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Dehydration synthesis or a condensation reaction

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Video transcript

- [Voiceover] In the previous video, we talked about the importance of glucose as a simple sugar. We talked about its molecular structure. What I wanna do in this video is study how glucose can be, how we can use it as a building block for more complex sugars and more complex carbohydrates. So this right over here, I've copy and pasted two glucose molecules, we can number their carbons. This is one, two, three, four, five, six. One, two, three, four, five, six. We have them in their cyclic form. And what we're going to do is explore what would happen if this oxygen right over here, I'll highlight it in this magenta color, were to use one of its lone pairs, one of its lone pairs, to do, what's in organic chemistry, referred to as a nucleophilic attack on the number one carbon on the left-hand glucose molecule. And the reason why that could happen is this number one carbon right over here, it's attached to two oxygens. Oxygens are very electronegative, they like to hog electrons when they're in a covalent bond. So that's gonna give this carbon a partially positive charge. And this oxygen is very electronegative. It's gonna hog the electrons from this hydrogen and the number four carbon on the right-hand glucose molecule, so it's gonna have a partially negative charge. And so it is going to be nucleophilic. It's going to be attracted to, I guess you could say, the carbon nucleus, to the partially positive charge right over here, and so as it does, it's gonna use a lone pair to form a bond. It's gonna share it with the carbon, and then the carbon can let go of another bond. So it could let go of, it could let go of both of these electrons in that bond. Now you could say maybe that just goes back to the oxygen and it forms a hydroxide anion. Or we could imagine, well, maybe it'll be used, maybe it forms a hydroxide anion first, or maybe that bond immediately goes and picks up a hydrogen ion out of the solution from another, from a hydronium ion sitting someplace. So this could be used to form a bond with this hydrogen ion, which is really, this is just a proton here. You take an electron away from hydrogen, it's just going to be a proton. Well, what's that going to do? Well, that's going to link these two glucose molecules. And it's gonna link it just like this, and it's important to keep track of our molecules here. So this oxygen is now going to be this oxygen. It's now going to be that oxygen. This bond between the number four carbon on the right-hand side of that oxygen is this bond right over here. This, where we took this electron pair to form this bond with the number one carbon, that is, let me do it in that magenta color. That is this bond, this bond right over here. The oxygen, this oxygen, is now this oxygen right over here. And this electron pair is now formed a bond with this hydrogen, so we could say, oh, that could be, let me do that blue, that could be, that could be this bond right over here. Now the one difference is, based on how I've drawn it, this oxygen, or sorry, this oxygen, the way I've drawn it, it's attached to the number one carbon here, the number four carbon here. We have that over, we've already done that over here. Number one carbon on the left molecule, number four carbon on the right molecule. But we also have it bonded, we also have it bonded to a hydrogen. So just the way I've done it right now, it's still bonded to a hydrogen. It's going to have a net positive charge. Over here, it was neutral. It was neutral right over here, but then it's now sharing its electrons. It's now sharing both of those electrons in a covalent bond, and so you can think of it as it's giving away an electron to this carbon, so it's going to have a net positive charge. But then to get back to neutral, you could imagine, well, maybe some type of a water molecule could grab that ion, so maybe this one right over here. This one right over here could grab that hydrogen, and then these electrons, both of them, and it would just grab the hydrogen nucleus of the proton, and so these two electrons could go back to this oxygen and then this oxygen would become neutral. And so what we would be left with, actually, let me just erase this, is that this hydrogen would now be attached to this oxygen, and we would have a hydronium ion. And this is reasonable. We essentially had some hydronium. We had a hydrogen proton out here before and we still do. Now it's attached to a water, so we've take a proton and we've given back a proton, so we have a net-net kind of added charge or taken charge away from the system. But the important thing that we just saw is as these two things essentially attached, we lost a water molecule, or I guess net-net, this system lost a water molecule. It took up a charge to do it, to build that water molecule, but the thing that really kind of escaped from both of these two molecules is this, is this right over here. This H is this H, this oxygen is this oxygen. And this hydrogen is this hydrogen right over here. And so this type of a reaction in which we're synthesizing a more complex molecule, a longer chain of glucose molecules, this is called a dehydration synthesis. So what we just did, this right over here is called a dehydration synthesis. Why are we calling it a dehydration synthesis? Well, we've just taken a water out. If you imagine losing water, we talk about you're getting dehydrated. And why synthesis? Well, we put two things together. We synthesized a larger molecule. Sometimes this would be called a condensation reaction. Condensation reaction. And by doing this, these two glucose molecules are able to form a disaccharide now. So each individually, they were monosaccharides, so this one on the right, that's a monosaccharide. What does monosaccharide mean? Well, it means, mono means single or one and saccharide comes from the Greek word for sugar. The Greek word for sugar is, I'm gonna mispronounce it, is sakcharon. When people talk about something being saccharine, they're saying something is very, very sweet. The Greek word for sugar is sakcharon. So saccharide means it's a sugar, it's a single sugar. So that meaning there, sugar. And the general term saccharide refers to not just the simple sugars, monosaccharides, but it could mean two of these things put together. And there's other simple sugars, fructose and others. Or it could mean a huge number of these put together. You could have polysaccharides. And that whole class, saccharides, we also associate with carbohydrates. Now we went from two monosaccharides to right over here. This is a disaccharide. This is a disaccharide, we have two. Two monosaccharides were involved. This is a disaccharide, and this particular disaccharide is maltose, or malt sugar. Maltose. So the whole point of this video is to see how you can start with these simple sugars, these monosaccharides, and form disaccharides. In fact, you could keep going. You could keep having dehydration synthesis, condensation reactions to keep adding more and more monosaccharides to build longer and longer chains. So if you were to keep doing that, it you were to keep building chains of these things, now you're getting into the world of polysaccharides. Polysaccharides, or many simple sugars, many monosaccharides, many monosaccharides put together. And this is the case for sugar, but this is something that you'll see often in chemistry, where you have a single unit, here's a single sugar, but if we talk in more general terms, we would call it a monomer. And then if we have a bunch of these monomers put together, we would call it a polymer. Now polysaccharides are super important, and you have probably eaten some polysaccharides today, and you probably have some, in fact, I'm sure you have some polysaccharides stored in your cells right now. If you put a bunch of glucose molecules, if we were to keep this process going and we were to have a bunch of glucose molecules together, when you find it in plants, it'll often be in the form of a starch. So a polysaccharide that you'll find in a plant is a starch, a bunch of glucoses put together in your own cells to have a immediate energy store, a bunch of glucoses put together is glycogen. So these macromolecules, these polysaccharides that are made up of a bunch of simple sugars, a bunch of monosaccharides put together, these are very common in biology. You have eaten them and you are storing them in your body right now.
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