Lecture on Structural Isomers of Alkanes

Definition of Structural Isomers

• Structural isomers have the same molecular formula but different structural arrangements.

Examples of Structural Isomers

Alkanes

• Methane, Ethane, Propane, Butane
  • First four alkanes.
  • Structural formulae differ despite the same molecular formula.

Butane

• Common drawing: Straight chain molecule.
• Alternate form with the same molecular formula, different structure.
• Longest chain: 3 carbon atoms (isomer of propane with a methyl group on the second carbon).
  • Name: 2-methyl propane
  • Still an isomer of butane due to containing four carbon atoms.

Drawing Structural Formulae

• Often omit hydrogen atoms, showing only carbon-carbon bonds for simplicity.

Historical Context: Lead-Free Petrol

• Petrol primarily composed of octane.
• Early petrol-air mixes exploded prematurely, causing engine knock.
• An isomer of octane helped petrol burn more smoothly.
• Longest chain in this isomer: 5 carbon atoms.
  • Name: 3,3-methyl ethyl pentane
• Lead tetraethyl was used to achieve similar effects.
  • Toxic lead oxide was a byproduct, took until the 21st century to eliminate.

Exercise: Isomers of Hexane (C6H14)

• Hexane Isomers
  • Two based on pentane
    • 2-methylpentane: Methyl group on the second carbon.
    • 3-methylpentane
  • Two based on butane
    • 2,3-dimethylbutane
    • 2,2-dimethylbutane

Summary

• Structural isomers: same molecular formula, different structural arrangement.
• Naming: Based on longest carbon chain and position of substituent groups.