5-6
Meso Compounds
Two identically substituted stereocenters give rise
to only three stereoisomers.
Consider the radical bromination of 2-bromobutane:
Since there are 2 chiral centers in the product, we might expect 4
distinct stereoisomers: RR, RS, SR, and SS.
These could then be organized into enantiomeric pairs: RR | SS
and RS | SR.
A closer look at the RS and SR pair of molecules, however, shows
that they are superimposable molecules and are therefore
identical.
A compound containing 2 or more stereocenters that is
superimposable with its mirror image is called a meso compound.
Meso compounds contain an internal mirror plane which divides
the two halves of the molecule which are mirror images of each
other.
The presence of a mirror plane in any energetically accessible
conformation of a molecule is sufficient to make it achiral.
2,3-Dibromobutane exists as three stereoisomers only: a pair of
enantiomers and an achiral meso diastereomer.
Examples of meso compounds having multiple chiral centers: