RACEMIC MIXTURE SEPARATION(MENTHOL)

Madhura N. Chincholi

14CHE2032

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RACEMIC MIXTURE?

A solution in which both enantiomers of a compound are present in equal amounts

Racemic mixtures can be symbolized by a (d/l)- or (+/-)- prefix in front of the substance's name.

The separation of a racemic mixture in to dextro and laevo components is termed resolution.

Due to identical physical properties their separation cannot be effected by simple physical methods.

Usual methods used for resolving racemic compounds are Mechanical Separation, Biochemical separation and by means of salt formation.  2

RESOLUTION BY CRYSTALLIZATION

The racemic compound is reacted with a resolving agent

So the mixture contains three chiral compounds: the two enantiomers and another chiral compound.

At this time the more stable diastereoisomer crystallizes and it can be separated from the

non racemic enantiomeric mixture by several methods.

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The essential processes of enantiomeric separation starting from the racemic compound

MENTHOL

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(+)-Menthol (-)- Menthol

IUPAC name : (1R,2S,5R)-2-Isopropyl-5-methylcyclohexanol

•Menthol has local anesthetic and counterirritant qualities.•Widely used to relieve minor throat irritation.

RESOLUTION OF RACEMIC MENTHOL(MEN)

1. Crystallization without any solvent The mixture of resolving agent(RA) and

racemic compound should be warmed until it melted

Then it should be cooled while one of the diastereoisomers crystallizes from the melt.

RA : O,O’-dibenzoyl-(R,R)-tartaric acid ((R,R)-DBTA).

The crystalline molecular complex (diastereoisomer) contains the (1R,2S,5R)-menthol (the Lmenthol(L-MEN))

While the remained melt is enriched in the other enantiomer.

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Crystallization without any solvent

2. Crystallization with solvent

RA : (R,R)-DBTA. Solvent : Hexane The crystalline molecular complex

(diastereoisomer) contains the (1R,2S,5R)-menthol (the Lmenthol(L-MEN))

While the other enantiomer reamined in the solvent.

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Crystallization with solvent (hexane)

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REFERENCE

E. Pálovics, F. Faigl and E. Fogassy, "Separation of the Mixtures of Chiral Compounds by Crystallization", Journal of Advances in Crystallization Processes, Page 1-37.

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THANK YOU!

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