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Stereogenic or Chiral Center

Article ID: 136
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

A stereogenic center is also known as a chiral center.  It is characterized by an atom which has (typically four) different groups bound to it in such a manner that its mirror image is non-superimposable.  Below are the enantiomers of 2-chlorobutane, notice the stereogenic center on both molecules.  Look at the 3D molecule on ChemSpider and try to match it to the R or S enantiomer of 2-chlorobutane.  If you're struggling, try the "Labeling Enantiomers" article linked at the bottom of this page.

     




    (R)-2-chlorobutane 
     
    (S)-2-chlorobutane 
     

Note you should be learning to relate the 3D model images to the 2D drawn images.  More importantly you should be able to reproduce and manipulate the 2D drawn images as that is what you will be expected to do on exams or assignments!

The presence of a single stereogenic center in a molecule always results in a chiral molecule (which will produce only two enantiomers).

Molecules can, however, possess more than one stereogenic center.  Such molecules may or may not be chiral.

Isomers of chiral molecules that possess two or more stereogenic centers, may be either enantiomers or diastereomers (stereoisomers that are not mirror images).  For example, the two molecules below are stereoisomers but are not mirror images of each other, known as diastereomers.  Note: the enantiomer of (S,R)-2-bromo-3-chlorobutane would be (R,S), where both stereogenic centers are opposite.

 




(S,R)-2-bromo-3-chlorobutane  (R,R)-2-bromo-3-chlorobutane 

Diastereomers can have quite different physical and chemical properties from one another.

Isomers of achiral molecules, that possess two or more stereogenic centers, are known as meso isomers (stereoisomers that are superimposable).  You should be able to draw a pair of meso isomers and see how one relates to the other by simple rotation.


Image A above is the mirror image of B, but can be seen to be the same as B once it has been rotated.  A quick way of recognizing whether a molecule is achiral is to look for a plane of symmetry.
 

The most prevalent stereogenic centers in organic chemistry are carbon atoms, which have four different groups bound to them.  However, any atom which has four different groups bound to it is a stereogenic center.  The more common of these atoms, with which an organic chemist should be familiar, are Si, N and P.  They may be tetrahedral molecules (where the four different groups are atoms) or trigonal pyramidal molecules (where a lone pair is included as one of the four different groups).

CAUTION:  many trigonal pyramidal molecules (especially those of nitrogen) exhibit rapid pyramidal inversion:


In this specific case, even though a stereogenic center is present, the molecules are optically inactive as the optical activity of the two extremes of inversion averages out due to the rapid inversion.

The larger the atom, though, the slower the pyramidal inversion and as a result many optically active compounds for P and S have been prepared.

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Enantiomers
Labeling Enantiomers

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Enantiomers     Optical Activity

Department of Chemistry & Chemical Biology