Classification of Carbohydrates

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Ethanoic acid, the acid in vinegar, has the formula C two H four O two. Now look closely: that is two carbon atoms, plus two molecules of water. Carbon plus water, a hydrate of carbon, so it must be a carbohydrate, right? Hold that thought; we will settle it soon. In this lesson we see what a carbohydrate really is, and the four ways to classify them. This classification comes up often in JEE, so let's begin.

0:34What is a carbohydrate?

So, what exactly is a carbohydrate? Carbohydrates are optically active polyhydroxy aldehydes or ketones. Optically active means they rotate the plane of polarised light. And polyhydroxy means many hydroxyl groups, the O H groups, along with the aldehyde or ketone group. They also include compounds that give such units on hydrolysis, that is, on breaking up with water. Okay? Now, many carbohydrates fit the formula on screen: C with the letter x written low, then H two O in brackets, with the letter y written low. The letter x counts the carbon atoms, and the letter y counts the water molecules. For example, C six H twelve O six is C six, with H two O in brackets, taken six times. That is why these compounds were called hydrates of carbon, the words in quotes on screen. Right? So here is my question for you. If a compound fits this formula, is it always a carbohydrate? And if it does not fit, can it still be one? Let us check the two cases the course gives. First, rhamnose, C six H twelve O five. Twelve hydrogens would need six oxygens to make six waters, but rhamnose has only five. So it does not fit the formula. And yet rhamnose is a carbohydrate! Clear? And now, our puzzle from the start. Ethanoic acid, C two H four O two, is C two, with H two O in brackets, taken two times. It fits the formula perfectly, yet it is not a carbohydrate, because it is not a polyhydroxy aldehyde or ketone. So the answer to our puzzle is no. Got it? So write this line down: the formula is not a test, and the structure decides. This idea comes up often in statement questions.

2:29Way 1 of 4: by hydrolysis

Now the classification. The first way is by hydrolysis. Monosaccharides cannot be hydrolysed to simpler polyhydroxy aldehydes or ketones. They are the single units, like glucose, fructose and ribose. With me? Mono means one, and oligo means a few. So tell me, what does an oligosaccharide give when we hydrolyse it? It gives a few monosaccharide units, yes, exactly, from two to ten of them. With two units, we call it a disaccharide, like sucrose, maltose and lactose. Okay? And polysaccharides, like starch, cellulose and glycogen, give a large number of units. So the test is really simple: how many units does hydrolysis give? Clear?

3:17Way 2 of 4: functional group and number of carbons

Way number two looks inside the molecule. If the carbonyl group is an aldehyde group, the sugar is an aldose. If it is a keto group, the sugar is a ketose. Okay? Then we add the number of carbon atoms, the numbers in brackets on screen. Tri for three, tetra for four, pent for five, and hex for six. So an aldose with three carbons is an aldotriose. Really simple, right? Now you try. Ribose is an aldose with five carbon atoms. So what do we call it? It is an aldopentose, very good! And glucose is an aldose with six carbons, so glucose is an aldohexose. With me? And fructose? It also has six carbons, but its carbonyl group is a keto group. So fructose is a ketohexose: the same carbons as glucose, but a different functional group.

4:11Way 3 of 4: by taste

Got it? Way number three is the easiest one, which is taste. Monosaccharides and oligosaccharides are sweet, crystalline and soluble in water. So we call them sugars. Okay? Polysaccharides, like starch and cellulose, are tasteless and amorphous, which means they do not form crystals. So they are called non-sugars. Starch is a carbohydrate, but it is not a sugar. Clear?

4:37Way 4 of 4: by reducing power

And way number four is reducing power. Sugars that reduce Tollens' reagent and Fehling's solution are called reducing sugars. Okay? Now, here comes a trap. Fructose is a ketose, so it has no aldehyde group at all. So fructose cannot be a reducing sugar, right? That is wrong, and this is where students slip. All monosaccharides, aldoses and ketoses alike, are reducing sugars. In these alkaline reagents, fructose rearranges into aldoses, as the bracket on screen says, so it reduces them too. Clear? Now the disaccharides, and here there is a condition. Such a sugar is reducing only if one of its units still has a free hemiacetal group, or a free hemiketal group. With me? So sucrose is non-reducing, while maltose and lactose are reducing. Look, sucrose is sweet, so it is a sugar, and still it is non-reducing. So never mix up taste and reducing power. Got it?

5:39Example 1: classify ribose, maltose, cellulose and more

Now Example one, an easy one to warm up. Classify each of these as a monosaccharide, a disaccharide or a polysaccharide. Ribose, maltose, cellulose, fructose, lactose, glycogen and galactose. Pause the video and try it first. Have you finished? Okay, let's check. Ribose and fructose are straight from our list of monosaccharides. Galactose is new, but it also cannot be hydrolysed further, so it is a monosaccharide too. Okay? Next, maltose gives two glucose units on hydrolysis, and lactose gives galactose plus glucose. Two units each, so both are disaccharides. Right? Now cellulose. Its name ends just like glucose and lactose. So is cellulose a simple sugar too? It is not, and the name was the trap here! Cellulose and glycogen each give many glucose units on hydrolysis, so both are polysaccharides. Never classify by the name; classify by what hydrolysis gives. Clear?

6:43Example 2: formula and molar mass of a disaccharide

Now Example two, a little harder, with numbers. Two molecules of a hexose, C six H twelve O six, join with loss of water to form a disaccharide. Give its formula and its molar mass. Pause the video and try it first. Have you got an answer? Okay, let's check. First, the hexose. Take twelve for carbon, one for hydrogen and sixteen for oxygen. Six times twelve, plus twelve times one, plus six times sixteen, gives one hundred and eighty. The unit on screen, g for grams and mol for mole, with a minus one written high, is read as grams per mole. Okay? Now two hexoses join. So the disaccharide is just two times one hundred and eighty, which is three hundred and sixty grams per mole, right? That is the trap, so be careful here! They join with loss of water, so one molecule of water, H two O, goes out, and that is the minus sign on screen. Two molecules of C six H twelve O six have twelve carbons, twenty four hydrogens and twelve oxygens. Take away two hydrogens and one oxygen, and we get C twelve H twenty two O eleven. With me? So capital M, the molar mass, is two times one hundred and eighty, minus eighteen for the water. Three hundred and sixty minus eighteen gives three hundred and forty two grams per mole. The same holds for sucrose, maltose and lactose. Nice, isn't it?

8:18Quick revision: mono, oligo, poly, aldose, reducing

So, quick revision. Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give them on hydrolysis, and the formula is not a test. Monosaccharides cannot be hydrolysed, oligosaccharides give two to ten units, and polysaccharides give many. Name a sugar as an aldose or a ketose, plus its carbon count, and remember, all monosaccharides are reducing. Now go and try the practice problems on this page. You will find them easy. All the best!