Optical Isomerism and Total Stereoisomers

Transcript

0:00Introduction

Hello everyone! Let me start with your own two hands. Same thumb, same fingers, in the same order, right? Your left hand is the mirror image of your right hand. But try putting a left glove on your right hand. It never fits, does it? Now here is a puzzle for you. From geometrical isomerism, you know that a cobalt complex with two e n ligands and two chlorides has two forms, cis and trans. Yet it has three stereoisomers, not two. So where does the third one come from? Today we study optical isomerism, and how to count all the stereoisomers of a complex. In JEE, counting questions like this come up often. So, let's begin.

0:47Chiral: not the same as its mirror image

So, what exactly happened with your hands? Your left hand cannot be placed exactly on its mirror image. We say it cannot be superimposed on its mirror image, and such an object is called chiral. Okay? Now let me ask you this. When can a complex match its mirror image exactly? What must it have? Symmetry, yes, exactly! If a mirror plane cuts the complex into two matching halves, the complex and its mirror image are the same. So a chiral complex has no plane of symmetry and no centre of symmetry, or, in general, no improper axis, the words in round brackets. In most questions, you simply look for a mirror plane. Clear? Such a chiral complex is optically active, which gives this topic its name. The complex and its mirror image are two different isomers, called enantiomers, labelled with the small letters d and l. So one chiral form always means a pair. Right?

1:46Square-planar and tetrahedral complexes

Now, let's apply this. Take a square-planar complex. The metal and all four ligands lie in one flat plane. So tell me, can such a complex be chiral? Almost never, and the reason is beautiful. The plane of the molecule is itself a mirror plane. Reflect the complex in that plane, and you get the same complex back. With me? So square-planar complexes are almost never chiral. Look, this is a common trap. Square-planar complexes can have cis and trans forms, but do not go looking for optical isomers there. Okay? Now take a tetrahedral complex, MABCD, a metal M with four different ligands, okay? That arrangement is chiral, but such complexes are rarely isolated. So the chiral complexes you will meet are mostly octahedral. Clear?

2:39Chiral octahedral complexes to know

So, octahedral complexes. First, two short names. Here, A A means a bidentate ligand with two identical donor atoms, like e n, ethylenediamine, or o x, oxalate. And A B means two different donor atoms, like g l y, glycinate, which binds through a nitrogen and an oxygen. Got it? Now the first chiral type: M, then A A in round brackets, all in square brackets. The small three written below means three A A ligands, so three chelate rings around the metal, like the blades of a propeller. It twists either to the left or to the right, and the two twists are mirror images, a pair of enantiomers. Okay? Next, cis, M, A A twice, B two, in square brackets. Two chelate rings, and the two B ligands side by side. That is what cis means. This one is chiral too. Right? Then M, A two, B two, C two, three pairs of monodentate ligands. Only its all-cis form, with every pair side by side, is chiral. With me? And the last type: M, A B three times. In fac, the three donor atoms marked A all sit on one face of the octahedron; in mer, they run round a meridian. So which of the two is chiral? Both of them are chiral, yes! Many students guess only one. But fac and mer M, A B three, are both chiral, so each gives a pair. Clear?

4:04Counting all the stereoisomers

Now the counting rule. Total stereoisomers equals the number of geometrical isomers, plus one extra for each chiral geometrical isomer. Okay? Let's try it on M, A two, B two, C two. Five geometrical isomers, and only the all-cis one is chiral. The chiral one is a pair, two isomers, so five plus two gives seven, right? No, no, wait, that is the trap! The all-cis isomer is already one of the five, and its mirror image adds only one more. So five plus one gives six stereoisomers. That is why the rule says one extra, not two. Got it?

4:41Example 1: stereoisomers of Co(en)2Cl2+, cis and trans

Now, remember our puzzle from the start? Let's solve it. How many stereoisomers does this complex have? Open square bracket, C o, then e n in round brackets, two times, then C l two, close bracket, with a plus charge written up high. Pause the video and try it first. Done? Okay, let's check. Here, e n has two identical donor atoms, so this is M, A A twice, B two. Two geometrical isomers: cis, with the two chlorides side by side, and trans, with them opposite. Right? Now check each one for a mirror plane. Start with trans. Is it chiral? No, it is not. In trans, the cobalt and the four nitrogen atoms of the two e n ligands lie in one plane, with one chloride above and one below. That plane is a mirror plane, so trans is achiral, just one isomer. Okay? And what about cis? We just saw that cis, M, A A twice, B two, is chiral, so it comes as a pair of enantiomers, two isomers. With me? So the total is one plus two, which gives three stereoisomers. Or by our rule: two geometrical isomers, plus one extra for the chiral cis, gives three. Puzzle solved: the third form is the mirror image of cis, just like your two hands. Clear?

6:11Example 2: stereoisomers of Co(en)(NH3)2Cl2+

Alright, now a harder one. How many stereoisomers does this complex have? Open square bracket, C o, then e n in round brackets, then N H three in round brackets, two times, then C l two, close bracket, with a plus charge. Pause the video and try it first. Done? Okay, let's check. One e n, two ammonias and two chlorides, so this is M, A A, B two, C two. Remember, e n can only span two cis positions, so fix it there. Right? Now the ammonias and the chlorides fill the other four positions, and only one pair of those is opposite each other. So which ligands can sit in that opposite pair? List the cases. Case one: the two ammonias trans to each other, one above and one below. Then the e n, the cobalt and both chlorides lie in one plane, a mirror plane. So this one is achiral. Okay? Case two: swap them, so the two chlorides are trans to each other. Same argument, a mirror plane, so achiral. Got it? Now case three, the tricky one. Neither pair takes the opposite positions, so both pairs are cis. Does this one have a mirror plane? No, it does not, very good! With both pairs cis, there is no mirror plane, so this isomer is chiral, a pair of enantiomers. With me? So, three geometrical isomers, and one of them is chiral. Three plus one extra gives four stereoisomers. If you wrote three, you forgot the mirror image. Always ask the chirality question at the end. Clear?

7:56Quick revision: chirality, chiral types, counting rule

So, quick revision. Chiral means no mirror plane and no centre of symmetry. Square-planar complexes are almost never chiral. The chiral octahedral types: M, A A three, cis M, A A two, B two, the all-cis M, A two, B two, C two, and both forms of M, A B three. And total stereoisomers equals the geometrical isomers, plus one for each chiral one. Now go and try the practice problem on this page. All the best!