What is enantiomeric excess?

1 Answer
Ernest Z.
Feb 13, 2017

Enantiomeric excess (#"ee"#) is 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) color(red)("ee" = |R - S|color(white)(a/a)|)))" "#

It is often expressed as percent enantiomeric excess, #"% ee"#, which is

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

This equation is equivalent to

#color(blue)(bar(ul(|color(white)(a/a)color(red)("% ee" = [α]_text(obs)/([α]_text(max)] × 100 %) color(white)(a/a)|)))" "#

EXAMPLE

A compound #"X"# has a specific rotation of -20 °. A mixture with its enantiomer has a specific rotation of -10 °. What is the enantiomeric excess of #"X"#?

Solution

The enantiomeric excess is

#% ee = ("-10" color(red)(cancel(color(black)(°))))/("-20" color(red)(cancel(color(black)(°)))) × 100 % = 50 %#

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