Basic Organic Nomenclature
7. Stereochemistry (iii)
|Although chiral molecules containing chirality
carbon centres are by far the most usual, chiral molecules without a
chirality centre exist.
Remember that the definition of chirality requires only that the mirror image of a molecule be non-superimposible on that molecule.
These pages give the briefest of introductions to these molecules.
1. Axial Chirality.
The allenes (C=C=C) are one group of coumpounds showing this type of chirality.
The asymmetry in this type of molecule arises because the pi orbitals of the two double bonds on the same carbon (the one represented by the dot in the above line diagrams) are at right angles to each other. This means that the sigma bonds at the ends of the allene system must be in planes which are also at right angles to each other, in effect a tetrahedral centre elongated into a line.
One difference from a chirality centre that occurs with axial chirality is that the substituents, which must be different when attached to one end of the axis, may be the same pair at each end of the axis (as above). (In some ways this is similar to the criterion for E/Z stereoisomers at a double bond.)
An example of an allene enantiomeric pair is given by penta-2,3-diene:
Following this through for the enantiomers of penta-2,3-diene:
See if you can follow the naming of the two enantiomers of 3-chloro-5-methylhepta-3,4-diene.
One other type of molecule with axial chirality is the 2,6,2,'6'-substituted biphenyl group in which the substituents interfere with the free rotation about the sigma bond between the two phenyl groups.
This phenomenon is named atropisomerism.
An example of such a molecule is 2-(2'Carboxy-6'-nitrophenyl)-3-nitrobenzoic acid:
In this molecule the carboxyl and nitro groups are physically bulky enough to interfere with each other such that they prevent free rotation about the benzene to benzene sigma bond and thus lead to axial chirality.
Can you work out that the enantiomer modelled is the M (Ra) one?
|Review R/S (ii) More than
One Chirality Centre.
Next page: Chiral Molecules with no Chirality Center (ii).
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