Introduction to Carbohydrate Monosaccharides

Concept: Concept

13m
Video Transcript

Hey everyone, I'm going to take you on a journey through one of the oldest and most elegant fields over again in 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 or saccharides or carbohydrates, 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 talked about carbohydrates and nutrition all the time, 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 is symbolizing. And in chemistry, we know that a hydrate is any atom or molecule thatÕs combined with water, so when we say that a sugar is a carbohydrate, what we're saying is that if 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 monosaccharides. We're really spending a lot of time in this course just talking about the properties of monosaccharides before we expand that further into the larger sugars, okay? All unmodified monosaccharides have the same general formula, so no matter what their shape, was their size, they always have the same formula and that's carbon, H2 and oxygen, all to the n; where n is a number greater than or equal to 3, 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 can always change a sugar to have other atoms on it later but, when it starts off, it is 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 three carbon sugar, it can be as big as you want, but it has to be at least three carbons, if it's less than three, -if itÕs two, it doesn't really count as a sugar anymore, because it just has a very different reactivity, it doesn't behave like other sugars use, we wouldn't call a two carbon molecule usually a sugar, okay? So, what else do we need to know about monosaccharides? We're just saying the general facts right now, okay? This is an introduction. Monosaccharides can be represented as either straight chains or rings. So, you're going to see that throughout this section I'm going to be showing them to you as straight chain, and then other times you're going to see them as rings, and we're going to explain more why they cyclize, okay? Now, something you should know is that by definition a carbohydrate needs to have one oxygen attached to every carbon to be a carbohydrate, you can't have two oxygens on one carbon and then zero on the other, that would not be a carbohydrate any more, so it needs to be a one to one ratio. Also an interesting fact about carbohydrates is that they're always going to have one IHD regardless of their form. Now, remember guys, what an IHD mean? It meant that you're missing hydrogens in order to be a saturated molecule, and remember that in IHD could either come from a double bond or a ring, right? Not necessarily six membered but a ring, okay? 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 180 from the ring, right? And then when it's a straight chain it must have 180 from a double bond, so just letting you know you're always going to have one, no matter what form it's in, okay? Now, monosaccharides always begin as either, let's write this down, aldehydes or ketones, okay? Now, once again I'm using this word ÒbeginÓ because you could always modify that function verb later, if you wanted to, maybe you could change the aldehyde into something else, but what I am 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, okay? And what we do is we call an aldehyde sugar an aldose, and we call a ketone sugar a ketose, okay? So, that's pretty easy. Let's look at two examples of monosaccharides. So, just looking at the definitions in the facts that I told you, we could prove in several different ways that these are monosaccharides, okay? So, first of all, do they have three or more carbons? Yes, I see that they both have I think six carbons, so we're good there. Does it have at least, I mean not at least, does it have exactly one oxygen on every carbon? Yeah, I see that, I see one O for every carbon, very good a one-to-one ratio. Do I see at least one IHD on each of these compounds? Well, remember guys, that a double bond comes as one IHD, so this would be my IHD over here, and also remember from our functional groups section back in orgo 1 that CHO is the condensed formula for an aldehyde, so it actually looks like is this, so there's a double bond there as well, that's an aldehyde for you, okay? So guys, we know these are monosaccharides and as you can see the first one is called an aldose, because it contains an aldehyde at the top, the second one contains, is called a ketose because it has a ketone of the second carbon.

Remember that a ketone needs to have R groups on both sides, you can't put a ketone all the way at the top, it needs to be at least on the second carbon. Very good. So, easy enough so far all the definitions that I gave you at the beginning aren't checking out. Now, let's talk about some more general features. There's so much to learn about carbohydrates but we have to start with the baby steps. Let's start out talking about naming. So, monosaccharides can be described both by generic names and by specific names, at this point you're probably more accustomed to hearing the specific names, so what's a specific name? Glucose, we've all heard of glucose before, it is a name that describes one specific monosaccharide but we need to know is not only, probably a few of those specific names, but we need to know the general categories and those are described by generic names. So, how do you name a sugar with a generic name? Well, the first thing you start off with is the carbonyl type, so is it an aldehyde or a ketone? That's going to give you either an alto- or a keto- prefix. Next, you would go to your carbon chain length, then you say how long is the carbon chain. Now, here this is where things get a little tricky. Remember, I said this is an old field of organic chemistry, meaning that the naming conventions were devised pre-IUPAC, before IUPAC. Remember, IUPAC was in 1919, a lot of this sugar chemistry was discovered in the 1800Õs; so what that means is that a few of the names sound familiar and a few of them don't really meet convention. So, it's pre- IUPAC names with an ÒoseÓ ending, so for example: -pentose and Ðhexose, that makes sense, that would be IUPAC, right? Because pent- is the prefix for five, hex- is the prefix for six, so that makes sense, but then a four carbon sugar isn't called butose as we would have expected, IUPAC it's called a -tetraose, why? It's random guys, someone named this before IUPAC got started and then they just stuck with it, and then a three carbon is called a -triose. So, just letting you guys know that you're not going to respon-, be responsible for all the names up to like 20 carbons, as long as you know those four you're probably, okay? Cool. So, we'll name more in a little bit but I want to talk about some stereochemistry as well. So, the total number of stereoisomers possible for a sugar is described by the same formula we used in our chirality section a long time ago, 2 to the n, where n is equal to your stereo genic centers. So, guys why is this important? Because sugars are notoriously complicated in terms of their stereochemistry, because they have so many chiral centers, okay? Remember that guys, these are Fischer projections that you're looking at here and remember Fischer projections are designed to show you the chirality of each atom. So glucose, for example, has how many chiral centers? Every single, like intersection is a chiral center, so this would be one, this would be another, this would be another, this would be another, the top one is not chiral because there isnÕt four groups, the bottom is not chiral because there isnÕt four different groups, so we have four chiral centers. So, that means just glucose, glucose has how many total possible stereoisomers? Well, it would have 2 to the 4, because we're using the 4 to the n formula, so that means that glucose has 16 possible stereoisomers, okay? So, it just shows you an example of using the 2 to the n formula, okay? Now, once again guys, I have to preface this by saying this is in old field of organic chemistry, so we're going to be learning a lot of new terminology that's specific to sugars, that it doesn't apply to any other types of molecules and one of those words, that's a new word is an Epimer, so what is an Epimer? So, an Epimer guys, is a stereoisomer of a monosaccharide that differs at only one chiral carbon. So, essentially, what an Epimer is, is it's a diastereomer. Remember that diastereomers were two molecules that were related by having some of the chiral centers changed but not all of them, right? They were non mirror image stereo isomers. So, what an Epimer is, is it's a specific type of diastereomer that differs at only one chiral carbon, so how could we look at an example? Well, let's look at one of the Epimers of glucose, if all you did was you kept this chiral center the same, you kept this chiral center the same and you kept this chiral center the same, notice that the OHÕs are all in the same place, but then you move the second OH, the C to OH from the O over to that side, that would be an Epimer and that will be an Epimer because the diastereomer that differs at only one carbon, alright? Cool guys. So, great job so far, we're just scratching the surface, there's so much more to learn from a monosaccharides but let's go ahead and do some practice problems.

Problem: Provide the generic name for the following monosaccharide.

2m

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

4m

Example: Practice: Identifying Types of Stereoisomers

4m