Chirality and Optical Activity

Transcript

0:00Introduction

Hello everyone! Let us start with your own two hands. Hold them up, palms facing you. They are mirror images, right? Now turn both palms down, and try to place your left hand exactly on top of your right. You cannot match them, can you? The thumbs point opposite ways. Many molecules are like your hands, and this decides whether they turn polarised light. It is called chirality, and it comes up often in JEE, so let's begin.

0:30Plane-polarised light and optical activity

So, first, the light. Ordinary light vibrates in all planes. Pass it through a Nicol prism, and what comes out vibrates in one plane only: plane polarised light. Okay? Now, some substances turn the plane of this light through an angle. They are called optically active. Right? If a substance turns the plane to the right, clockwise, it is dextrorotatory, written as a plus sign in brackets, or the small letter d, for dextro. If it turns it to the left, it is laevorotatory, written as a minus sign in brackets, or the small letter l, for laevo.

1:07Chirality: the hand test

Clear? Now, back to our hands. An object that cannot be superimposed on its mirror image, that is, placed exactly on top of it, is called chiral, like a hand. On the right is lactic acid and its mirror image, and the grey dashed line is the mirror. The solid wedge is a bond coming towards you, and the dashed bond goes behind the page. With me? Here is the rule that matters: a molecule is optically active only if it is chiral. So look at the central carbon of lactic acid. The small three written low in C H three is a subscript, for three hydrogens. Now tell me, how many different groups are attached to that carbon? Yes, exactly, four different groups! COOH, a hydrogen, O H, and C H three. Any carbon bonded to four different groups is a chiral carbon, also called an asymmetric carbon, or a stereocentre. Okay? And a molecule with just one chiral carbon is always chiral. So these two lactic acids can never be matched, just like your hands. Got it?

2:13The symmetry test

Now, a tricky question. If a molecule has two or more chiral carbons, it must be optically active, right? No, no, wait, that is not always true! Here is the symmetry test. If a molecule has a plane of symmetry or a centre of symmetry, it can be superimposed on its mirror image. Clear? Then it is achiral and optically inactive, even if it contains chiral carbons. We will meet such molecules in the lesson on two or more chiral centres. Right? And a chiral molecule has neither a plane nor a centre of symmetry. So judge the whole molecule, not one carbon. With me?

2:53Enantiomers and the racemic mixture

The two lactic acids we saw are enantiomers: stereoisomers that are non superimposable mirror images of each other. Okay? And here is the surprise: they are almost identical. Same melting point, boiling point and solubility, and the same reactivity towards achiral reagents. Got it? They differ in only two ways. They rotate plane polarised light by equal angles in opposite directions, and they behave differently towards other chiral substances. Now mix the two in equal amounts. What happens to the light? Yes, exactly, nothing happens to it! An equimolar mixture of two enantiomers is a racemic mixture, written plus minus in brackets, or small d l, and it is optically inactive. Clear? It is inactive by external compensation: the rotation by one enantiomer cancels the other. And separating a racemic mixture into its enantiomers is called resolution. Right?

3:52Specific rotation and optical purity

Now let us put a number on the rotation. Alpha in square brackets is the specific rotation. It equals alpha observed, the measured angle, over small l times c; obs, written small below alpha, means observed. Small l is the tube length in decimetres, and c is the concentration in grams per millilitre; the minus one written high is a superscript meaning per. Okay? For a mixture of enantiomers, we want the optical purity, also called the enantiomeric excess. It is the specific rotation of the mixture, over that of the pure enantiomer, times one hundred. Got it? And here is a topper's trick: the same number is simply the percentage of the major enantiomer, minus that of the minor one. We will use this in Example two. Clear? Now a common trap. The sign of rotation, plus or minus, is found only by experiment. It has no fixed relation to the labels R and S, or capital D and capital L, which describe the arrangement of groups. So never guess it from the label, okay?

4:57Example 1: which of five compounds have a chiral carbon

Alright, let us do Example one. Which of these have a chiral carbon: two chloro butane, two chloro propane, butan two ol, two methyl pentane, and three methyl hexane? Pause the video and try it first. Look for a carbon with four different groups. Done? Okay, let's check. Two chloro butane is C H three, CHC l, C H two, C H three, and each dash is a single bond. C dash two means carbon two. It carries hydrogen, chlorine, methyl, and ethyl, C two H five, so it is chiral. With me? Butan two ol has O H in place of chlorine, written in brackets as a branch on carbon two. Hydrogen, hydroxyl, methyl and ethyl: four different groups, so it is chiral too. Now, two chloro propane also has chlorine on carbon two. So it is chiral as well, right? No, careful, that is the trap! Two chloro propane is C H three, CHC l, C H three. Carbon two carries two identical methyl groups, so it is not chiral. Got it? Two methyl pentane has the same problem. The C H three in brackets is a branch on carbon two, and carbon one is another methyl on carbon two. So it is not chiral either. Clear? And in three methyl hexane, carbon three carries hydrogen, methyl, ethyl, and propyl, C three H seven. Four different groups, so it is chiral. So the answer is two chloro butane, butan two ol and three methyl hexane. With me?

6:37Example 2: optical purity and percent of each enantiomer

Now a harder one. Here, a sample of a compound has a specific rotation of plus twelve point six degrees; the small circle written high is the degree sign. The pure plus enantiomer has plus thirty degrees. Find the optical purity, and the percentage of each enantiomer. Pause the video and try it first. Done? Okay, let's check. Using our formula, twelve point six over thirty gives zero point four two, and times one hundred, we get forty two percent. Okay? So forty two percent of the sample is the plus form, and the other fifty eight percent is the minus form, right? No, no, careful, this is the trap! Optical purity is the excess of one enantiomer, not its total amount. One hundred minus forty two leaves fifty eight percent, and that part is racemic: half plus, half minus, so twenty nine percent each. Clear? So the plus form is forty two plus twenty nine, which is seventy one percent, and the minus form is twenty nine percent. Check with our topper's trick: seventy one minus twenty nine gives forty two, exactly the optical purity.

7:52Quick revision: chirality, symmetry test, optical purity

Got it? So, quick revision. First, a molecule is optically active only if it is chiral, not superimposable on its mirror image. Second, a plane or centre of symmetry makes it achiral, even with chiral carbons. And third, optical purity is the percentage of the major enantiomer minus the minor one, and the rest is racemic. Now go and try the practice problems on this page. All the best!