Enantiomeric excess

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

definition

Schematic representation of a 40% enantiomeric excess ( ee = 40%).

${\displaystyle ee={\frac {|m_{1}-m_{2}|}{m_{1}+m_{2}}}\cdot 100\,\%}$

With

• ${\displaystyle m_{1}}$: Mass of enantiomer 1
• ${\displaystyle m_{2}}$: Mass of enantiomer 2.

In the example from the figure, with mass fractions of the enantiomers of 70% to 30%, each 30 percentage points of the enantiomers can be present as a racemic mixture and the remainder of the excess enantiomer can be present in pure form, which means a 40% enantiomeric excess for the enantiomeric mixture ${\displaystyle \left(ee=40\,\%\right).}$

The limiting cases for the enantiomeric excess are:

• ${\displaystyle ee=0\,\%}$for a racemate (1: 1 mixture of enantiomers 1 and 2)
• ${\displaystyle ee=100\,\%}$in the case of an enantiomerically pure compound .

Enantiomeric excess and rotation value

In practice, a certain enantiomeric excess often means that when measuring the rotation value of an optically active solution, only the corresponding percentage of the theoretical rotation value of the enantiomerically pure solution is measured.

Strictly speaking about the rotation but only the optical purity measured (English optical purity ). Contamination of the examined sample with by-products (irrespective of whether they are achiral or enantiomerically pure) can lead to the optical purity not corresponding to the enantiomeric excess: ${\displaystyle op}$

${\displaystyle op\neq ee}$

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

Enantiomeric excess and enantiomeric ratio

According to the proposal of the IUPAC , the enantiomeric excess should increasingly be replaced by the enantiomeric ratio , which can be calculated from the enantiomeric excess: ${\displaystyle er}$

${\displaystyle er={\frac {1+ee}{1-ee}}}$

In the example from the figure, the enantiomeric ratio is therefore ${\displaystyle er={\tfrac {7}{3}}.}$

The above formula can be changed to:

${\displaystyle \Leftrightarrow ee={\frac {er-1}{er+1}}.}$

The enantiomeric ratio can also be measured directly , mainly with

• chromatographic methods ( thin layer , gas or high pressure liquid chromatography using chiral stationary phases )
• special NMR techniques.

See also

• Enantioselective synthesis
• Enantioselective Catalysis

Individual evidence

1. ↑ James D. Morrison, Harry S. Mosher: Asymmetric Organic Reactions. Prentice-Hall, Englewood Cliff, New Jersey, 1971 ( ISBN 0-13-049551-4 ).
2. ↑ Reinhard Brückner : reaction mechanisms , spectrum Akademischer Verlag, 3rd edition, 2004, ISBN 978-3-8274-1579-0 , p. 110.
3. ↑ Volker Schurig : Contributions to the theory and practice of the chromatographic separation of enantiomers. In: Chirality 17 (2005) 205-226, doi: 10.1002 / chir.20133 .
4. ↑ K. Günther, J. Martens, M. Schickedanz: Thin-layer chromatographic separation of enantiomers by means of ligand exchange. In: Angew. Chem. 1984, 96, pp. 514-515, doi: 10.1002 / anie.19840960724 .
5. ↑ Teresa Kowalska , Joseph Sherma (editor): Thin Layer Chromatography in Chiral Separations and Analysis (= Chromatographic Science Series. Volume 98). CRC Press Taylor & Francis Group, 2007, ISBN 978-0-8493-4369-8 .
6. ↑ Kurt Günther, Jürgen Martens , Maren Messerschmidt: Gas Chromatographic Separation of Enantiomers: Determination of the Optical Purity of the Chiral Auxiliaries (R) - and (S) -1-Amino-2-methoxymethylpyrrolidine. In: Journal of Chromatography A . 1984, 288, pp. 203-205, doi : 10.1016 / S0021-9673 (01) 93696-9 .
7. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds, John Wiley & Sons, 1994, ISBN 0-471-05446-1 , pp. 221-240.