Organic Chemistry: Stereoisomers


Racemic Mixtures and Enantiomeric Excess

Summary Racemic Mixtures and Enantiomeric Excess
Figure %: Biological functions of molecules and their enantiomers

When chemists want to synthesize compounds that are important for biological usage, they almost always need one enantiomer in high purity. The degree of enantiomeric purity of a solution is measured by its enantiomeric excess, or ee. The enantiomeric excess is found by dividing the observed optical rotation by the optical rotation of the pure enantiomer and multiplying by 100 to obtain a percentage. This number represents the percentage of one enantiomer in excess of the other. For instance, a 75/25 mixture has a 75 - 25 = 50 % ee, while a 50/50 racemic mixture has a 50 - 50 = 0 % ee. One strategy to make a pure enantiomer is to produce the racemic mixture, resolve the racemate using one of the techniques above, and toss away the undesired half. However, this strategy is not viable for expensive syntheses that require multiple steps. The waste is special to the syntheses of complex molecules that have several stereocenters. If we threw away half the product at every stereogenic step, our yield would decrease exponentially!

A better solution is to employ a reagent that selectively produces one enantiomer over another. Of course such reagents must be chiral. The problem with this approach is that the precious chiral reagent is used up once the reaction is complete. An even better approach is to use a chiral catalyst that can be used over and over again. The field of chiral catalysis is a relatively new and exciting venture in organic chemistry that holds much promise for enhancing the power of organic synthesis.