Stereoisomerism (Edexcel A Level Chemistry): Revision Note

E/Z Isomerism

• Stereoisomers are compounds that have the same atoms connected to each other, however the atoms are differently arranged in space

E/Z isomerism

• E/Z isomerism is seen in unsaturated (double bond containing) or ring compounds that have the same molecular formula and order of atoms (the atoms are connected similarly to each other) but different shapes

• E/Z nomenclature is used to distinguish between the isomers

  • Z isomers have groups on the same side of the double bond/carbon ring

  • E isomers have groups on opposite sides of the double bond/carbon ring

• You may see this type of isomerism referred to in other sources as cis/trans isomerism

  • This is a special case of E/Z isomerism

  • A "cis" isomer would essentially be the same as a "Z" isomer and a "trans" isomer would also essentially be the same as an "E" isomer

Geometrical isomerism in unsaturated compounds

Geometrical isomerism in cyclic compounds

• This causes the compounds to have different chemical and physical properties

• For example, they may have different reaction rates for the same reaction (chemical property) or different melting/boiling points (physical property)

Limitation of Cis/Trans Naming

• The cis / trans naming system cannot be used with three or four atoms / groups of atoms when they are all different

  • This requires the use of the E / Z naming system

1-bromo-2-chloropropene cannot be named using cis / trans 

E / Z isomers

• To discuss E / Z isomers, we will use an alkene of the general formula C₂R₄:

The general alkene, C₂R₄

• When the groups R₁, R₂, R₃ and R₄ are all different (i.e. R₁ ≠ R₂ ≠ R₃ ≠ R₄), we have to use the E / Z naming system

  • This is based on Cahn-Ingold-Prelog (CIP) priority rules

• To do this, we look at the atomic number of the first atom attached to the carbon in question

  • The higher the atomic number; the higher the priority

• For example, 1-bromo-1-propen-2-ol has four different atoms or groups of atoms attached to the C=C bond

  • This means that it can have two different displayed formulae:

1-bromo-1-propen-2-ol (compounds A and B)

Compound A - is it E or Z?

• Step 1: Apply the CIP priority rules

  • Look at R₁ and R₃:

    • Bromine has a higher atomic number than hydrogen so bromine has priority

  • Look at R₂ and R₄:

    • Oxygen has a higher atomic number than carbon so oxygen has priority

• Step 2: Deduce E or Z

  • E isomers have the highest priority groups on opposite sides of the C=C bond, i.e. one above and one below

    • The E comes from the German word "entgegen" meaning opposite

  • Z isomers have the highest priority groups on the same side of the C=C bond, i.e. both above or both below

    • The Z comes from the German word "zusammen" meaning together

  • In compound A, the two highest priority groups are on opposite sides (above and below) the C=C bond

    • Therefore, compound A is E-1-bromo-1-propen-2-ol