1. CH 5-6: Meso Compounds – A new type of Stereoisomer
Are any stereoisomers of 2,3-dibromobutane superimposable?
(2R,3R)
(2S,3S)
(2R,3S)
(2S,3R)
Hmmm…the (R,S) and (S,R) stereoisomers look suspicious!

2. The (2R,3S) and (2S,3R) stereoisomers are identical!
flip 180o
The (2R,3S) and (2S,3R) stereoisomers are identical!
WHY?……….
Plane of symmetry!

3. (2R,3S)
(2S,3R)
The (R,S) and (S,R) stereoisomers are identical and is called a Meso compound.
Meso compounds have 2 or more chiral centers, but the (R,S) and (S,R) stereoisomers have a plane of symmetry.
Meso compounds are achiral.
Compounds with a Meso stereoisomer will have fewer than the maximum possible number of stereoisomers.

4. How many stereoisomers for 3,4-dimethyl hexane?
Draw (3R, 4S)-3,4-dimethyl hexane 6
(R)
(S)
We predict a max of 4 stereoisomers, but we do see a plane of symmetry! 1

5. Is this molecule a Meso compound? YES!3
(R) 4
(S)