Have you ever tried to put your right shoe on your left foot? Or your left shoe on your right foot? If you can do either, the result will be very uncomfortable! But the width, length and height of both shoes are identical. And the outer, inner and upper curvatures are all the same. So why is there a problem putting a shoe on the wrong foot?
Look at the reflection of your right shoe in a mirror. The reflection looks like your left shoe – it is not like your right shoe. Your shoes are objects that are not identical to their mirror images. Objects that are not identical to their mirror images are chiral.
Look at the cup in the picture above – it is identical to its mirror image. An object that is identical to its mirror image is achiral.
Another way of saying the same thing is to say that you cannot rotate a chiral object to be identical to its mirror image. But you can rotate an achiral object to be identical to its mirror image.
Remember that an atomic orbital can accommodate two electrons provided they spin in opposite directions (post 16.29). In the picture above, a ball has a right-handed spin around the green axis. It doesn’t matter how we rotate the axis, the direction of spin doesn’t change – it’s always right-handed (post 17.10). But if we look at the spinning ball in a mirror, the reflection will be spinning to the left. Spin of a particle, like an electron (or any other fermion, post 20.26), exhibits chirality.
There are other examples of chirality in science but, in the rest of this post, I am going to concentrate on one – chiral molecules. So, it might be helpful to read post 16.30.
The picture above shows the formula of a molecule of alanine. This simply shows, in two dimensions, the atoms that bond to each other. It doesn’t imply anything about the three-dimensional structure of the molecule. (So, if you know about Fischer projections – this isn’t one!) In three dimensions, the four bonds formed by the central carbon atom (indicated by a green arrow in the pictures above and below) point towards the corners of a tetrahedron (post 16.30). So the three-dimensional structure of alanine looks like the molecule below. Here a black ball represents a carbon atom, a red ball oxygen, a blue ball nitrogen and a white ball hydrogen.
But the picture below shows that this molecule is not identical to its mirror image. The two chiral forms of alanine are called D alanine and L alanine. The two forms of alanine are called optical isomers or enantiomers.
You may find it difficult to see that the D and L forms are different. So look at the first three-dimensional picture of the alanine molecule; the carbon atom indicated by the green arrow is bonded to three different groups. In the picture below, they are labelled as the purple, pink, brown and blue groups. In both the D and L forms, the pink group points towards you and the purple away. In the D form, the brown group is to your left and the blue group to your right. But, in the L form, the brown group is to your right and the blue group is too your right. So, the D and L forms of alanine are not identical.
You might find it simpler to make some models, instead of looking at pictures. In the picture above, I have copied the simple flat picture of the alanine molecule and outlines the purple, pink, brown and blue groups.
In the picture above, I have drawn two pictures on thin card. All the small triangles in these pictures have sides that are the same length.
In the picture above, I have cut them out. Then I folded them along the lines, put glue on the flaps that I then folded into the inside of the resulting regular tetrahedra. The results are shown below.
If you make these models, you will find that there is no way to rotate them so that the four different colours appear in the same relative positions, Therefore, the tetrahedra, and the alanine molecules that they represent, are chiral.
What do the letters D and L represent? We can think of them as arbitrary labels. But they do convey some meaning, that is not very important unless you want to synthesise a chiral molecule from another chiral molecule. Here is a warning – you will sometimes see optical isomers labelled as d or l forms. The d form may not be the same as the D form and the l form may not be the same as the L form – so be careful!
Is the existence of D and L forms important? Vitamin C is the L form of ascorbic acid. It is involved in cross-linking (post 20.8) the polymer molecules (post 20.7) that help hold our bodies together. It may also help to guard against some infections. But the D form of ascorbic acids doesn’t do anything for us. Both natural and synthetic vitamin C consist of L ascorbic acid.
The picture above shows L ascorbic acid – vitamin C. See if you can convince yourself that ascorbic acid is a chiral molecule!
20.29 Properties of optical isomers