Home Molecules Isomerism Cis-trans isomerism

Creative Chemistry Home Page

Cis–trans isomerism

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.

mouse_icon.gifDouble tap to stop and restart each animation.
×

Inorganic compounds may also have cis–trans isomers.

cisplatinFor example, cisplatin (an anticancer drug) is the complex ion [Pt(NH3)2Cl2].

Transplatin does not have the same medical effect.

chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

cis-but-2-ene

trans-but-2-ene

Structures

Name of isomer
Structure
cis-but-2-ene cis-but-2-ene.jpg
trans-but-2-ene trans-but-2-ene.jpg
but-1-ene but-1-ene

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.

Nomenclature

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.