Introduction

Stereochemistry is the study of 3D structures of molecules. How are the atoms of a molecule arranged in space? This may seem boring, but it is literally a matter of life and death. In the late 1950’s, thalidomide was an over-the-counter drug prescribed to pregnant women in West Germany. This drug helped with nausea associated with morning sickness. It worked well, but soon, nearly 7000 babies were born with birth defects.  They had a malformation of their arms and legs. More than half of them died in infancy. Because of this, thalidomide was pulled from the market in the early 1960’s.


Thalidomide is a compound that exists as two different mirror images. One of these mirror images helped fix nausea. The other mirror image caused birth defects. Many times, when a medicine is produced that has two different mirror images, one is bioactive while the other is not. After thalidomide, we now only sell the bioactive enantiomer unless it is proven that the other one does not cause harm. Now, we try to produce only one of the mirror images, or if we produce a mixture of the two, we separate them. Unfortunately, for thalidomide, it would not have mattered if only the good mirror image was sold. In the body, the good thalidomide mirror image gets converted into a mixture of the good and bad.

Thalidomide mirror images

If early some morning you groggily put your right foot into your left shoe, you would be able to tell because your foot would hurt. Why? Your feet are mirror images of each other. Your shoes are mirror images of each other. But, there is a difference between your left and right foot. They are not identical. There is a difference between your left and right shoe. They are not identical. Your feet are chiral. Your shoes are chiral. They have a handedness. Hands and gloves are also chiral. You cannot put your right hand in your left glove. A chiral object has a non-superimposable mirror image. Another way to think about it is it is an object whose mirror image is different or not identical.


Not every object is chiral. Most socks are not. You are equally comfortable if you put a tube sock on either foot. It makes no difference. They do not have handedness. Socks are achiral. Achiral means “not chiral”. It is an object with a mirror image that is identical. The chairs in the picture below are achiral. The two green rocking chairs are mirror images of each other. If you sit in either chair, you could tell no difference. They are identical. Not all chairs are achiral. Some desks with a writing surface are chiral. They are right or left-handed. You can tell the difference if you sit in one or the other.

All of the previous compounds with one stereocenter are chiral. This is a big help in helping identify if a compound is chiral or not! A compound is chiral if it has only one stereocenter. If a compound has zero or two stereocenters, we need to look more carefully. These molecules can be either chiral or achiral. Let’s look at how compounds with two stereocenters can be either. The carbon atoms labeled with * are stereocenters.

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