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Chapter 7 : Stereochemistry |  |
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Chapter 7 : Stereochemistry | |
So what structural feature allows a molecule to exist as a pair of enantiomers
?
It requires the presence of a chirality center which we will
explore here. Terms such as an asymmetric, stereogenic or
chiral
center have been used in the past.
In its simplest and most common case, chirality center is characterised
by an atom that has four different groups bonded to it in such a
manner that it has a non-superimposable mirror image. The enantiomers
of 2-chlorobutane we saw previously are shown below.
Which atom is the chirality center ?
What are the four groups that are attached ?
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 since you may be expected to do this on examinations or assignments!
(R)-2-chlorobutane
(S)-2-chlorobutane
The presence of a single chirality centre in a molecule results in a chiral molecule fundamentally because the top, bottom, left, right, front and back faces are all different.
Molecules can (of course) possess more than one chirality center. Such molecules may or may not be chiral.
Chiral molecules that possess two or more chirality centers, have stereoisomers that may be either enantiomers or diastereomers. Diastereomers are stereoisomers that are not mirror images.
Diastereomers can have quite different physical and chemical properties from one another. This is important as it allows them to be separated. Do you remember the properties of enantiomers ?
(S,R)-2-bromo-3-chlorobutane (R,R)-2-bromo-3-chlorobutane
Isomers of achiral molecules, that possess two or more chirality 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 recognising whether a molecule is achiral is to look for a plane of symmetry.
Carbon-based Chirality Centers
The most prevalent chirality centers in organic chemistry are carbon atoms, which have four different groups bonded to them.
Chirality Centers other than Carbon
Any atom which has four different groups bonded to it is a chirality center. The more common of these atoms, with which an organic chemist should be familiar, are Si, N and P (note: a lone pair is included as one of the four different groups) They may be tetrahedral molecules 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 chirality center is present in each molecule, the sample is optically inactive since the optical activity of the two extremes of 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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