Isomerism in Hydrocarbons (WJEC GCSE Science (Double Award)): Revision Note

Isomerism in Hydrocarbons

Higher Tier

• Isomers are chemicals with the same molecular formula but a different structural formula

  • This means that isomers have the same type and number of atoms but they are arranged differently

Alkane isomers

• The following alkanes do not have any isomers:

  • Methane, CH₄ 

  • Ethane, C₂H₆ 

  • Propane, C₃H₈ 

• This is because carbon atoms must always have four bonds and hydrogen atoms have one bond

• Therefore, there is no way to rearrange the atoms in methane, ethane and propane to give a different arrangement of atoms

• The molecular formula of the first alkane that can form isomers is C₄H₁₀

• It forms 2 isomers:

  • One isomer is the simple, straight-chain alkane butane

  • To form the other isomer, a CH₃ (methyl) group from the end of the butane chain is snapped off and switches places with a hydrogen atom on the second carbon of the remaining chain

Isomers of C₄H₁₀

There are two isomers with the molecular formula C₄H₁₀ 

• The molecular formula of the second alkane that can form isomers is C₅H₁₂

• It forms 3 isomers:

  • Isomer 1 is the simple, straight-chain alkane pentane

  • To form isomer 2, a CH₃ (methyl) group from the end of the pentane chain is snapped off and switches places with a hydrogen atom on the second carbon of the remaining chain

  • To form isomer 3, the CH₃ (methyl) groups from both ends of the pentane chain are snapped off and switch places with both hydrogen atoms on the central carbon of the remaining chain

Isomers of C₅H₁₂

There are three isomers with the molecular formula C₅H₁₂ 

Alkene isomers

• The following alkenes do not have any isomers:

  • Ethene, C₂H₄ 

  • Propene, C₃H₆ 

• There is no way to rearrange the atoms in ethene and propene to give a different arrangement of atoms

• The molecular formula of the first alkene that can form isomers is C₄H₈ 

• To determine the isomers, be aware that the carbon atoms in the longest chain are numbered from left to right

• C₄H₈ forms 3 isomers:

  • Isomer 1 is a chain of 4 carbons with the carbon-carbon double bond between carbons 1 and 2

  • Isomer 2 is a chain of 4 carbons with the carbon-carbon double bond between carbons 2 and 3 

  • Isomer 3 is formed from isomer 1, a CH₃ (methyl) group from the end of the carbon chain is snapped off and switches places with a hydrogen atom on the second carbon of the remaining chain

Isomers of C₄H₈

There are three isomers with the molecular formula C₄H₈