Lecture on Alkanes

General Formula of Alkanes

• Alkanes have a general formula of CnH2n+2.
• They consist of aliphatic alkanes with open carbon chains.

Types of Alkanes

Aliphatic Alkanes

• Simple chains of carbon atoms.

Cyclic Alkanes

• Forms rings.
• Examples:
  • Cyclopropane: C3H6
  • Cyclohexane: C6H12
  • Methylcyclohexene: C7H14
• General formula: CnH2n.

Characteristics of Alkanes

Saturated Hydrocarbons

• Contain only single carbon-carbon bonds.
• Composed of hydrogen and carbon.
• Examples:
  • Ethane: Two carbons
  • Butane: Four carbons

Molecular Shape

• Alkanes have a tetrahedral shape.
• Bond angle is approximately 109.5°.
• Example: Methane shows the tetrahedral structure.

Boiling Points of Alkanes

Chain Length Influence

• Longer chains have higher boiling points.
• Example:
  • Propane Boiling Point: -42°C
  • Hexane Boiling Point: 68°C

Explanation

• Alkanes exhibit induced dipole-dipole interactions (London forces).
• Nonpolar molecules; longer chains have stronger intermolecular forces.
• More energy is required to overcome these forces, hence higher boiling points.

Branching Influence

• Structural isomers show variations in boiling points.
• More branches lead to lower boiling points due to reduced intermolecular contact.

Examples of Structural Isomers

• No branching (strong intermolecular forces)
• One branch (intermediate intermolecular forces)
• Two branches (weakest intermolecular forces)

Combustion of Alkanes

Complete Combustion

• Occurs with ample oxygen supply.
• Produces carbon dioxide and water.
• Example Equation (Butane):
  • Balance carbon first, then hydrogen, followed by oxygen.

Incomplete Combustion

• Occurs with limited oxygen.
• Produces carbon monoxide and water or carbon and water.
• Two scenarios:
  • Producing CO: Balance involves carbon monoxide.
  • Producing C: Balance involves elemental carbon.