Stereoisomerism (AQA A Level Chemistry): Revision Note

Stereoisomerism: Geometrical Isomerism

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

E/Z isomerism

• E/Z isomerism occurs as a result of restricted rotation about the planar carbon–carbon double bond

• The C=C double bond (the π bond) prevents rotation, so the groups are locked in position — and E–Z isomerism only occurs when each carbon of the double bond carries two different groups

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

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

  • E isomers have the highest priority groups on opposite sides of the double bond

• 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

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

• When the groups R₁, R₂, R_3, 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:

Compound A

• 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

Compound B

• 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

  • In compound B, the two highest priority groups are on the same side (both below) the C=C bond

    • Therefore, compound B is Z-1-bromo-1-propen-2-ol

More complicated E / Z isomers

• Compound X exhibits E / Z isomerism:

• Step 1: Apply the CIP priority rules

  • Look at R₁ and R₃:

    • Carbon is the first atom attached to the C=C bond, on the left-hand side

  • Look at R₂ and R₄:

    • Carbon is the first atom attached to the C=C bond, on the right-hand side

  • This means that we cannot deduce if compound X is an E or Z isomer by applying the CIP priority rules to the first atom attached to the C=C bond

    • Therefore, we now have to look at the second atom attached

  • Look again at R₁ and R₃:

    • The second atoms attached to R₁ are hydrogens and another carbon

    • The second atoms attached to R₃ are hydrogens and bromine

    • We can ignore the hydrogens, as both R groups have hydrogens

    • Bromine has a higher atomic number than carbon, so bromine is the higher priority

    • Therefore, the CH₂Br group has priority over the CH₃CH₂ group

  • Look again at R₂ and R₄:

    • The second atoms attached to R₂ are hydrogens

    • The second atoms attached to R₄ are hydrogens and an oxygen

    • Oxygen has a higher atomic number than hydrogen, so oxygen is the higher priority

    • Therefore, the CH₂OH group has priority over the CH₃ group

• Step 2: Deduce E or Z

  • In compound X, the two highest priority groups are on the same side (both below) the C=C bond

    • Therefore, compound X is the Z isomer

Related topics

• Reactions of Alkenes

• Optical Isomerism