Enantiomeric excess

Enantiomeric excess (ee) is a measure of the purity of a chiral compound in terms of the proportion of one enantiomer relative to the other. Enantiomers are pairs of molecules that are non-superimposable mirror images of each other, and they often differ in their biological activity and properties.

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The enantiomeric excess of an enantiomer pair is the magnitude of the difference between their relative mole fractions,

ee = ∣ x_{+} - x_{-} ∣

, often reported as a percentage. Here

x_{+} + x_{-} = 1

counts the pair only, excluding solvent; the signs label optical rotation under the specified conditions, not R and S configuration.

In the model above,

x_{+} = 0.75

and

x_{-} = 0.25

give

ee = 0.50 = 50%

. Keep the same total pair concentration

c = 0.200 g m L^{- 1}

, path length, solvent, temperature and wavelength. The two contributions are

c_{+} = 0.150, c_{-} = 0.050 (g m L^{- 1}), α = 40 (1) (0.150 - 0.050)^{\circ} = + 4^{\circ} .

The

+ 8^{\circ}

pure-member reference therefore gives

α / α_{pure, +} = + 0.50

. This ratio measures signed population imbalance only for a pure pair with additive linear responses under matching conditions. Adding unrelated solutes, changing total concentration, or changing chemical equilibrium can invalidate that inference. For

x_{+} = x_{-} = 0.50

, the racemate gives zero net optical rotation by cancellation; each molecule remains chiral. A zero reading alone proves neither a racemate nor molecular achirality.