In chemistry, 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.
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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, 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 allenes 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 are held in place and do not slide past each other.
Diastereomers have similar chemical properties yet have differing physical properties. Via processes such as crystallization one diastereomer can be isolated.
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 is often dispensed as racemic salts while the more active dextroamphetamine is reserved for refractory cases or more severe indications; another example is methadone, of which one isomer has activity as an opioid agonist and the other as an NMDA antagonist.
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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