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Chapter 7: Stereochemistry



Summary | Isomer types | Enantiomers | The Stereogenic center | Optical Activity | Absolute Relative Configuration | Cahn-Ingold-Prelog R/S notation | Fischer projections | Self Assessment | Quiz |


The Stereogenic Center

Chapter 7 : Stereochemistry

A stereogenic center is also known as a chiral center.  It is characterized by an atom which has  different groups bound to it in such a manner that its mirror image is non-superimposable.  Below are the enantiomers of 2-chlorobutane.

     
    (R)-2-chlorobutane 
     
    (S)-2-chlorobutane 
     
Note you should be learning to relate the chime images to the drawn images.  More importantly you should be able to reproduce the 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 results in a chiral molecule.

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).

 
(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.
 

Carbon-based Stereogenic Centers:

The most prevalent stereogenic centers in organic chemistry are carbon atoms, which have four different groups bound to them.
 

Stereogenic Centers other than Carbon:

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 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.

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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