Lecture on Structural Isomers of Alkanes

Introduction to Structural Isomers

• Structural isomers have the same molecular formula but different structural arrangements.
• Examples include different forms of alkanes like butane and octane.

Alkanes and Their Structural Isomers

• First Four Alkanes:
  • Methane (CH₄)
  • Ethane (C₂H₆)
  • Propane (C₃H₈)
  • Butane (C₄H₁₀)
• Butane Structural Isomers:
  • Straight chain molecule.
  • Alternative arrangement: 2-methylpropane (has a methyl group on the second carbon atom).

Drawing Structural Formulae

• Structural formula often omits hydrogen atoms to simplify.
• Bonds connecting carbon atoms are emphasized.

Historical Context: Lead-Free Petrol

• Octane and Its Isomers:
  • Used in petrol to prevent engine knocking.
  • Isomer example: 3,3-methyl ethyl pentane.
• Lead Tetraethyl:
  • Used to enhance petrol performance but was toxic.
  • Lead from car exhausts was a global environmental issue until the 21st century.

Identifying Isomers

• Hexane Isomers (C₆H₁₄):
  • Based on Pentane:
    • 2-methylpentane
    • 3-methylpentane
  • Based on Butane:
    • 2,3-dimethylbutane
    • 2,2-dimethylbutane
  • Naming depends on the length of the longest carbon chain and position of branches.

Conclusion

• Structural isomers are defined by having the same molecular formula but differing structural arrangements.
• The naming of isomers is based on the longest continuous carbon chain and the position of functional groups.