In chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, racemization refers to the converting of an enantiomerically pure mixture (one where only one enantiomer is present) into a mixture where more than one of the enantiomers are present. If the racemization results in a mixture where the enantiomers are present in equal quantities, the mixture is described as racemic.


Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

 molecules have two forms (at each point of asymmetry), which differ in their optical characteristics: The levorotatory form (the (−)-form) will rotate the plane of polarization of a beam of light to the left, whereas the dextrorotatory form (the (+)-form) will rotate the plane of polarization of a beam of light to the right. The two forms, which are non-superimposable when rotated in 3-dimensional space, are said to be enantiomers.

A common misconception is that a point of asymmetry exists only if there are four different groups attached to the central atom (the chiral centre). Although this particular case always gives rise to chirality, other configurations may also make something chiral. When considering chemical stereochemistry, it is important to recall that molecules exist in 3-dimensional space, and as such can have different arrangement of the atoms in that space. Notable examples include substituted allene
An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are...

s in which the pendants of one end are oriented at right angle with those of the other end. Also, due to steric barriers, the two nathphyl groups of BINAP
BINAP is an abbreviation for the organophosphorus compound 2,2'-bis-1,1'-binaphthyl. This chiral ligand is widely used in asymmetric synthesis. It consists of a pair of 2-diphenylphosphinonaphthyl groups linked at the 1 and 1´ positions. This C2-symmetric framework lacks stereogenic atom, but...

 are held in place and do not slide past each other.

Physical properties

Diastereomers have similar chemical properties yet have differing physical properties. Via processes such as crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

 one diastereomer can be isolated.

Biological importance

In general, only one form of a chiral molecule will participate in biochemical reactions while the other simply does not participate or can cause side-effects in the form of side reactions. Of note, the L form of amino acids and the D form of sugars (primarily glucose) are usually the biologically reactive form. Additionally, many psychotropic drugs show differing activity or efficacy between isomers, e.g. amphetamine
Amphetamine or amfetamine is a psychostimulant drug of the phenethylamine class which produces increased wakefulness and focus in association with decreased fatigue and appetite.Brand names of medications that contain, or metabolize into, amphetamine include Adderall, Dexedrine, Dextrostat,...

 is often dispensed as racemic salts while the more active dextroamphetamine
Dextroamphetamine is a psychostimulant drug which is known to produce increased wakefulness and focus as well as decreased fatigue and decreased appetite....

 is reserved for refractory cases or more severe indications; another example is methadone
Methadone is a synthetic opioid, used medically as an analgesic and a maintenance anti-addictive for use in patients with opioid dependency. It was developed in Germany in 1937...

, of which one isomer has activity as an opioid agonist and the other as an NMDA antagonist.

Formation of racemic mixtures

Substitution reactions that proceed through a carbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...

 intermediate (such as unimolecular substitution reactions
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...

) lead to the non-stereospecific addition of substituents. While unimolecular elimination reactions
Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...

 also proceed through a carbocation, they do not result in a chiral center. Rather, they result in a set of geometric isomers in which trans/cis or E/Z forms will result

The rate of racemization (from L-forms to a mixture of L-forms and D-forms) has been used as a way of dating biological samples in tissues with slow rates of turnover, forensic samples, and fossils in geological deposits .
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