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 diastereomers.
In order for a point of asymmetry to exist there must be four different groups attached to the central atom (the chiral center). 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.
Diastereomers have similar chemical properties yet have differing physical properties. Via processes such as crystallization one diastermer can be isolated.
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.
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