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Pictures of the Day CH320M/CH328M Fall 2012

Enantiomers and Diasteromers - Molecules With Two Chiral Centers
These structures match the structures in the back of the web handout on stereochemistry, so you should refer to that. This molecule, 2,3-dihydroxybutanoic acid, has two chiral centers (one at carbon 2, and one at carbon 3, so there are four possible stereoisomers. The 2R,3R and 2S,3S stereoisomers are enantiomers of each other, and the 2R,3S and 2S,3R are enantiomers of each other. These are enantiomer pairs becuase they are nonsuperimposable mirror images of each other. Any other pair, such as 2R,3R and 2S,3R, are diastereomers of each other, since they are stereoisomers that are not enantiomers (not mirror images of each other). Thus, you use the terms "enantiomers" or "diastereomers" to describe the stereochemical relationship when comparing different stereoisomers
Meso Compounds -Molecules with Chiral Centers AND Planes of Symmetry
A meso compound at first sounds like a contradiction. A meso compound is not chiral even though it contains chiral centers. The key idea is that meso compounds contain a plane of symmetry, so cannot be chiral. This occurs in the special case that a molecule has chiral centers (in the above case 2 chiral centers) that have the same four substituents attached. In these cases, the R,S/S,R stereoisomer will be a meso compound. As an example, the tartaric acid isomers are shown about. The upper two are the S,S and R,R stereoisomers, these are non-superimposable mirror images of each other so they are enantiomers. There is no plane of symmetry in either of these stereoisomers. On the other hand, the R,S/S,R stereoisomer shown below has a plane of symmetry as indicated in the drawing. Thus, it is superimposable on its mirror image (its mirror image is the same molecule), it is not chiral, and the R,S stereoisomer is the same molecule as the S,R stereoisomer! The bottom line is that symmetric molecules with two chiral centers like tartaric acid only have three different stereoisomers, a pair of enantiomers (will always be the R,R and S,S compounds) and a meso compound (will always be the R,S/S,R stereoisomer).