Optical purity

The optical purity (English optical purity ) describes the stereochemistry of the composition of a mixture of enantiomers , ie a mixture of enantiomers (mirror image to each other chiral molecules ): ${\displaystyle op}$

${\displaystyle op={\frac {[\alpha ]_{\text{gemessen}}}{[\alpha ]_{\text{maximal}}}}\cdot 100\,\%}$

In it is

• [α] _measured the specific angle of rotation of the chemically pure enantiomers mixture
• [α] _maximum the specific rotation of a pure enantiomer under the same measurement conditions.

The numerical value of can be between 0 and 1: ${\displaystyle op}$

${\displaystyle 0\leq op\leq 100\,\%=1}$

“Optical purity” = enantiomeric purity?

Under the condition of ideal behavior (no interaction between the enantiomers and validity of the Lambert-Beer law ), the optical purity is equal to the enantiomeric purity  ee ( enantiomeric excess ). This led to the widespread but misconception that the terms optical purity and enantiomeric purity can be used synonymously . Therefore, the traditional method for the stereochemical characterization of mixtures of enantiomers was the measurement of the rotation value .

Since optical and enantiomeric purity are not generally equivalent ( ), nowadays the following is used to determine the enantiomeric purity of enantiomeric mixtures: ${\displaystyle op\neq ee}$

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

Examples against a general equivalence of optical and enantiomeric purity:

• The specific rotation value of enantiomerically pure 1-phenylethanol can be increased by contamination with acetophenone .
• The relationship between optical and enantiomeric purity is not linear , as Horeau showed 1969 pure ( S ) -2-ethyl-2-methyl succinic acid has at 22  ° C , a specific rotation of [α] _D = + 4.4 ° (c = 15 , CHCl ₃ ). From this, Horeau calculated a specific rotation value of [α] _D = + 2.2 ° (c = 15, CHCl ₃ ) for a mixture of 75% of the ( S ) form and 25% of the ( R ) form, corresponding to an enantiomeric excess  ee of 50%. However, the experimental check showed a measured specific rotation value of [α] _D = + 1.6 ° (c = 15, CHCl ₃ ) for this enantiomer mixture, which corresponds to an optical purity of 36%. This effect only occurred when using slightly polar solvents ( methylene chloride , chloroform , benzene ); the discrepancy between optical and enantiomeric purity did not occur when polar solvents ( ethanol , pyridine , acetonitrile ) were used.

The term enantiomeric excess  ee (Engl. Enantiomeric excess ) was coined in 1971 by Morrison and Mosher. Nowadays, the term enantiomeric excess is increasingly the term enantiomer  he (Engl. Enantiomeric ratio replaced), while a mixture of enantiomers is characterized by the ratio [S]: [R] or [R]: [S].

With the same argument, the vague term diastereomeric excess  de (English. Diastereomeric excess ) is increasingly being replaced by the term diastereomeric  ratio dr (English. Diastereomeric ratio ).

Individual evidence

1. ↑ Bernhard Testa: Fundamentals of organic stereochemistry , Verlag Chemie, Weinheim, 1983 , ISBN 3-527-25935-X , pp. 144-145.
2. ↑ K. Günther, J. Martens, M. Schickedanz: Thin-layer chromatographic separation of enantiomers by means of ligand exchange. In: Angew. Chem. 96, 1984 , pp. 514-515, doi : 10.1002 / anie.19840960724 .
3. ^ K. Günther: Thin-layer chromatographic enantiomeric resolution via ligand exchange. In: Journal of Chromatography. 448, 1988 , pp. 11-30, doi : 10.1016 / S0021-9673 (01) 84562-3 .
4. ↑ K. Günther, M. Schickedanz, J. Martens: Thin-Layer Chromatographic Enantiomeric Resolution. In: Natural Sciences . 72, 1985 , pp. 149-150, doi : 10.1007 / BF00490403 .
5. ↑ Teresa Kowalska , Joseph Sherma (editor): Thin Layer Chromatography in Chiral Separations and Analysis. CRC Press Taylor & Francis Group, Chromatographic Science Series Volume 98, 2007 , ISBN 978-0-8493-4369-8 .
6. ↑ Kurt Günther, Jürgen Martens and 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 . 288, 1984 , pp. 203-205, doi : 10.1016 / S0021-9673 (01) 93696-9 .
7. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds. John Wiles & Sons, 1994 , ISBN 0-471-05446-1 , pp. 221-240.
8. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds. John Wiles & Sons, 1994 , ISBN 0-471-05446-1 , pp. 219-221.
9. ^ Morrison, James D .; Mosher, Harry S .: Asymmetric Organic Reactions , Prentice-Hall, Englewood Cliff, New Jersey, 1971 ( ISBN 0130495514 ).
10. ^ Robert E. Gawley: Do the Terms "% ee" and "% de" Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity? In: J. Org. Chem. 71, 2006 , pp. 2411-2416, doi : 10.1021 / jo052554w .