Question

What is the definition of ‘enantiomeric excess’? How may the enantiomeric excess of a compound be determined using a chiral chromatographic approach?

Answer 1

Enantiomeric excess (`"ee"`) is defined as the absolute difference between the mole fractions of two enantiomers.

Explanation:

If we represent the mole fractions as `R` and `S`,

`color(blue)(|bar(ul(color(white)(a/a) "ee" = |R - S|color(white)(a/a)|)))" "`

It is often presented as percent enantiomeric excess, `"% ee"`, which is calculated as

`color(blue)(|bar(ul(color(white)(a/a) "% ee" = |R-S|/(R+S) × 100 %color(white)(a/a)|)))" "`

Chiral Chromatography

Chemists can use chiral chromatography to determine the `"% ee"`, because the areas of the peaks are proportional to moles of each enantiomer.

A team of Brazilian chemists prepared and hydrogenated the `R` and `S` enantiomers of the sex pheromone of a particular beetle.

They analyzed the products by chiral gas chromatography:

If the `"S"` isomer gave areas of `"R = 120"` and `"S = 2880"`, we can calculate that

`"% ee" = |R - S|/(R+S) × 100 % = |120-2880|/(120+2880) × 100 % = 2760/3000 × 100 % = 92 %`