Stereoisomerism occurs when substances have the same molecular formula, but a different arrangement of their atoms in space. Cis-trans isomerism is one type of this isomerism. It applies to:
- alkenes and other organic compounds that contain C=C bonds
- cyclic alkanes.
In A Level Chemistry, you only need to know about cis-trans isomerism due to the presence of a C=C bond.
What is here?
You can see models of the two cis-trans isomers of but-2-ene. This alkene has cis-trans isomers because each carbon atom involved in the C=C bond has two different groups attached.
For comparison, you can also see a model of but-1-ene.
You should be prepared to identify cis-trans isomers for simple organic compounds like these for your examinations, and you should also be able to name them.
Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.
The name cis or trans depends on where the identical groups are located:
- cis if they are on the same side of the C=C bond
- trans if they are on opposite sides of the C=C bond.
So in cis-but-2-ene, the two —CH3 groups are on one side and the two H atoms are on the other side. In trans-but-2-ene, each of these are on opposite sides.
What happens if there are more than different groups?
In this case, you cannot use the cis–trans naming system. Instead, you must use the E–Z naming system. This is more complicated but much more flexible.
But-1-ene does not have geometrical isomers, even though it has a C=C bond. One of the C atoms in this bond has two identical groups (H atoms) attached.