Reference

The formula to calculate enantiomeric excess is: ee = ( [R] - [S] ) / ( [R] + [S] ) x 100% where [R] and [S] are the concentrations of the two enantiomers in the sample. Enantiomeric excess is an important parameter in asymmetric synthesis, which is the production of chiral compounds from achiral starting materials. It is also used in drug ...

In stereochemistry, enantiomeric excess (ee) is a measurement of purity used for chiral substances. It reflects the degree to which a sample contains one enantiomer in greater amounts than the other. A racemic mixture has an ee of 0%, while a single completely pure enantiomer has an ee of 100%. A sample with 70% of one enantiomer and 30% of the other has an ee of 40% (70% − 30%).

Enantiomeric excess tells us how much more of one enantiomer is present in the mixture. In this example, the ee is determined by the difference of percentages of the two enantiomers: % ee (R) = enantiomer R - enantiomer S = 80% - 20% = 60%. We can visualize this by looking at the boxes representing the mixture of the enantiomers.