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

(ii) Helicity and Planar Chirality

Index
Reminder:

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:

(M)-Hexahelicene
(P)-Hexahelicene
(P)-Tetradecahelicene

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

Index

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