Molecules with Multiple Chiral Centers

Recall that a chiral center in a molecule, such as in (R)-2-butanol, is a tetrahedral atom with four different substituents. Some molecules, such as butane-2,3-diol, have multiple chiral centers.

While molecules with only one chiral center are always chiral, molecules with multiple chiral centers may have achiral configurations.

A molecule with n chiral centers has 2ⁿ possible configurations. If any configuration can be superposed on another, those two configurations represent the same molecule. Thus, there may be fewer distinct stereoisomers than possible configurations.

For example, butane-2,3-diol has two chiral centers and four possible configurations. The RR and SS configurations are mirror images of each other, as each chiral center’s configuration is inverted.

Although their lowest-energy conformations do not appear to be mirror images overall, each chiral center is still the mirror image of its partner.

Here, there is no way to rotate the RR and SS configurations to make them superposable, even in their less energetically favorable but exactly mirrored conformations. Thus, they are an enantiomeric pair.

The other pair of configurations, RS and SR, are superposable after a 1