AIEEE Questions: Di- and Polysaccharides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Cane sugar, sucrose, is sweet, and it dissolves easily in water. The cotton in your shirt is mostly cellulose, and it has no taste at all. Yet chemists call both of them carbohydrates. So, can one single test in the lab pick out both of them? Keep that question in mind, because our first question today answers it. We will solve two short A I triple E questions on disaccharides and polysaccharides: one on a test for sugars, and one on what cotton is really made of. Questions like these come up often in JEE Main, so let's begin.

0:42Tests for carbohydrates: who gives which?

So, let us first meet the tests, starting with Molisch's test. We add alpha naphthol, with the Greek letter alpha in front, and then pour concentrated sulphuric acid down the side of the test tube. The short form before the acid means concentrated, and in H two S O four, the small lowered numbers count the atoms. Okay? The result is a violet ring. And this test is given by all carbohydrates: monosaccharides, disaccharides and polysaccharides, every one of them. Remember this line, because it is the key to our first question. Clear? Now compare two tests you already know. Tollens' reagent gives a silver mirror, and Fehling's solution gives red C u two O; C u is copper, and the small lowered two means two copper atoms. But these two tests are given only by reducing sugars. Right? Now, sucrose and starch are both non-reducing, so neither of them gives the red precipitate with Fehling's solution. So tell me, do sucrose and starch fail Molisch's test as well? They do not, and that is exactly the trap. Molisch's test is not a test for reducing sugars. It is given by all carbohydrates, so sucrose and starch both give the violet ring. With me? And here are two more tests to note. Seliwanoff's test gives a red colour with ketoses, like fructose, and iodine turns starch blue. Each of these picks out only some carbohydrates, while Molisch's test picks out all of them. Got it?

2:16Polysaccharides: long chains of D-glucose

Now, the polysaccharides. In the figure, each little hexagon is one glucose unit. The top chain is amylose and the branched one is amylopectin, the two parts of starch; glycogen is like amylopectin, only more branched; and the orange chain at the bottom is cellulose. All of them are polymers of D-glucose, and we will read that capital D in a moment. Okay? Look at the formula on screen: C six H ten O five in brackets, with a small lowered n outside, meaning n units, a very large number. Glucose itself is C six H twelve O six, so each unit has lost one H two O, a water molecule, in its glycosidic linkage. Hydrolysis simply adds that water back. Right? In starch, the units are alpha glucose, with the Greek letter alpha, and glycogen, the animal starch, is built in the same way. Clear? But cellulose is different. Its units are beta D-glucose, with the Greek letter beta, in straight chains. C dash one means carbon number one, so carbon one of each unit is joined to carbon four of the next. With me? Now think about this. Starch and cellulose both give only glucose on hydrolysis, and we digest starch easily. So why can our body not digest cellulose? Yes, exactly, it is the linkage. Humans have no enzyme to hydrolyse the beta linkage of cellulose. So the same glucose, linked a different way, simply passes through us. Got it?

3:50D or L? Reading a sugar's name

Now, one more idea before the questions: what that capital D means. The figure shows four Fischer projections. Each cross on the vertical line is an asymmetric carbon, with CHO at the top of the aldoses and C H two O H at the bottom. Find C dash five, carbon five, which is the lowest asymmetric carbon in a hexose. If its O H group is on the right, the sugar is D, and if it is on the left, the sugar is L, okay? Now look at the names under the figure. The plus or minus sign in brackets tells us the rotation, dextrorotatory or laevorotatory, and it has nothing to do with D or L at all. So fructose, the last one, is a D sugar that is laevorotatory. Students often mix these two labels up, so watch out. Clear? And most natural sugars belong to the D-series, glucose among them. So the glucose units in starch and cellulose are all D-glucose. Right? So then, what is L-glucose? It is the mirror image of D-glucose, the second projection in the figure, with every asymmetric carbon reversed. Compare it with the first one, and you will see every O H group has moved to the other side. Okay?

5:03Example 1: what does Molisch test detect? All carbohydrates

Time for Example one, from A I triple E two thousand and twelve. Which can be detected by Molisch's test: nitro compounds, sugars, amines, or primary alcohols? Pause the video and try it first. Done? Okay, let's check. Molisch's test gives a violet ring, and as we saw, it is given by all carbohydrates. Okay? Now here is where students slip. Sugars are full of O H groups, and a primary alcohol has an O H group too. So, should a primary alcohol also give the violet ring? It should not, and that is exactly the trap. Remember the definition: carbohydrates are polyhydroxy aldehydes or ketones, which means many O H groups together with an aldehyde or keto group, or compounds that give such units on hydrolysis. Right? So a simple primary alcohol is not a carbohydrate, and neither are amines or nitro compounds. None of them gives Molisch's test. Clear? So the answer is sugars. And it works whether the sugar is reducing or not. Now remember our puzzle? Cane sugar and cotton are both carbohydrates, so this one test picks out both of them, and our puzzle is solved.

6:21Example 2: complete hydrolysis of cellulose gives D-glucose

Now Example two, from A I triple E two thousand and three. What does complete hydrolysis of cellulose give: D-ribose, D-glucose, L-glucose, or D-fructose? Pause the video and try it first. Done? Okay, let's check. Cellulose, the main material of cotton, is a polymer of beta D-glucose units, as we saw. Okay? Complete hydrolysis means water is added back at every glycosidic linkage. So every linkage breaks, and only single units are left. Right? Now let us remove the wrong options one by one. D-ribose is an aldopentose, a sugar with an aldehyde group and only five carbons. Every glucose unit has six carbons, so ribose cannot come out of cellulose. With me? D-fructose is a ketohexose, six carbons with a keto group. In the last projection of the figure, it is the C equals O group on carbon two, with two short lines for the double bond. Cellulose has no fructose units at all, so that option goes too. Clear? Now two options are left, and both of them say glucose. Glucose is glucose, so many students feel either one is fine. So tell me, does that capital letter really matter here? It matters completely, and that is the trap. L-glucose is the mirror image of D-glucose, and cellulose, like natural glucose everywhere, is built only from D-glucose units. Got it? So the answer is D-glucose. Cotton, broken down completely, gives D-glucose, the same unit that builds starch, only linked the beta way. Clear?

8:05Quick revision: Molisch test, starch vs cellulose, D-glucose

So, quick revision. First, Molisch's test gives a violet ring with all carbohydrates, reducing or not. Second, starch and glycogen, with alpha linkages, and cellulose, with beta linkages, are all polymers of D-glucose. And third, the complete hydrolysis of cellulose gives D-glucose, not L-glucose. Now go and try the practice problems in this chapter. All the best!