4.6 : 具有多个手性中心的分子

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 180° rotation; that is, they are the same molecule. Hence, butane-2,3-diol has only three distinct stereoisomers, one of which is achiral. This achiral stereoisomer is called a meso compound.

Nearly all meso compounds have a plane of symmetry or a center of symmetry. Such molecules are transformed into their mirror images by a 180° rotation.

However, a molecule with multiple chiral centers that lacks those elements still may be superposable on its mirror image by a rotation of less than 180°. If any such rotation exists, both configurations represent the same meso compound.