Basic Organic Nomenclature

An Introduction

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

7. Stereochemistry (iii)

V. Naming Enantiomers
Chiral Molecules
With No Chirality Centre

(ii) Helicity and Planar Chirality



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.

2. Helicity.

Helicity is the chirality due to a helical (propeller or screw) shaped structure for a molecule.

Here are two examples with their full names:


As you see from the examples, you arrange the molecule so that you are looking down its chiral axis, then follow the spiral:
clockwise to give P
anticlockwise to give M.

3. Planar Chirality

In this case the molecule contains a group of bonds in a plane with the chirality resulting from the arrangement of the out of plane molecules.

An example of this is given by the molecule (E)-cyclooctene:

(E,P)-Cyclooctene (E,M)-Cyclooctene

In this case, put the plane of the molecule on the top and see that the bonds spiral away and down from the double bond in a clockwise direction for the left model, and in an anticlockwise direction for the right model.

Incidentally, this molecule has featured in work that seems to give a clue as to how an exess of one enantiomer over another could have arisen naturally. See chiral hydrocarbon.

Review Chiral Molecules with no Chirality Center (i)

Next Page: Heterocyclics (i) Common Names


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