Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

7. Stereochemistry (iii)
Chirality

V. Naming Enantiomers
Chiral Molecules
With No Chirality Centre

(i)Axial Chirality

Index


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.

If a chirality centre is considered a point causing chirality, axial chirality can be imagined as being a line causing chirality, as if the point had been pulled into a line.

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 name these two enantiomers the symbols Ra/Sa (a for axial) or P/M (for plus/minus) are used.

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 Ra or Sa:

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 Ra; an anticlockwise (left) turn is assigned Sa.

 
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 (Ra) one?

Review R/S (ii) More than One Chirality Centre.

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


Index

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