Disaccharides

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take sucrose, which is cane sugar, maltose, which is malt sugar, and lactose, the milk sugar. All three have exactly the same molecular formula, and the same molar mass. Yet maltose and lactose reduce Fehling's solution, and sucrose does not. Same formula, different behaviour. Strange, isn't it? Today we find out why. Disaccharides come up often in JEE chemistry, so let's begin.

0:33The glycosidic linkage

So, let us first see what a disaccharide is. It is two monosaccharide units, joined through an oxygen atom, the capital O on screen. In COC on screen, the short dashes are bonds: a carbon of one unit, the oxygen bridge, then a carbon of the other unit. In the figure, each ring is one sugar unit, and the O between two rings is that bridge. Okay? Now, let us see how this bridge forms. An O H group of one unit reacts with an O H group of the other, and only one oxygen stays in the bridge. So tell me, which small molecule must leave? Yes, exactly, a molecule of water leaves. That is H two O on screen, and the small lowered two means two hydrogen atoms. On the first unit, it is the hemiacetal O H that meets an O H group of the other unit. This bond is called the glycosidic linkage. Clear? Now, this glycosidic linkage is an acetal linkage. Dilute acid, or an enzyme, breaks it with water, and we get the two monosaccharides back. That is hydrolysis. With me?

1:40Three disaccharides, one formula

Now meet the three disaccharides in the figure: maltose, lactose and sucrose. All three have the formula C twelve, H twenty two, O eleven; the small lowered numbers count the atoms. Okay? Now, glucose has a molar mass of one hundred and eighty, from the last chapter, and fructose and galactose are its isomers, so they have the same. Two such units would make three hundred and sixty. So, is three hundred and sixty the molar mass of a disaccharide? Careful here, because the answer is no. Remember the water that left when the bridge formed. Two times one hundred and eighty, minus eighteen for the water, gives three hundred and forty two. The cross means times, and in grams per mole, the small raised minus one means per. Got it? Sucrose, with its common name, cane sugar, in brackets, is made of alpha glucose and beta fructose. Alpha and beta are the Greek letters on screen; they tell us which anomer of each unit is used. The capital D in the figure gives the configuration, as in the last chapter. Right? Maltose, or malt sugar, is simply two alpha glucose units joined together. With me? And lactose, the milk sugar, is beta galactose and beta glucose. Watch out here, because its first unit is galactose, not glucose. Clear?

3:03Which carbons are joined?

Now let us see which carbons the bridge joins. C dash one on screen is short for carbon number one. In sucrose, carbon one of glucose is joined to carbon two of fructose. In the figure, look at the small numbers one and two beside the bridge. Okay? In maltose, carbon one of one glucose unit is joined to carbon four of the other, the small one and four in the figure. We call it a one-four linkage, and in maltose it is an alpha linkage. Right? Now look at lactose in the figure. It is also linked from carbon one to carbon four. So tell me, how is lactose different from maltose? Yes, exactly, there are two differences. Its carbon one comes from galactose, the unit named in brackets, and its one-four linkage is beta, not alpha. Students often mix these up, so note it down. Got it? And each one is hydrolysed by its own enzyme: invertase for sucrose, maltase for maltose, and lactase for lactose. Clear?

4:04Reducing or non-reducing?

Now, back to our puzzle. The rule from the last chapter is this: a disaccharide is reducing only if one of its units still keeps a free hemiacetal group, okay? Now here is the trap. Sucrose is made of glucose and fructose, and every monosaccharide is a reducing sugar. So sucrose must be a reducing sugar too, right? Careful, because sucrose is not reducing at all. Its linkage joins carbon one of glucose to carbon two of fructose, and those are exactly the reducing groups of the two units. Both are used up in the bridge. With me? So no free reducing group is left, and sucrose is non-reducing. Mutarotation, where the alpha and beta forms interconvert in solution, also needs a free hemiacetal group, so sucrose shows none. That is half of our puzzle solved. Clear? Now maltose and lactose. Their bridge uses carbon one of the first unit, but carbon one of the second unit, a glucose unit, stays free as a hemiacetal. The brackets on screen name that second unit. Got it? That free hemiacetal can open to an aldehyde, the CHO group on screen, and the aldehyde reduces Fehling's solution. So maltose and lactose are reducing sugars, and both show mutarotation. And now our puzzle is fully solved. Okay?

5:27Example 1: glucose from hydrolysis of sucrose and maltose

Time for the first example. Find the mass of glucose obtained by the complete hydrolysis of thirty four point two grams of sucrose, and of the same mass of maltose. Use the molar mass we just found. Pause the video and try it first. Done? Okay, let's check, with the moles first. The sign on screen is the division sign, so thirty four point two grams over three hundred and forty two grams per mole gives zero point one mole. It is the same for both sugars, okay? Now take sucrose. One molecule of a disaccharide gives two units, so zero point one mole of sucrose should give zero point two mole of glucose, right? That is exactly the trap, so be careful. Look at the arrow, which means gives: sucrose gives glucose plus fructose. Only one of its two units is glucose, so we get zero point one mole of glucose. Zero point one times one hundred and eighty gives eighteen grams. With me? Now take maltose. Both of its units are glucose, so zero point one mole of maltose gives zero point two mole of glucose. Zero point two times one hundred and eighty gives thirty six grams. Same mass of sugar, but twice the glucose. That's it!

6:43Example 2: maltose with bromine water, then hydrolysis

Now let us try a harder one. Maltose is oxidised with bromine water, and the product is then hydrolysed. What are the final products? Pause the video and try it first. Done? Okay, let's check. Bromine water oxidises glucose to gluconic acid, remember? And maltose is just two glucose units. So we get two molecules of gluconic acid, right? That is exactly the trap here, so wait. Bromine water oxidises only the unit with the free hemiacetal group, the one that can open to an aldehyde. In maltose, that is just one glucose unit. Okay? Now look at the other glucose unit. Its carbon one is held in the glycosidic linkage, which is an acetal, so it cannot open, and it is not oxidised. Right? Now we hydrolyse the product. The linkage breaks, the oxidised unit comes off as gluconic acid, and the other as glucose. So the final products are glucose and gluconic acid, one of each, in equal amounts. Clear?

7:47Quick revision: linkages, reducing sugars, mutarotation

So, quick revision. The glycosidic linkage joins two units through an oxygen atom, with loss of a water molecule. Sucrose is linked carbon one to carbon two; maltose, alpha, and lactose, beta, are linked carbon one to carbon four. And a free hemiacetal group makes a disaccharide reducing and lets it show mutarotation; sucrose has none. Now go and try the practice problems on this page. All the best!