racemization

In chemistry racemization refers to partial conversion of one enantiomer into another.

Stereochemistry

Chiral 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, while 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's 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 allenes where 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 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 one diastereomer can be isolated.

Biological importance

Generally only one form of a chiral molecule will participate in biochemical reactions while the other simply does not participate or can cause sideffects in the form of side reactions. Of note, the L form is usually the biologically reactive form.

Formation of racemic mixtures

Substitution reactions that proceed through a carbocation intermediate (such as unimolecular substitution reactions) lead to the non-stereospecific addition of substituents. While unimolecular elimination reactions 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.

See also

• Racemic

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