Sucrose and Invert Sugar

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. Take a solution of ordinary sugar, sucrose, the sugar in your kitchen. Shine plane-polarised light through it, and it turns the light to the right. Now add a little acid, wait for some time, and test it again. This time, it turns the light to the left! Nothing was taken out, yet the direction flipped. How is that possible? The answer also explains honey, and a trap that comes up often in JEE questions. So, let's begin.

0:32Sucrose: glucose and fructose, joined

So, on screen is the figure from the last lesson. The top two are maltose and lactose, and maltose comes back later in a trap. Today, look at the bottom one, sucrose, which is cane sugar. Its formula is C twelve H twenty two O eleven; the small lowered numbers count the atoms. Its molar mass is three hundred and forty two, okay? Now look at the structure. On the left, a six-membered ring, alpha D-glucose. On the right, a five-membered ring, beta D-fructose. The Greek letters alpha and beta tell us which anomer each unit is. Right? The two rings are joined through one oxygen, from C one of glucose to C two of fructose. Both reducing groups are used up in this link, so sucrose is a non-reducing sugar. Clear?

1:20Inversion: the sign of rotation flips

Now, what happens when we break that link? Sucrose plus water, with H plus over the arrow, which means a little acid, or the enzyme invertase. The products are glucose plus fructose, one molecule of each, an equimolar mixture. Okay? Sucrose has a specific rotation of plus sixty six point five degrees. The alpha in square brackets is the symbol for specific rotation, and the small raised circle means degrees. The plus sign means it turns the light to the right, so sucrose is dextrorotatory. With me? Now the products. D-glucose carries a plus sign in brackets, plus fifty two point five degrees. D-fructose carries a minus sign in brackets, minus ninety two point four degrees. Remember, the letter D gives the configuration, and the sign in brackets gives the rotation; the two are independent. Got it? So here is my question. One product turns the light to the right, the other to the left. Which way will the mixture turn it? Yes, exactly, it turns the light to the left! Fructose turns the light more strongly, ninety two point four against fifty two point five. So fructose wins, and the mixture is laevorotatory. Right? So the sign has flipped, from plus to minus. That is why this hydrolysis is called inversion, and the mixture is called invert sugar. And that solves our puzzle: the sugar split into two parts, and the left-turning part is stronger. Clear?

2:47Specific rotation of invert sugar

Now, what is the specific rotation of invert sugar itself? Glucose and fructose are both C six H twelve O six, with molar mass one hundred and eighty. So one mole of each means equal masses. Okay? So, should we simply add the two rotations, or do something else? Yes, exactly, we take the mean, because each sugar is only half of the mass. Plus fifty two point five and minus ninety two point four add up to minus thirty nine point nine. Then we divide by two, which gives about minus twenty degrees; the wavy equals sign means approximately. If you wrote minus thirty nine point nine, that was the trap. Clear?

3:29Invert sugar: honey, sweetness and tests

Now, invert sugar is not just a laboratory mixture. You meet it in honey, because honey is largely invert sugar, and invert sugar is sweeter than sucrose. That is worth remembering, right? The order of sweetness comes up often. Fructose first, then sucrose, then glucose, then lactose. The greater-than sign here simply means sweeter than. Okay? Remember, sucrose itself is a non-reducing sugar. So tell me, does invert sugar reduce Fehling's solution? Yes, it does, because invert sugar is a mixture of two free monosaccharides. So it reduces Fehling's solution, and it also shows mutarotation, while sucrose does neither. With me? Next, bromine water oxidises aldoses only. So the glucose changes into gluconic acid, which is what the arrow means, and the fructose, a ketose, stays unchanged. Got it?

4:21Trap: is every fall in rotation an inversion?

Now, a trap that comes up often. Maltose has a specific rotation of plus one hundred and forty degrees. On hydrolysis, its rotation falls to that of glucose, plus fifty two point five degrees. So, is this an inversion too? No, no, careful, it is not an inversion. The rotation drops, but it stays positive. Inversion means the sign itself flips, from plus to minus. Okay? In fact, of the three common disaccharides, sucrose, maltose and lactose, only sucrose gives a laevorotatory product mixture. So always check the sign. Clear?

4:56Example 1: invert sugar and water from 68.4 g of sucrose

Now, Example one. Read it with me. Sixty eight point four grams of sucrose is completely inverted; the small g after the number means grams. Find the mass of invert sugar formed, and the mass of water used. Pause the video and try it first. Are you done with it? Okay, let's check. Moles of sucrose: sixty eight point four over three hundred and forty two, the molar mass, which gives zero point two; the short form mol means moles. Now tell me, one sucrose molecule gives how many sugar molecules? It gives two, exactly, one glucose and one fructose. So zero point two moles of sucrose gives zero point four moles of C six H twelve O six in total. Right? Each has molar mass one hundred and eighty, and the cross means times. Zero point four times one hundred and eighty gives seventy two grams of invert sugar. If you used zero point two moles here, you got thirty six grams, only half the answer, so be careful with that step. And the water? One water molecule for each sucrose, so zero point two times eighteen gives three point six grams. Now check: sixty eight point four plus three point six gives seventy two. The mass balances perfectly!

6:14Example 2: fraction hydrolysed when rotation becomes zero

Now a harder one. Read it with me. We have a sucrose solution that is being hydrolysed. Treat the rotation as the weighted mean of plus sixty six point five degrees for the sucrose left, and minus twenty degrees for the part inverted, and neglect the small gain in mass. Find the fraction hydrolysed when the solution shows no rotation. Pause the video and try it first. Are you done with it? Okay, let's check. We can follow this reaction with a polarimeter, the instrument that measures rotation. The reading falls steadily from plus sixty six point five, through zero, to minus twenty. So at zero, half the sucrose is gone, right? Let us not guess; let us calculate it. Let the letter x stand for the fraction inverted. The sucrose left is one minus x, and it gives sixty six point five times one minus x. The inverted part gives minus twenty times x. And the total is zero. Okay? Open the bracket and collect the terms: sixty six point five equals eighty six point five times x; the double arrow means which gives. So the fraction x equals sixty six point five over eighty six point five, which is about zero point seven seven. Right? So about seventy seven percent is hydrolysed, not fifty percent! That is because sucrose turns the light more strongly, sixty six point five against just twenty, so more of it must go before the two cancel. Okay?

7:41Quick revision: inversion, rotation signs and invert sugar

So, quick revision. Sucrose, plus sixty six point five, splits into glucose, plus fifty two point five, and fructose, minus ninety two point four. Fructose wins, so invert sugar is laevorotatory, about minus twenty degrees. And inversion means the sign flips, not just a fall in rotation. Now try the practice problems on this page. All the best!