Meso Compounds

A compound that contains exactly one stereocenter is always chiral. But compounds that contain more than one stereocenter are not necessarily chiral. This occurs when multiple stereocenters create an internal plane of symmetry. For example, consider 2,3-dichlorobutane. There are four possible stereochemical permutations since there are two stereocenters:

Figure %: Stereoisomers of 2,3-dichlorobutane
Note, however, that compounds 3 and 4 are not enantiomers. Rather, they are identical (flip 3 over to obtain 4). The reason for this is that the molecule has an internal plane of symmetry that bisects the 4-carbon chain. Therefore, it is achiral and is identical to its mirror image. A compound that contains multiple stereocenters but is achiral due to an internal mirror plane is called a meso compound. So there are really only three stereoisomers of 2,3-dichlorobutane: a pair of chiral molecules (enantiomers of each other) and the achiral meso compound.