Chapter 4 : Stereochemistry B.sc 1st Year Page : 3 Organic Chemistry
Resolution of Enantiomers
The process of separation of components of racemic mixture into its two enantiomers is called resolution’. Since individual enantiomers have identical physical and chemical properties, separation of those have. been a difficult process with ordinary methods. They can only be distinguished by a chiral environment.
Resolution by Mechanical separation Method :
Provided that the crystals of enantiomers e visually distinct, it should be possible to pick them apart by means of tweezers and thus to effect resolution. The first resolution even to be brought about was achieved in this way by Louis Pasteur 848). He prepared sodium ammonium salt of racemic tartaric acid and allowed it to crystallise in large Drystals by slow evaporation of the aqueous solution. Then he picked apart the two kind of crystals. This method of mechanical separation is not a practical way of separating enantiomers. It is tedious, it can not be applied to racemic compounds or solid solutions and can be applied only to those mixtures in which the crystals of enantiomers are visually distinct.
Resolution via Molecular complexes :
A recemic mixture is treated with a chiral reagent with which it can form a crystalline complex compound. Resulting two diastereomeric complexes are of different solubilities are obtained. Thus, these complexes are easily separated. Decomposition of separated complexes by heating, dissolution, chromatography or chemical treatment will yields pure arrantiomers. 1) Resolution by Chromatography : Chromatographic separation emplies on a difference in nity between a stationary phase (often silica) and a mobile phase (solvent travelling through the Eationary phase). The stationary phase is made chiral by bonding to an enantiomerically pure sempound, it can be used to separate enantiomers. One of the enantiom med more tightly than the other. The difference in adsorption process may be due to different degree of meraction(hydrogen bond, van der Waal) between stationary phase and enantiomers. The enantiomer will be held tightly, will pass through the column more slowlly.
Biochemical Resolution :
Recently enzymes are used to resolve enantiomers. Racemic mixture of some amino acids are separated with an enzyme amylase after this amino acids being amylated. Only L- isomer of acylated amino acid undergoes hydrolysis with enzyme acylase. The D- isomer remains rhyrolysed. Now free L- amino acid can be separated using chemical or other conventional methods.
Chemical Resolution using Diastereoisomers:
A racemic mixture is treated with an enantiomerically pure component -a resolving reagent, which form diastereoisomers with both enantiomers. These diastereomer mixture of salt or esters is initially separated by crystallization from which pure enantiomers are recovered. For example( + )’racemic alcohol(pentan-2-ol) on treatment with enantiomerically pure acid Mandelic acid terms two diastereomeric products. These diastereomers are separated chromatographically.
If we hydrolyze each diastereomer separately, we have managed to separate two enantiomers of the starting pentan-2-ol.