Structural Isomers of Alkanes

Introduction

• Structural Isomers: Compounds with the same molecular formula but different structural formulas.
• Alkanes: Methane, ethane, propane, and butane are simple alkanes.
• Aim: Understand structural isomers using examples, particularly focusing on butane to octane.

Structural Isomers of Butane

• Butane (C4H10) can be drawn as a straight chain.
• An alternative arrangement: a propane chain with a methyl group on the second carbon, known as 2-methylpropane.
• Despite having different structures, both forms are isomers of butane.
• Drawing Tip: Often, hydrogen atoms are omitted, and only carbon bonds are shown.

Historical Context: Lead-free Petrol

• Petrol (Octane C8H18): Originally caused engine knocking due to rapid explosions.
• Isomers of Octane: Burn more smoothly than straight-chain octane.
• Example: 3,3-methyl-ethyl-pentane has a longest chain of 5 carbons.
  • Features an ethyl group and a methyl group on the third carbon.
  • Is a structural isomer of octane.
• Lead Tetraethyl: Used to improve petrol performance but produced toxic lead oxide.
  • Lead emissions persisted into the 21st century, affecting global environments.

Example: Isomers of Hexane (C6H14)

• Pentane-based Isomers:
  • 2-methylpentane: Methyl group on the second carbon of a pentane chain.
  • 3-methylpentane: Methyl group on the third carbon of a pentane chain.
• Butane-based Isomers:
  • 2,3-dimethylbutane: Methyl groups on the second and third carbons of a butane chain.
  • 2,2-dimethylbutane: Two methyl groups on the second carbon of a butane chain.

Key Takeaways

• Structural Isomers:
  • Same molecular formula, different structural arrangements.
  • Naming depends on the longest carbon chain and the positioning of additional groups.
• Example applications include improving fuel efficiency and reducing environmental impact from leaded petrol.