Separation of Enantiomers (Resolution of Racemates)
Resolution of Racemates (Separation of Enantiomers)
Hands and gloves
How could a mixture of gloves be sorted out into left-hand and right-hand gloves with your eyes closed?
A pile of the same amount of left-hand and right-hand gloves represents a racemate. The enantiomers, here either a left-hand or right-hand glove, cannot be distinguished by their physical properties, such as their weight and density. In the case of sorting out gloves with one's eyes shut, the gloves may be distinguished by putting each glove on the left hand. If it fits, it is a left-hand glove; otherwise it is a right-hand glove. That is, the left hand serves as an enantiomerically pure, chiral auxiliary to separating the racemate into the enantiomers. This is called the resolution of racemates.Pairs of enantiomers - a left-hand and a right-hand glove, for instance - have the same physical properties. In the case of molecules, however, enantiomers differ in one physical property: their optical activity, that is, they rotate the plane of polarized light by the same amount in opposite directions. Unfortunately, this cannot be used to separate them. However, several physical as well as chemical methods were developed for the resolution of racemates.
Resolution of racemates by crystallization
The oldest method of separating pairs of enantiomers was introduced by Louis Pasteur. As he was crystallizing a racemate of sodium ammonium tartrate, he realized that different crystals had been formed. Some were left-handed, while some were right-handed. Racemic mixtures of enantiomers almost always crystallize from solution yielding racemic crystals; that is, each crystal (and each unit cell) contains the same amounts of each enantiomer. However, in some cases, a spontaneous crystallization of a mixture of enantiomerically pure crystals occurs. An example of such a crystallization is evident by Louis Pasteur's stroke of luck, when he worked with racemic sodium ammonium tartrate, that crystallizes below 27 °C from an aquaeous solution resulting in a 1:1 mixture of enantiomerically pure crystals of (+) and (-) enantiomers. These crystals have a mirror-image relationship; they are hemihedral. Therefore, these two kinds of crystals can be macroscopically distinguished and separated with a pair of tweezers. In this way, Louis Pasteur managed to establish the first resolution of racemates in 1848.
Chemical methods of resolution of racemates with previous formation of diastereomers
Today, chemical methods are preferably employed in the resolution of racemates. For this matter, the enantiomers of the racemic mixtures are converted into diastereomers through a reaction with an enantiomerically pure, chiral auxiliary. Due to their different physical and chemical properties, these diastereomers can be separated by common separation techniques. Examples of such techniques are:
- chromatography or
- selective crystallisation of one diastereomer with the other one kept in the solution.
After the separation, the pure enantiomers of the racemic mixture and the chiral auxiliary are recovered.
The recovery of the pure enantiomers and the chiral auxiliary is easier if no covalent bonds but salts or complexes are formed during the conversion of the enantiomers into diastereomers. In a standard method of the separation of racemic organic acids, for example, they are treated with a chiral, enantiomerically pure nitrogenous base and form diastereomeric salt pairs. Nitrogenous bases that are frequently used for this purpose are the alkaloids morphine, strychnine, and brucine. They can be isolated from the chiral pool of nature as enantiomerically pure compounds.
Chromatography with a chiral stationary phase
Additionaly, a common technique of the separation of enantiomers is the chromatography in association with a chiral stationary phase. Such a separation is based on the differently strong interaction of the enantiomers with the chiral stationary phase. As a result, one enantiomer is more retarded than the other, so that they pass through the chromatographic column at different times.