Introduction to Determining Chirality

Stereoisomerism can be a lot trickier to identify than constitutional isomerism. With this in mind, and to some extent because I am too busy to make really good posts right now but also want to keep this project moving, there will be a series of short posts on the subject, starting with this one.

In order for any of this to make sense, you need to understand what it means for a particular atom to be a tetrahedral stereogenic center. Don't panic. Just peruse the previous entry and make sure you grasp the concept I am describing. The pictures are probably best for this, but what we're basically dealing with are atoms attached four different groups. This is because when an atom (usually carbon) is attached to four different groups, it is not superimposable on its mirror image. And, if it helps any, this concept can be extended to macroscopic things in our everyday lives. The textbook contrasts gloves and socks. In a pair of socks, the two individuals are identical (usually). But in a pair of gloves, the right glove and the left glove are not interchangeable.

Chiral molecules are like gloves (or shoes, for that matter). Even though the properties of the isomers are virtually identical, they are, in principle different from each other and these differences can manifest in ways that are relevant to us. An obvious demonstration of this is in pharmaceuticals, where often only one of the isomers has the desired effect, but the drug is sold and administered as a mixture of both versions. I should do a post on the thalidomide incident. Not right now, though. But maybe later.

Anyway, this isomerism can show up in other types of situations and hopefully I'll soon get to some of them, but for now, we shall focus on chirality that arises from tetrahedral stereogenic centers. Here are some points to keep in mind about these types of chiral molecules.

1. A molecule for which the mirror image is superimposable is achiral. A molecule for which the mirror image is not superimposable is chiral.

2. A carbon that is bonded to four groups, none of which are identical to each other, is a stereogenic center (aka chiral center). This does not necessarily mean that the molecule itself is chiral as we shall see.

3. A molecule that contains exactly one stereogenic center is chiral. Always. No exceptions.

4. A molecule that contains more than one stereogenic center might be chiral, but it might not. More on this later.