11.4 Alkanes

2026 Syllabus Objectives

Core:

  1. State that the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons
  2. Describe the properties of alkanes as being generally unreactive, except in terms of combustion and substitution by chlorine

Supplement: 3. State that in a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms 4. Describe the substitution reaction of alkanes with chlorine as a photochemical reaction, with ultraviolet light providing the activation energy, EaE_a, and draw the structural or displayed formulae of the products, limited to monosubstitution

Structure and Bonding in Alkanes 🔑

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Alkanes are saturated hydrocarbons containing only single covalent bonds between carbon atoms. The term "saturated" indicates that each carbon atom forms four single bonds and cannot accommodate any additional hydrogen atoms.

Key Point: All bonding in alkanes consists of single covalent bonds (C–C and C–H bonds), making them saturated compounds.

General Properties of Alkanes

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Reactivity Profile

Alkanes are generally unreactive compounds due to the strength and stability of their C–C and C–H single covalent bonds. They exhibit the following characteristics:

  • Unaffected by acids or alkalis
  • Cannot participate in addition reactions (because they are saturated)
  • Generally inert under normal conditions

However, alkanes do undergo two important types of reactions:

  1. Combustion reactions ⚡
  2. Substitution reactions with chlorine

Combustion of Alkanes 🔥

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Complete Combustion

When alkanes burn in a good supply of air (excess oxygen), they undergo complete combustion to produce:

  • Carbon dioxide (CO2\mathrm{CO_2})
  • Water vapour (H2O\mathrm{H_2O})

The reactions are highly exothermic, releasing large amounts of energy, which makes alkanes excellent fuels.

General word equation:

alkane+oxygen→carbon dioxide+water\text{alkane} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}

Examples of complete combustion:

Methane:

CH4(g)+2O2(g)→CO2(g)+2H2O(g)\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}

Ethane:

2C2H6(g)+7O2(g)→4CO2(g)+6H2O(g)\mathrm{2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(g)}

Butane:

2C4H10(g)+13O2(g)→8CO2(g)+10H2O(g)\mathrm{2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(g)}

Important: When balancing combustion equations, balance the oxygen atoms last to ensure correct stoichiometry.

Incomplete Combustion

When alkanes burn in a limited air supply (insufficient oxygen), incomplete combustion occurs, producing:

  • Carbon monoxide (CO\mathrm{CO}) - a poisonous gas
  • Water vapour (H2O\mathrm{H_2O})
  • Carbon particulates (soot) in extreme cases

Example of incomplete combustion of methane:

2CH4(g)+3O2(g)→2CO(g)+4H2O(g)\mathrm{2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(g)}

Hazards of Incomplete Combustion:

  • Carbon monoxide is toxic because it interferes with oxygen transport by red blood cells in the body
  • Carbon particulates (fine particles of carbon/soot) contribute to air pollution
  • These carbon particles can glow yellow in flames, giving characteristic yellow colour to candle flames or Bunsen burner safety flames

Environmental Impact: Incomplete combustion of fossil fuels is a major cause of air pollution.

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