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.

The allenes (C=C=C) are one group of coumpounds showing this type of chirality.

a tetrasubstituted allene	and its mirror image

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:

To determine which enantiomer is which:

1. Separately, assign the two groups at each end of the axis a priority using the Cahn-Ingold-Prelog rules.

 

2. Look at the molecule down the chiral axis.

 

3.1. To assign the symbol R_a or S_a:

Trace from the group of higher priority on the front carbon to the group with lower priority on the front carbon, then on to the group of higher priority on the back carbon and thence to the group of lower priority. A clockwise (right) turn is assigned R_a; an anticlockwise (left) turn is assigned S_a.

 

3.2. To assign the symbol P or M:

Trace from the group of higher priority on the front carbon to the group of higher priority on the back carbon. A clockwise (right) turn is assigned P; an anticlockwise turn is assigned M.

 

Following this through for the enantiomers of penta-2,3-diene:

Step	Left Model	Right Model
-
1	At each end: methyl higher, hydrogen lower priority
2
3.1
	Motion is clockwise Ra enantiomer.	Motion is anticlockwise Sa enantiomer.
3.2
	Motion is anticlockwise M enantiomer.	Motion is clockwise P enantiomer.

See if you can follow the naming of the two enantiomers of 3-chloro-5-methylhepta-3,4-diene.

(P)- or (Sa)-3-Chloro-5-methylhepta-3,4-diene		(M)- or (Ra)-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 (R_a) one?

Next page: Chiral Molecules with no Chirality Center (ii).