E/Z cis / trans – geometric isomers

The E/Z or cis / trans system can be used to name geometric isomers. Geometric isomers are molecules that have groups above and below the plane of a ring or double bond. It is important to consider and be aware of these isomers as even though the structural formula is the same, the molecules will have different physical properties and can have different chemical properties.

So to make life easier, we have two systems for distinguishing between them. The cis trans system is great – it is easy to understand and allows you to quickly decide what a molecule is. However, it has some limitations. If you have two of the same groups to consider it is fairly obvious what the isomer is – if both groups are on the same side, the type of isomer will be cis, if the groups are on opposite sides the isomer will be trans.

A classic example is but-2-ene where there are two methyl groups that can be on the same side or different sides.

However, this model begins to break down if the groups are not the same, or if there are more groups to consider.

For example, take 3,4-dimethylhept-3-ene (!):

Is the example shown cis or trans? Do we consider the methyl groups when naming this or the propyl and ethyl group as they are larger? Our system has broken down.

To counter these difficulties, we use the E / Z system. This was a system put together by Cahn, Ingold and Prelog and is sometimes referred to as the CIP system. A simplified version of the system (one I explain to my classes) involves assigning ‘priorities’ to the two groups attached to a carbon atom. Essentially, the group with the highest Mr is given priority. Once you have carried this out for both carbon atoms that are adjacent to each other, look to see where the groups with priority are located with regards to the ring or double bond. If they are on the same side, use ‘Z’ and on opposite sides, use ‘E’.

The letters are actually from the German word for together (zusammen) and opposite (entgegen)

As I wrote, this is a slightly simplified version of how to use the system – the actual system is slightly more complicated, but my way of using it seems to work for IB.

On the whole, it is better to use the E/Z system, but interestingly, there are some example of molecules that can be both cis and E or tans and Z (ie, together and opposite at the same time). Take for example 3-fluoro-4-chlorohex-3-ene:

Using the cis trans system, this molecule is trans as the ethyl groups are opposite each other but with the CIP system, the molecule is Z as the two highest priority groups are on the same side!

And the point of this? Well, I suspect at some time in the future there will be an exam question on this, asking you to discuss / evaluate the strength of each naming system.

Are there any tricky topics like this that you predict will come up in future exams? If you have any ideas, please do share them with us below!