What is Racemic Mixture

What is Racemic Mixture?

In chemistry, a racemic mixture, or racemate, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.

If you are not getting the point from the above definition, than let’s start with the basics.

• ·       An organic compound that contains a chiral carbon atom usually shows two non-superposable structures.
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• ·       These two structures are mirror images of each other and are, thus, commonly called enantiomers.
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• ·       These enantiomers refer identical physical and chemical properties except for their ability to rotate plane-polarized light (+/−) by equal amounts but in opposite directions.  
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• ·       The enantiomer that rotates plane polarized light to right side is known as dextro or right handed isomer. Such enantiomer is denoted by the prefix (+) or D Or d. On the other hand, the enantiomer that rotates light to left side is known as levo or left handed isomer. Such enantiomer is denoted as prefix (-) or L or l.

 

Now when equal amounts of left and right handed enatiomers of a chiral molecule are mixed together they form a racemic mixture.

Nomenclature

Since in a racemic mixture, ratio of the both right and left handed enatiomers are equal i.e. 1:1, hence it is denoted by the prefix (±)- or dl.   (for sugars the prefix dl- may be used),

Also the prefix rac- (or racem-) or the symbols RS and SR (all in italic letters) are used.

If the ratio is not 1:1 (or is not known), the prefix (+)/(−), d/l- or d/l- (with a slash) is used instead.

However the usage of d and l is strongly discouraged by IUPAC. 

Properties

• ·       Although the two enantiomers rotate plane-polarized light in opposite directions, a racemate or racemic mixture is optically inactive, meaning that there is no rotation of plane-polarized light. This is because, rotation of plane polarized light into two opposite directions and in equal amounts, cancels the net rotation. 
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• ·       In contrast to the two pure enantiomers, which have identical physical properties except for the direction of rotation of plane-polarized light, a racemate sometimes has different properties from either of the pure enantiomers.
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• ·       Different melting points are most common, but different solubilities and boiling points are also possible.
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• ·       Pharmaceuticals may be available as a racemate or as the pure enantiomer, which might have different potencies.

Crystallization

There are four ways in which a racemate can crystallize, three of which H. W. B. Roozeboom had distinguished by 1899:

• Conglomerate (sometimes racemic mixture or racemic conglomerate)

• Racemic compound (sometimes true racemate)

• Pseudoracemate (sometimes racemic solid solution)

• Quasiracemate

Resolution

The separation of a racemate into its components, the pure enantiomers, is called achiral resolution. There are various methods, including crystallization, chromatography, and the use of enzymes.

 The first successful resolution of a racemate was performed by Louis Pasteur, who manually separated the crystals of a conglomerate.

Synthesis

• Without a chiral influence i.e. without a chiral catalyst, solvent or starting material, a chemical reaction that makes a chiral product will always yield a racemate.
• That can make the synthesis of a racemate cheaper and easier than making the pure enantiomer, because it does not require special conditions.
• Thr reagents of, and the reactions that produce, racemic mixtures are said to be “not stereospecific” or “not stereoselective,” for their indecision in a particular stereoisomerism.