Resolution of racemic mixture (dextro and laevo)

Separation of Dextro and Laevo components

The synthesis of optically active compounds in the laboratory usually results in racemic mixture. The d and l forms can be separated from the racemic mixture. The separation of a racemic mixture in to dextro and laevo components is termed resolution. Due to identical physical properties of optical isomers their separation cannot be effected by simple physical methods. Usual methods which have been used for resolving racemic compounds are Mechanical Separation, Biochemical separation and by means of salt formation.

Methods of separation of a racemic mixture in to dextro and laevo components

1. Mechanical Separation

When the two varieties of isomers form well defined crystals they can be separated by hand picking. The crystals of Sodium ammonium racemate can be separated by this method.

2.Biochemical Separation

In this method certain micro organism such as mould, bacteria or fungi when allowed grow in a solution of racemic mixture destroy one of the optical isomers at a much quicker rate than the other due to selective assimilation. When penicillium glaucum is allowed to grow in a solution of ammonium tartrate, it destroys the dextro isomer leaving the laevo isomer.

3. By means of Salt Formation

This is an effective method for resolution. Here the isomers of racemic mixture are converted to their salts with an optically active acid or base. The two salts obtained often differ in their solubilities  and can be sseparated by fractional crystallisation. The salts on treatment with acid or base regenerate the optically active reagent.

For more visit Optical Isomerism

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