Meso compounds are achiral : Compounds that contain stereogenic centers but are achiral.
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| • | Tartaric acid, like threonine, contains two stereogenic centers so again we might expect four stereoisomers: two diastereoisomers, each existing as a pair of enantiomers: |
| • | However, we actually find there are only three stereoisomers of tartaric acid (refcodes: TARTAC, TARTAL04 and TARTAM). Can you determine why this is? Examine all three structures closely. For each structure, assign the configuration at both stereogenic centers and match the structure with the corresponding stereoisomer in the diagram above. |
| • | You should find that TARTAM can be matched against both the R,S and S,R configurations shown in the diagram above. R,S-Tartaric acid and S,R-tartaric acid are identical, this can be seen by rotating one structure 180 degrees. The identity of the R,S and S,R structures results from the fact that the molecule has a plane of symmetry. This plane cuts through the C2-C3 bond, making one half of the molecule a mirror image of the other. |
| • | Compounds that contain stereogenic centers but are achiral (due to a symmetry plane) are called meso compounds. Tartaric acid therefore exists as three stereoisomers: two enantiomers (refcodes: TARTAC and TARTAL04) and one achiral meso form (refcode: TARTAM). |