Identifying Chiral Molecules, Meso Compounds, and Diastereomers

A meso compound contains a plane of symmetry and so is achiral, regardless of whether the molecule has a chiral center. A plane of symmetry is a plane that cuts a molecule in half, yielding two halves that are mirror reflections of each other.

By definition, a molecule that's not superimposable on its mirror image is a chiral molecule. Compounds that contain chiral centers are generally chiral, whereas molecules that have planes of symmetry are achiral and have structures that are identical to their mirror images.

For example, cis-1,2-dibromocyclopentane (shown in the first figure) is meso because a plane cuts the molecule into two halves that are reflections of each other. Trans-1,2-dibromocyclopentane, however, is chiral because no plane splits the molecule into two mirror-image halves.

Now look at the mirror images of these two molecules in the second figure to prove this generality to yourself.

Even though the cis compound has two chiral centers (indicated with asterisks), the molecule is achiral because the mirror image is identical to the original molecule (and is, therefore, superimposable on the original molecule). Molecules with planes of symmetry will always have superimposable mirror images and will be achiral. On the other hand, the trans stereoisomer has no plane of symmetry and is chiral.

In organic chemistry, you need to be able to spot planes of symmetry in molecules so you can determine whether a molecule with chiral centers will be chiral or meso. For example, can you spot the planes of symmetry in each of the meso compounds shown in the last figure?

When more than one chiral center is present in a molecule, you have the possibility of having stereoisomers that are not mirror images of each other. Such stereoisomers that are not mirror images are called diastereomers.

Typically, you can only have diastereomers when the molecule has two or more chiral centers.

The maximum number of possible stereoisomers that a molecule can have is a function of 2ⁿ, where n is the number of chiral centers in the molecule. Therefore, a molecule with five chiral centers can have up to 2⁵ or 32 possible stereoisomers! As the number of chiral centers increases, the number of possible stereoisomers for that compound increases rapidly.

For example, the molecule shown here has two chiral centers.

Because this molecule has two chiral centers, it can have a total of 2², or 4, possible stereoisomers, of which only one will be the enantiomer of the original molecule.

Enantiomers are stereoisomers that are mirror images of each other.

Because both chiral centers in this molecule are of R configuration, the enantiomer of this molecule would have the S configuration for both chiral centers. All the stereoisomers of this molecule are shown in the next figure. Those molecules that are not enantiomers of each other are diastereomers of each other.