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

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Answer 1 · 2016-07-11T21:20:28

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

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:

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

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