Posted by: Mark Foreman | January 28, 2010

Symmetry how to spot it in a molecule

Proper rotation

Proper rotation, imagine benzene which is a hexagonal molecule, if you rotate the molecule you can see how it is possible to rotate the molecule by 60 degrees to get the atoms in exactly the same place. In the diagram below the axis which the object can be rotating about is marked out with the gold dumbbell.


This is a C6 axis because the object can be rotated six times before it has been rotated a total of 360 degrees (2p radians). When you look at a shape search for an axis of rotation, if you can find one then the object is symmetric.

NB Ignore the idea of a C1 axis, all objects can be spun around by 360 degrees to make exactly the same object.

Mirror planes (plane of symmetry)

Look at the object and think is the object symmetric about a plane or line. For the purposes of teaching let us think about naphthalene.

For instance naphthalene has three mirror planes two of which are marked on the diagram in blue and green. The final mirror plane (red) runs through all the atoms.




NB Flat objects are symmetric as if all the atoms in an object are in the same plane, then this plane is a mirror plane.

Improper rotation

This is a hard one; it may be redundant if another form of symmetry exists in a molecule.

But is it as follows, if you can rotate the object and then reflect it in a plane perpendicular to the axis of rotation to get the same shape as before then you have an improper rotation axis. We will use tetrahedral methane as an example. We will ignore the four C3 axes and concentrate on the improper rotation only.

If the methane is rotated through 180o (pi radians) about the golden axis and then reflected using a mirror plane placed where the blue frame indicates a plane to exist then the methane can be converted into its self.

Centre of inversion

This is another hard one. We will use 1,2-dichloro-1,2-difluoroethane as a teaching example to help explain this.

Imagine a point at the centre of a molecule, where each atom on one side of the molecule has another atom diametrically opposite this point at equal distance. For example the anti conformation of 1,2-dichloride-1,2-difluoroethane has a centre of symmetry. Below the centre of symmetry has been added to the molecule and lines linking the atoms and the centre have been added.

I will try and soon give you a guide to how to rotate molecules on paper and look at their stereocentres.


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