Enantiomeric ratio

The enantiomeric ratio ( English enantiomeric ratio ) is in the stereochemistry of the relative amount ratio of excess to deficiency enantiomer in a mixture of enantiomers to. ${\displaystyle er}$

definition

${\displaystyle er={\frac {m_{\text{Überschuss}}}{m_{\text{Unterschuss}}}}={\frac {n_{\text{Überschuss}}}{n_{\text{Unterschuss}}}}\geq 1}$

This stands for the mass and the amount of substance of the respective enantiomers. ${\displaystyle m}$ ${\displaystyle n}$

For example, if the enantiomers were in mass fractions of 80% to 20%, the enantiomeric ratio would be

${\displaystyle er={\frac {0{,}8}{0{,}2}}=4.}$

In the case of a racemate (also called a racemic mixture ), the enantiomeric ratio is by definition

${\displaystyle er=1.}$

history

Traditionally, mixtures of (two) enantiomeric compounds were characterized by their optical purity (English:  optical purity , abbr.  Op ), since the analysis was based on the determination of the specific angle of rotation by means of polarimetry .

The enantiomeric excess (engl.  Enantiomeric excess , abbr.  Ee ) is often mistakenly considered to be identical with the optical purity and, therefore, also often used.

present

According to the proposal of the IUPAC , the term enantiomeric excess should increasingly be replaced by the term enantiomeric ratio.

The enantiomeric ratio can be calculated from the enantiomeric excess:

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

This formula can be converted to:

${\displaystyle \Leftrightarrow ee={\frac {er-1}{er+1}},}$ in the example from the introduction would be ${\displaystyle ee={\frac {4-1}{4+1}}=0{,}60=60\,\%.}$

For the analysis of the enantiomeric ratio one mainly uses today

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

Application example

The analysis of the enantiomeric ratio is used, for example, to test natural fruit flavors and foods that are flavored with them . Some flavoring substances have natural chiral properties and therefore occur in enantiomeric forms. Chiral flavoring substances of natural origin usually have a characteristic distribution of enantiomers. Therefore, with the help of food analysis, it can be determined whether only natural flavorings are present and whether the products are labeled in accordance with the regulations .

Individual evidence

1. ^ Robert E. Gawley: Do the Terms "% ee" and "% de" Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity? In: The Journal of Organic Chemistry 71 (2006) pp. 2411-2416; doi: 10.1021 / jo052554w .
2. ↑ Volker Schurig : Contributions to the theory and practice of the chromatographic separation of enantiomers. In: Chirality . 17 (2005) pp. 205-226, doi: 10.1002 / chir.20133 .
3. ↑ 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 .
4. ↑ 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 .
5. ↑ 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 .
6. ^ Ernest L. Eliel, Samuel H. Wilen: Stereochemistry of Organic Compounds. John Wiley & Sons, 1994, pp. 221-240, ISBN 0-471-05446-1 .
7. ↑ Flavorings - legal basis and analysis , website of the Bavarian State Office for Health and Food Safety