Resolution of Racemic Mixtures

Resolution of Racemic Mixtures


1 Answers

Sachin Tyagi
31 Points
14 years ago

when an optically active compound is synthesized, the product formed is found to be an optically inactive racemic miture containing equal proportions of both d and l forms. The process of separation of a recemic mixture into d and l forms is called resolution.

The main difficulty in a process of resolution is that d and l forms have identical physical and chemical properties, so they cannot be separated by ordinary methods. However, the following methods can be used for this purpose.

(i)Mechanical separation:- if the d and l forms of a substance exits in well defined crystalline forms, the separation can be done by hand picking with the help of magnifying lens and a pair of tweezers. For example, the d and l forms of sodium ammonium tartarate can be separated by this method. The method has very limited application and applies to only few crystalline constituents having different shape.

(ii) Biochemical separation:- in this method, the resolution is done by the use of micro organisms. When certain bacteria or moulds are added to a solution of a racemic mixture, they decompose one of the optically active forms more rapidly than the other. For example, when the mould, racemic ammonium tartarate, the mould completely decomposes the d form white l form is left practically unaffected. The main drawback of the method is that half of the material is destroyed during resolution. The process is very slow and only small amounts of the materials can be separated.

(iii) chemical separation:- this is probably the best method of resolution. The racemic mixture is made to combine with another optically active compound and the resulting solubility in various solvents. By fractional crystallization from a suitable solven, they can be separated. For example, the racemic mixture of lactic acid is allowed to combine with the optically active base (-) strachnine or (+) brucine.

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