Lecture Notes: Structural Isomers of Alkanes

Introduction to Structural Isomers

• Structural isomers: Compounds with the same molecular formula but different structural arrangements.
• Example: C4H10 has two structural isomers.

First Four Alkanes

• Methane, Ethane, Propane, Butane are the first four alkanes.
• Structural formulae provided for first three.
• Butane typically drawn as a straight chain molecule.

Example of Structural Isomer

• Alternative arrangement for butane: A form of propane with a methyl group on the second carbon, named 2-methyl propane.
• Demonstrates the concept of structural isomers.

Simplified Drawing of Structural Formula

• Hydrogen atoms often omitted.
• Focus on bonds connecting carbon atoms.

Lead-Free Petrol and Isomers of Octane

• Petrol is mostly octane.
• Isomers of octane: Used historically to reduce engine knocking.
• Longest chain of analyzed isomer is 5 carbon atoms, named 3,3-methyl ethyl pentane.
• Lead tetraethyl historically used, but very toxic.

Environmental Impact of Leaded Petrol

• Lead oxide is toxic; pollution has affected global environments, including Antarctica.

Hexane and Its Isomers

• Hexane (C6H14) has five isomers.
• Two based on pentane:
  • 2-methyl pentane: Methyl group on second carbon.
  • 3-methyl pentane.
• Two based on butane:
  • 2,3-dimethylbutane.
  • 2,2-dimethylbutane.

Key Concepts

• Structural isomers have identical molecular formulas but vary in structure.
• Naming isomers based on the longest carbon chain and position of additional groups.