Enantiomeric excess

The enantiomeric excess ee (English: enantiomeric excess, term coined in 1971 by Morrison and Mosher ) in stereochemistry is the excess of one enantiomer in a mixture of enantiomers on.

Definition

With

• : Mass of enantiomer 1
• : Mass of the enantiomer 2

In the example of the figure may be present as a racemic mixture and the rest of the excess enantiomer in pure mass fractions of the enantiomers of 70 % to 30 % for each 30 percentage points of the enantiomers, which means a 40% enantiomeric excess for the enantiomers

The limiting cases for the enantiomeric excess are:

• For a racemate ( 1:1 mixture of the enantiomers 1 and 2)
• At an enantiomerically pure compound.

Enantiomeric excess and optical rotation

In practice, a certain enantiomeric excess often means that when measuring the optical rotation of an optically active solution only the relevant percentage of the theoretical value of the rotational enantiopure solution is measured.

Only the optical purity Strictly speaking about the rotation value, however measured (english optical purity ). Contamination of the sample examined with by-products (whether achiral or enantiomerically pure ) can cause the optical purity does not coincide with the enantiomeric excess:

Both a higher and a lower enantiomeric excess than is actually present can be faked.

Enantiomeric excess and enantiomeric

According to the IUPAC proposal therefore is the enantiomeric excess are increasingly being replaced by the enantiomeric ratio, which can be calculated from the enantiomeric excess:

In the example of the figure, the enantiomeric ratio is therefore

The above formula can be changed to:

The enantiomer ratio can also be measured directly, mainly with

• Chromatographic techniques (thin layer, gas or high performance liquid chromatography using a chiral stationary phase )
• Special NMR techniques.