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E-Z isomerism

What is it?

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.

You will be able to identify some of the organic compounds you meet at A Level as cis or trans isomers. However, this naming system 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.

E isomer or Z isomer?

You need to work out which of the two groups attached to each C atom in the C=C bond has the higher priority. (Jump to: 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.

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Structure of E-but-2-ene


Structure of Z-but-2-ene



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.

Jump to: E isomer or Z isomer?


Structure of E-1-bromo-2-chloroprop-1-ene


Structure of Z-1-bromo-2-chloroprop-1-ene


E-Z vs cis-trans

You may recognise E-but-2-ene as trans-but-2-ene, and 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.

In the E-Z naming system:

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