Organic Lecture 5: Epimers, Anomers, and Meso Compounds

These topics are skimmed over in most of our organic classes but, nevertheless, it's important to have a cursory understanding for the MCAT.

Epimers: differ in their absolute configuration at a single chiral center; they are a subclass of diastereomers (see picture).

The prefix D on the name of molecules (i.e. D-Galactose) refers to the orientation of the hydroxyl group (-OH) on the highest-numbered chiral center in a Fischer projection. When the hydroxyl group is on the right of this carbon in the Fischer projection, the molecule is a D sugar. When it's on the left, the molecule is an L sugar.

**Note: D and L are entirely unrelated to optical activity. Distinctions between D and L (or between R and S) can be made just by looking at a drawing of the molecule, but distinctions between (+) and (-) can be made only by running experiments in a polarimeter

Once again:
R or S = absolute configuration (structure)
D or L = relative configuration (structure)
(+) or (-) = observed optical rotation (property)

Epimeric carbon: place at which stereochemistry is different between two molecules

Anomers: epimers that form as a result of ring closure; for MCAT it pertains to only sugar chemistry (i.e. glucose)

Anomeric carbon in glucose: can assume two forms--with hydroxyl group down, it is alpha...with the hydroxyl group up, it is Beta.

Meso compounds: internal plane of symmetry in a molecule that contains chiral centers; are not optically active because one side of the molecule is a mirror image of the other (this means optical activity imparted by one side is canceled out by its other side)