Enantiomeric excess

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%).

Definition

Enantiomeric excess is defined as the absolute difference between the mole fraction of each enantiomer:

e

e

=

|

F

R

−

F

S

|

{\displaystyle \ ee=|F_{R}-F_{S}|}

where

F

R

+

F

S

=

1

{\displaystyle \ F_{R}+F_{S}=1}

In practice, it is most often expressed as a percentage.

Enantiomeric excess is used as one of the indicators of success of an asymmetric synthesis. For mixtures of diastereomers, there is an analogous definition for diastereomeric excess.

For example, a sample with 70% of R isomer and 30% of S has an enantiomeric excess of 40%. This can also be thought of as a mixture of 40% pure R and 60% of a racemic mixture (which contributes 30% R and 30% S to the overall composition).

If given the enantiomeric excess

e

e

{\displaystyle \ ee}

of a mixture, the fraction of the main isomer, say R, can be determined using

F

R

=

(

1

+

e

e

)

/

2

{\displaystyle \ F_{R}=(1+ee)/2}

and the lesser isomer

F

S

=

(

1

−

e

e

)

/

2

{\displaystyle \ F_{S}=(1-ee)/2}

.