E-Z isomerism

Stereoisomerism occurs when substances have the same molecular formula, but a different arrangement of their atoms in space. E-Z isomerism is one type of this isomerism. It applies to:

  • alkenes and other organic compounds that contain C=C bonds
  • cyclic alkanes.

The cistrans naming system for geometrical isomers cannot cope with complex situations. For example, where it is not obvious which groups are on the same side of the C=C bond, or opposite to each other. This is where the E-Z system comes to the rescue.

In A Level Chemistry, you only need to know about E-Z isomerism due to the presence of a C=C bond.

You need to work out which of the two groups attached to each C atom in the C=C bond has the higher priority. When this is done:

  • in an E isomer, the higher priority groups are on either side of the C=C bond
  • in a Z isomer, the higher priority groups are on the same (‘zame’) side of the C=C bond.

What is here?

You can see models of the E-Z isomers of:

  • but-2-ene
  • 1-bromo-2-chloroprop-1-ene

These are chosen to show some of the ideas behind the E-Z naming system. A Level students should be able to work out whether each isomer is the E isomer or the Z isomer.

atom labels
ball and stick model
space-filling model
perspective view

Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.




To work out which group has the higher priority:

  • look at the atom directly attached to a C atom
  • the atom with the higher atomic number has the higher priority.

In but-2-ene, the —CH3 groups have the higher priority on each side of the C=C bond because the atomic number of carbon is 6, but the atomic number of hydrogen is 1. The rules do get more complex but this is enough to explain the substances here.

E-Z vs cis-trans

You may recognise:

  • E-but-2-ene as trans-but-2-ene
  • Z-but-2-ene as cis-but-2-ene.

However, the cis-trans naming system is insufficient for substances like 1-bromo-2-chloroprop-1-ene.

The models below show the E and Z isomers of this compound.



In the E-Z naming system:

  • on the left-hand C atom in C=C, the Br atom has the higher priority (atomic number = 35) compared to hydrogen (atomic number = 1)
  • on the right-hand C atom in C=C, the Cl atom has the higher priority (atomic number = 17) compared to carbon (atomic number = 12).