Illustrated Glossary of Organic Chemistry

Enantiomeric excess (ee): The excess of one enantiomer over the other in a mixture of enantiomers. Expressed mathematically: enantiomeric excess = % of major enantiomer - % of minor enantiomer. Example: A mixture composed of 86% R enantiomer and 14% S enantiomer has 86% - 14% = 72% ee. A substance that is a single, pure enantiomer (i.e., has 100% ee) is called homochiral or optically pure.
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R enantiomer
90% of product mixture
Major product
S enantiomer
10% of product mixture
Minor product
The product of this reaction, one possible step in the manufacture of naproxen (an analgesic), is a mixture of 90% S enantiomer and 10% R enantiomer. For this example, 90% - 10% = 80% enantiomeric excess.