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.
- · These two structures are mirror images of each other and are, thus, commonly called enantiomers.
- · These enantiomers refer identical physical and chemical properties except for their ability to rotate plane-polarized light (+/−) by equal amounts but in opposite directions.
- · 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.
- · 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.
- · Different melting points are most common, but different solubilities and boiling points are also possible.
- · 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.
