Enantiomeric Excess Definition:

A mixture that contains unequal amounts of a pair of enantiomers is said to have an enantiomeric excess which is the – difference between the percent concentration of the major enantiomer and the percent concentration of its mirror image.

Enantiomeric Excess Explained:

The enantiomeric excess of a solution is measured in percentage terms using the following formula.

ee is the enantiomeric excess and alpha is the specific rotation.

Another method to visualize the calculation is by assuming a hypothetical mixture of a pair of enantiomers E. ( +E and –E). Consider % conc. of +E to be 60% and the % conc. of –E to be 40%. In this case, the %ee is 20% and the net optical rotation of the solution will be + or clockwise.