Editing Racemization (section)

== Formation of racemic mixtures ==
Racemization can be achieved by simply mixing equal quantities of two pure enantiomers.  Racemization can also occur in a chemical interconversion.  For example, when (''R'')-3-phenyl-2-[[butanone]] is dissolved in aqueous ethanol that contains [[NaOH]] or [[HCl]], a racemate is formed.  The racemization occurs by way of an intermediate [[enol]] form in which the former stereocenter becomes planar and hence achiral.<ref name = "Streitwieser_1985">{{cite book| vauthors = Streitwieser A, Heathcock CH |author-link1 = Andrew Streitwieser |author-link2 = Clayton Heathcock|title = Introduction to Organic Chemistry|edition = 3rd|publisher = [[Macmillan Publishers|Maxwell MacMillan]]|year = 1985|isbn = 978-0-02-946720-6 |url-access = registration|url = https://archive.org/details/introductiontoor0003stre}}</ref>{{rp|373}} An incoming group can approach from either side of the plane, so there is an equal probability that [[protonation]] back to the chiral ketone will produce either an ''R'' or an ''S'' form, resulting in a racemate.

Racemization can occur through some of the following processes:
* Substitution reactions that proceed through a free [[carbocation]] intermediate, such as [[SN1 reaction|unimolecular substitution reactions]], lead to non-stereospecific addition of substituents which results in racemization. 
* Although [[Elimination reaction|unimolecular elimination reactions]] also proceed through a carbocation, they do not result in a chiral center.  They result instead in a set of [[Cis–trans isomerism|geometric isomers]] in which ''trans''/''cis'' (''E''/''Z'') forms are produced, rather than racemates.
* In a unimolecular aliphatic [[electrophilic substitution]] reaction, if the [[carbanion]] is planar or if it cannot maintain a pyramidal structure, then racemization should occur, though not always.<ref name = "March_1985">{{cite book| vauthors = March J |author-link = Jerry March|title = Advanced Organic Chemistry: reactions, mechanisms, and structure|publisher = [[John Wiley & Sons]]|edition = 3rd|year = 1985|isbn = 978-0-471-85472-2 }}
</ref>{{rp|517–518}} 
* In a [[free radical substitution]] reaction, if the formation of the free radical takes place at a chiral carbon, then racemization is almost always observed.<ref name = "March_1985" />{{rp|610}}

The rate of racemization (from <small>L</small>-forms to a mixture of <small>L</small>-forms and <small>D</small>-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. This technique is known as [[amino acid dating]].