Two Chiral Centers - Symmetrical Groups

First, notice that the stereoisomers shown here have some symmetry, due to the three equivalent groups attached to the two chiral compounds. Convince yourself that the pair of structures on the right are non-superimposable mirror images by trying to line up all the groups in space. You will recognize that you can line up the molecules so that they appear to be mirror image reflections of each other, because they are enantiomers. Superimposing them on each other would be like lining up two right hands on top of each other.

The pair of structures on the left are also mirror images. However, these mirror images can be superimposed on each other due to the symmetry of this compound. These stereoisomers each have a mirror plane of symmetry dissecting the compound, making the mirror images superimposable. For this reason, there are only three stereoisomers of 2,3-dichlorobutane. (Remember that 2-bromo-3-chlorobutane has four stereoisomers.)

Again, if you compare one structure from the left pair and one from the right pair, you will find that they are non-superimposable, non-mirror images known as diasteromers.