Someday in the distant future, I hope to be able to explain my own research project to my parents without making up lies about how I’m curing cancer. Perhaps if they understand my chemistry a bit, they won’t feel the need to bring a novel to read during my thesis defenseshould they be able to come, of course, no pressure : ), assuming I ever get to that point.
So here’s a start – a discussion about mirrors.
You’re standing in front of a mirror (Figure 1) looking at your reflection, perhaps admiring the look of the new gold Rolex that graces your wrist. (OK, in my family maybe it’s a new GPS watch or something). Then, like Carroll’s Alice, you step into the mirror. You turn around to face out, now standing shoulder to shoulder with your mirror twin (Figure 2). But something is amiss. The watch that you wear on your left wrist is instead worn on your twin’s right. You part your hair on the right, but your twin parts hers on her left. The two of you aren’t identical, and try as you might to twist and rotate your body, you can’t make yourself match up with your reflection.
A person who knows you well can discern between you and your reflection, perhaps by recalling the placement of a dimple or a freckle. Some objects, however, are completely indistinguishable from their reflections. Take the example of a simple, perfectby perfect I mean symmetrical, not that this is the ideal look for household furnishing kitchen chair:
You can perform the same exercise with this chair – push it through the mirror and turn it around to face front. As you stand back and scrutinize the two chairs that stand side by side, you will not be able to tell the difference between the two. The original chair and its reflection are completely superimposable.
The difference between you and the chair is one of symmetry. The chair is perfectly symmetrical from one side to the other – the left side is exactly like the right. In fact, the left side is the mirror image of the right side. You can imagine a vertical plane slicing right through the center of the chair, such that everything on the right side of the plane is the mirror image of everything on the left.
This is not the case for you (“you” being the stick figure above). Your left side is not the mirror image of your right, because your left wrist has a watch on it and your right does not. Of course, in reality, you have many slight imperfections that make even your unaccessorized body asymmetricalAnd in truth, a real life chair would also have such imperfections..
The concept that some things are identical to their mirror images and others are not is a fundamental principle in organic chemistry. The adjective “chiral” (ch is pronounced like k, and the word rhymes with eye-roll) is used to describe molecules that are not identical twins with their mirror images.
A chiral molecule can’t be superimposed on its mirror image, no matter how you rotate it. You could say that your body is chiral.
An achiral molecule cannot be distinguished from its mirror image. The chair in the picture above is achiral.
There’s another key vocabulary word related to mirrors and chemistry. The noun enantiomer is a slightly shorter way of saying “non-superimposable mirror image.” Your body and your reflection are enantiomers of each other. All chiral molecules have enantiomers, which are unique molecules in their own right. An achiral molecule doesn’t have an enantiomer; its mirror image has the exact same identity as itself.
Why do chemists care about mirror images? It turns out that nearly every molecule in biological systems (e.g., in your body) is chiral. Often only one enantiomer – either a molecule or its mirror image – works properly to carry out a given biological function.
This is important to consider when medicinal chemists design pharmaceutical drugs. For example, the active ingredient in the sleep aid Lunesta is a chiral molecule. Though this molecule helps people with insomnia, the mirror image version of this molecule doesn’t do a darn thing. In other cases, the mirror image of a medicine’s active ingredient is actually harmful. Therefore, even before getting to the drug-design stage, a lot of fundamental research (cough cough mine) in chemistry labs deals with developing new methods to selectively make one chiral molecule without making much of its enantiomer.