Stereoisomerism in Alkenes (OCR A Level Chemistry)

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Types of Stereoisomerism

  • One of the features of alkenes is their stereoisomerism, which can be classed as E / Z or cis / trans isomerism
    • Cis / trans isomerism occurs where two of the atoms or groups of atoms attached to each carbon atom of the C=C bond are the same
    • E / Z isomerism is an example of stereoisomerism where different atoms or groups of atoms are attached to each carbon atom of the C=C bond 

Cis / trans isomers

  • In saturated compounds, the atoms / groups attached to the single, σ-bonded carbons are not fixed in their position due to the free rotation about the C-C σ-bond

  • In unsaturated compounds, the groups attached to the C=C carbons remain fixed in their position
    • This is because free rotation of the bonds about the C=C bond is not possible due to the presence of a π bond

  • Cis / trans nomenclature can be used to distinguish between the isomers
    • Cis isomers have two groups on the same side of the double bond / carbon ring, i.e. both above the C=C bond or both below the C=C bond
    • Trans isomers have two groups on opposite sides of the double bond / carbon ring, i.e. one above and one below the C=C bond

 

An Introduction to AS Level Organic Chemistry Geometrical Isomers Unsaturated Compounds, downloadable AS & A Level Chemistry revision notes

The presence of a π bond in unsaturated compounds restricts rotation about the C=C bond forcing the groups to remain fixed in their position and giving rise to the formation of certain configurational isomers

Deducing E / Z Isomers

  • Cis / trans naming is relatively straightforward when you only have two different groups on either side of the double bond
  • However, with three or four different groups the cis / trans system doesn't work, so it has been superseded by E / Z isomerism, which covers all possibilities when 2, 3 or 4 different groups are involved.

Naming cis / trans isomers

  • For cis / trans isomers to exist, we need two different atoms or groups of atoms on either side of the C=C bond
    • This means that 2-methylpropene cannot have cis / trans isomers as the methyl groups are both on the same side of the C=C bond:

2-methylpropene, downloadable IB Chemistry revision notes

2-methylpropene molecules do not have cis / trans isomers

  • However, moving one of the methyl groups to the other side of the C=C bond causes cis / trans isomerism:


20-3-2-cis-and-trans-isomers-of-but-2ene  

But-2-ene does have cis / trans isomers

  • The atoms or groups of atoms on either side of the C=C bond do not have to be the same for cis / trans isomers:


20-3-2-cis-and-trans-isomers-of-1-chloroprop-1-ene

1-chloroprop-1-ene also shows cis / trans isomerism

  • However, the cis / trans naming system starts to fail once we have more than one atom or group of atoms on either side of the C=C bond
    • The cis / trans naming system can still be used with three atoms / groups of atoms but only if:
        • Two of the three atoms or groups of atoms are the same
        • These two atoms or groups of atoms are on opposite sides of the double bond
      20-3-2-cis-and-trans-12-dichloropropene

1,2-dichloropropene can be named using cis / trans 

    • The cis / trans naming system cannot be used with three atoms / groups of atoms when they are all different
      • This requires the use of the EZ naming system

1-bromo-2-chloropropene, downloadable IB Chemistry revision notes

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

E / Z isomers

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


20-3-2-general-alkene-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 EZ 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:

2-bromo-1-propen-1-ol, downloadable IB Chemistry revision notes

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

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 EZ isomers

  • Compound X exhibits EZ isomerism:


20-3-2-compound-x

Compound X

  • 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 atoms 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 R3 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

Examiner Tip

You may come across the term geometric isomerism in older exam papers and textbooks. The term used to refer to cis/trans isomerism, but is no longer in use and would not be required in exams.

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.