World Library  
Flag as Inappropriate
Email this Article

Racemization

Article Id: WHEBN0000518791
Reproduction Date:

Title: Racemization  
Author: World Heritage Encyclopedia
Language: English
Subject: Stereochemistry, Protein primary structure, Post-translational modification, Aminolysis, Hydroxybenzotriazole
Collection: Chemical Reactions, Posttranslational Modification, Proteins, Stereochemistry
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Racemization

In chemistry, racemization refers to the conversion 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 resulting sample is described as racemic or a racemate.[1]

Contents

  • Stereochemistry 1
  • Physical properties 2
  • Biological significance 3
  • Formation of racemic mixtures 4
  • References 5
  • See also 6

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, 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.

Racemization occurs when one pure form of an enantiomer is converted into equal proportion of both enantiomers, forming a racemate. When there are both equal numbers of dextrorotating and levorotating molecules, the net optical rotation of a racemate is zero.[1]

Physical properties

Racemate may have different physical properties from either of the pure enantiomers because of the differential intermolecular interactions. The change from a pure enantiomer to a racemate can change its density, melting point, solubility, heat of fusion, refractive index, and its various spectra. Crystalization of a racemate can result in separate (+) and (−) forms, or a single racemic compound.

Biological significance

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. 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.

Racemization of pharmaceutical drugs, however, can occur in vivo. An example is thalidomide: its (R) enantiomer is effective against morning sickness, while the (S) enantiomer is teratogenic, causing birth defects. If only one enantiomer is administered to a human subject, both forms may be found later in the blood serum.[2] The drug is therefore not considered safe for use by women of child-bearing age, and while it has other uses, its use is tightly controlled.[3][4]

Formation of racemic mixtures

On a trivial level, 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.[5] 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 unimolecular substitution reactions, lead to non-stereospecific addition of substituents which results in racemization.
  • Although unimolecular elimination reactions also proceed through a carbocation, they do not result in a chiral center. They result instead in a set of geometric isomers in which trans/cis or E/Z forms are produced, rather than racemates.
  • In an 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.[6]
  • 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.[7]

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. This technique is known as amino acid dating.

References

  1. ^ a b Andrew Streitwieser, Clayton H. Heathcock (1985). Introduction to Organic Chemistry (3rd ed.). Maxwell MacMillan. pp. 122–124.  
  2. ^ Teo SK, Colburn WA, Tracewell WG, Kook KA, Stirling DI, Jaworsky MS, Scheffler MA, Thomas SD, Laskin OL (2004). "Clinical pharmacokinetics of thalidomide". Clin Pharmacokinet. 43 (5): 311–327.  
  3. ^ Sheryl Gay Stolberg (17 July 1998). "Thalidomide Approved to Treat Leprosy, With Other Uses Seen". New York Times. Retrieved 8 January 2012. 
  4. ^ "Use of thalidomide in leprosy". WHO:leprosy elimination. WHO. Retrieved 22 April 2010. 
  5. ^ Andrew Streitwieser, Clayton H. Heathcock (1985). Introduction to Organic Chemistry (3rd ed.). Maxwell MacMillan. p. 373.  
  6. ^ Jerry March (1985). Advanced Organic Chemistry: reactions, mechanisms, and structure (3rd ed.). John Wiley & Sons. pp. 517–518.  
  7. ^ Jerry March (1985). Advanced Organic Chemistry: reactions, mechanisms, and structure (3rd ed.). John Wiley & Sons. p. 610.  

See also

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from Hawaii eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.