Stereoisomerism

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Stereoisomerism is defined as two molecules which have the same molecular formula but the atoms are arranged differently in space. This is due to the restricted rotation of the carbon carbon double bond. There are two main types of stereoisomerism- Cis and Trans. Cis (or Z) isomers (Z standing for Zusammen, the German word for together) occurs when the same group or groups of a high priority are both found on the same side of the restricted double bond. Trans (or E) isomers (E standing for Entgegen, the German word for opposite) occurs when the same groups or groups of a high priority are across the double bond and are so found on opposite sides.

The other major form of stereoisomerism is optical isomerism. Optical isomers arise when a chiral carbon is present, this is a carbon atom attached to four different groups. Optical isomers are non-superimposable mirror images of each other and are called enantiomers[1]. One enantiomer is in a D-form and will rotate plane polarized light in a clockwise direction, to the right (dextro) while the L-form will rotate plane polarized light in an anticlockwise direction, to the left (laevo)[2].

Ensuring the correct enantiomer is present is critical in pharmaceuticals, as different isomers can have very different effects. It was the presence of the incorrect enantiomer in Thalidomide that caused deformities in the foetus to develop when it was given to pregnant women for morning sickness.

There are a number of different methods used to separate enantiomers. For instance, the alteration of enantiomers into diastereomers by salt formation or covalent derivatisation allows separation due to variation in the physical and chemical properties that diastereomers possess. Secondly, enantiomers can also be separated by gas chromatography, high-pressure liquid chromatography and thin-layer chromatography. In addition microorganisms can be used to separate enantiomers by enzyme action[3].

References

  1. Mathews CK, Van Holde KE, Ahern KG. Biochemistry, 3rd Ed, San Francisco: Benjamin Cummings. 2000 (9, 280)
  2. Mathews CK, Van Holde KE, Ahern KG. Biochemistry, 3rd Ed, San Francisco: Benjamin Cummings. 2000 (9, 281)
  3. Dr Uwe Hofker, Prof. Dr. Gregor Fels, Separation of Enantiomers (Resolution of Racemates), http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/stereochemie/trennung_enantiomere.vlu.html, [Date accessed: 04/12/16]
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