'll move on to the other definitions that we know from the chirality section of this textbook. So which ones here would be epimers of each other? Remember an epimer has to differ at only one carbon, so these two would be epimers. Everything that I'm going to put with blue arrows is going to be an epimer. So I'm going to put here blue is equal to epimer. Cool? So those would be epimers, these would be epimers, oops. And that's actually pretty much it. Those are the only epimers that we see.

Now what does it mean to be an enantiomer? Remember that an enantiomer has all the chiral centers changed. So you can't have any the same, they have to have changed completely. So some enantiomers that I see are these. That's an enantiomer because I see that the 2 -OH groups used to face to the right and now they face to the left. I also see that these are enantiomers. Cool. I think I see another epimer guys. I see some more epimers. So what else was an epimer here? This is an epimer, right, because what I noticed is that here these 2 are the same, but in the top they change, right? So those were epimers as well. And it looks like these A to D are also related as epimers. Cool. So there might be more relationships guys, I'm going to keep looking at them as we keep going through this, okay?

So, I don't see any more enantiomers though, so I'm going to move on to the next definition which is diastereomers. So which of these are diastereomers? Actually guys, every epimer by definition is a diastereomer, so it's going to be all the same arrows as epimers. Now you might be saying, okay Johnny, well if they're the same thing, why do you have 2 different words for the same thing? Well guys, this molecule happens to meet the definitions of diastereomers and epimers happen to be the same because it's so small, but if it was bigger you would actually have diastereomers that are not epimers. Okay? In this case, since there are only 2 chiral centers, they happen to line up perfectly, but if you had 3 chiral centers then it would be possible to have a diastereomer that is an epimer that changed at one position and it would be possible to have a diastereomer that is not an epimer because it changed at 2 different positions. Okay? So just so you know, in this case they happen to have the same definition but they won't always. I don't see any other relationships that we missed so I think this question is over with. Good job. Let's move on to the next video.

Here’s what students ask on this topic:

What is the general formula for monosaccharides?

The general formula for monosaccharides is C_n(H₂O)_n, where n is a number greater than or equal to 3. This formula indicates that monosaccharides are hydrates of carbon, meaning each carbon atom is associated with a water molecule. For example, glucose, a common monosaccharide, has the formula C₆H₁₂O₆. This formula reflects the basic structure of monosaccharides, which can exist as either straight chains or rings and always have one oxygen atom for each carbon atom.

What are the differences between aldoses and ketoses?

Aldoses and ketoses are two types of monosaccharides distinguished by their functional groups. Aldoses contain an aldehyde group (–CHO) at the end of the carbon chain, while ketoses have a ketone group (C=O) typically at the second carbon atom. For example, glucose is an aldose because it has an aldehyde group, whereas fructose is a ketose because it has a ketone group. These functional groups influence the chemical reactivity and properties of the sugars.

How do you determine the number of stereoisomers for a monosaccharide?

The number of stereoisomers for a monosaccharide is determined using the formula 2ⁿ, where n is the number of chiral centers in the molecule. Chiral centers are carbon atoms with four different groups attached. For example, glucose has four chiral centers, so the number of possible stereoisomers is 2⁴ = 16. This formula helps in understanding the complexity and diversity of monosaccharide structures.

What is an epimer in carbohydrate chemistry?

An epimer is a type of diastereomer specific to carbohydrates. It refers to two monosaccharides that differ in configuration at only one chiral carbon. For example, glucose and mannose are epimers because they differ only at the C-2 carbon. Epimers are important in carbohydrate chemistry because they highlight the subtle differences in structure that can significantly impact the properties and functions of sugars.

Why do monosaccharides cyclize to form rings?

Monosaccharides cyclize to form rings to achieve a more stable structure. This cyclization occurs when the hydroxyl group on one of the carbon atoms reacts with the carbonyl group (aldehyde or ketone) within the same molecule, forming a hemiacetal or hemiketal. For example, glucose can cyclize to form a six-membered ring called a pyranose. This ring formation is favored because it reduces the molecule's energy and increases stability.