MCAT Content / Separations And Purifications / Racemic Mixtures Separation Of Enantiomers

Racemic mixtures, separation of enantiomers

Topic: Separations And Purifications

Racemic mixtures are difficult to separate due to the similar properties of the two chiral enantiomers, however chemical and chromatographic methods have been developed for this purpose.

A racemic mixture is a 50:50 mixture of two enantiomers. Because they are mirror images, each enantiomer rotates plane-polarized light in an equal but opposite direction, so the racemic mixture is optically inactive. If the enantiomers are separated, the mixture is said to have been resolved.

Since enantiomers have identical physical properties, such as solubility and melting point, resolution is extremely difficult. One method for resolving a racemic mixture is the reaction of that racemate with an enantiomerically-pure chiral reagent produces a mixture of diastereomers.  Diastereomers have different physical properties meaning they can be separated easily.

For example, if a racemic mixture of chiral alcohols is reacted with an enantiomerically pure R-carboxylic acid, the result is a mixture of diastereomers: in this case, a mixture of (R-R) and (R-S) diastereomeric esters, which can, in theory, be separated by their different physical properties. Subsequent hydrolysis of each separated ester will yield the ‘resolved’ (enantiomerically-pure) alcohols.

Chromatography can also be used to separate a racemic mixture. Using chiral column chromatography or gas chromatography, a chiral stationary phase that will only bind to the R or S confirmation is used to isolate one of the confirmations. This can be used with a suitable solvent to resolve the racemic mixture.

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Sample Test C/P Section Passage 8 Question 41

Key Points

• Enantiomers have similar physical and chemical properties so racemic mixtures are hard to separate.

• One method of separation (resolving) involves reacting the racemic mixture with a chiral compound to create diastereomers which have different physical and chemical properties and can be separated. Then a reverse reaction is carried out to isolated the initial enantiomer.

• Chiral chromatography can also be used to separate racemic mixtures. A chiral stationary phase is used to react with one of the enantiomers to separate the racemic mixture in the stationary phase.

Key Terms

Racemic mixture: has equal amounts of left- and right-handed enantiomers of a chiral molecule. Also called a racemate

Enantiomer: each of a pair of chiral molecules that are mirror images of each other, but not superimposable

Plane-polarized light: light whose electric field oscillates in just one plane

Siastereomers: stereoisomers that are not mirror images of one another and are non-superimposable on one another

Optically Inactive: a compound incapable of optical rotation

Resolution: a process for the separation of racemic compounds into their enantiomers

Chiral: asymmetric in such a way that the structure and its mirror image are not superimposable

Hydrolysis: breaking of a bond using water

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