What is it?
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, which does not form these isomers.
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.
⚠ 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.
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 three or four 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.