Racemic

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In chemistry, a racemate is a mixture of equal amounts of left- and right-handed enantiomers of a chiral molecule, such a mixture is called racemic.

The first known racemate was 'racemic acid', which Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.

Contents

1 Properties
2 Crystallization
3 Resolution
4 Synthesis
5 See also

Properties

A racemate is optically inactive: because the two isomers rotate plane-polarised light in opposite directions, a racemic mixture does not rotate plane-polarised light.

In contrast to the two separate enantiomers, which generally have identical physical properties, a racemate often has different properties compared to either one of the pure enantiomers. Different melting points and solubilities are very common, but different boiling points are also possible.

Pharmaceuticals may be available as a racemate or as pure enantiomer, which might have different potencies. It is nowadays regulated by law that both enantiomers have to be tested separately for their pharmacological effects (see Enantiomer).

Crystallization

There are three ways a racemate can crystallize, this is important for the resolution of a racemate by crystallisation. Precise ways to distinguish between these crystal forms were summarised already in 1899 by H. W. B. Roozeboom.

Conglomerate (sometimes racemic mixture or racemic conglomerate)

A mechanical mixture of enantiomerically pure crystals of one enantiomer and its opposite. Molecules in the crystal structure have higher affinity to the same enantiomer than to the opposite enantiomer. The melting point of the racemic conglomerate is always lower than the pure enantiomer. Addition of a small amount of one enantiomer to the racemic compound increases the melting point.

Racemic compound (sometimes true racemate)

Molecules have a higher affinity to the opposite enantiomer than to the same enantiomer, the substance forms a single crystalline phase in which the two enantiomers are present in an ordered 1:1 ratio in the elementary cell. By addition of small amount of one enantiomer to the racemic compound, the melting point decreases. But the pure enantiomer can have higher or lower melting point than the racemic compound.

Pseudoracemate (sometimes racemic solid solution)

Unlike the racemic compound or conglomerate, there is no big difference in affinity between the same and opposite enantiomers. Overall, both enantiomers occur in equal proportions in the crystal but they coexist in an unordered manner in the crystal lattice. By addition of small amount of one enantiomer to the racemic compound, the melting point changes just little bit, or not at all.

Resolution

The separation of a racemate into its components, the pure enantiomers, is called a 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.

Synthesis

Without a chiral influence (for example 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. This fact also leads to the question how biological homochirality evolved on a presumably racemic primordial earth.

The 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.

Racemic

Properties

Crystallization

Resolution

Synthesis

See also