Question

Why do carbon centres with 4 different substituents generate a pair of optical isomers?

Answer 1

Suppose you have an identical twin (and you may have one for all I know!)..............

Explanation:

So you take your right hand, and your identical twin's right hand, and you can lay one on top of the other (interestingly, the twins would have different sets of fingerprints, but this is by the by). You shake hands with your clasped right hands. This tells us that your right hand could potentially occupy the same space as your twin's right hand; they could be `"superimposed"`. So what?

Now take your left hand, and attempt to lay it on your twin's right hand. Can you do it; can you even shake hands with left and right hands? The answer is NO, for the reason that hands are handed. While they are structurally similar, they have a chiral component to their structure, their symmetry, and we may extend this idea all the way down to the molecular level.

So get a model set, and take a tetrahedron, and cap the tetrahedron with FOUR different groups. (You could do this with plasticene and toothpicks if you have no access to models; all you need are blobs of plasticene with 4 different colours). So you have made a representation of a chiral carbon, `CR_1R_2R_2R_4`; it does not matter what the `R` groups are; it matters that they are different.

So I have given you an example. And believe me, try as you might, you will not be able to get the isomer on the LHS (which in fact is the left-handed isomer with respect to carbon) to occupy the same space as the right-handed isomer, the isomer on the RHS. Exchange ANY two groups on either carbon, and you get the mirror image, and thus this isomer is the mirror image of a mirror image, and thus will be IDENTICAL to the initial so-called optical antipode.

As another example of optical isomerism, consider a pair of shoes. These are structurally identical. And yet can you fit your right hoof into your left shoe, and your left hoof into your right shoe? Perhaps you can, but I am sure that you will find it uncomfortable, because your feet also have a chiral component.

There should be many answers here that deal with these problems. Here is a start. It is a good idea to use model sets; they are always allowed in examinations. Of course, you have to learn how to use them, and learn how to represent a three dimensional structure on the printed page.