Alkanes — Combustion and Free Radical Substitution

Alkanes are generally unreactive, but they do undergo two important classes of reaction: combustion (burning in oxygen to release energy) and free radical substitution with halogens in ultraviolet light. Both reactions involve homolytic bond fission and radical intermediates. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.2 (e)–(h).

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1. Combustion of Alkanes

Alkanes release large amounts of energy when they burn in oxygen, which is why they are used as fuels.

1.1 Complete Combustion

In complete combustion, alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and water vapour only:

CₙH₂ₙ₊₂ + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

Worked examples:

• Methane: CH₄ + 2O₂ → CO₂ + 2H₂O
• Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
• Octane: C₈H₁₈ + 25/2 O₂ → 8CO₂ + 9H₂O (or 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O)

These reactions are highly exothermic — that is why alkanes are used as fuels.

1.2 Incomplete Combustion

If the oxygen supply is limited (e.g., in a poorly ventilated boiler, an old car engine, or a blocked flue), incomplete combustion occurs. Products can include carbon monoxide (CO), soot (C), and water.

Examples:

• 2CH₄ + 3O₂ → 2CO + 4H₂O
• CH₄ + O₂ → C + 2H₂O

1.3 Why Carbon Monoxide is Dangerous

• Colourless, odourless, tasteless — it cannot be detected without a sensor.
• Binds irreversibly to haemoglobin in the blood, forming carboxyhaemoglobin, which prevents oxygen transport.
• Causes headaches, dizziness, unconsciousness, and death at moderate concentrations.
• Responsible for hundreds of accidental deaths each year in the UK.

1.4 Other Pollutants from Alkane Combustion

Sulfur dioxide (SO₂): Crude oil contains sulfur impurities. When the fuel burns, the sulfur burns with it:

S + O₂ → SO₂

SO₂ dissolves in atmospheric water to form sulfurous acid, which is oxidised to sulfuric acid, contributing to acid rain:

SO₂ + H₂O → H₂SO₃ ; 2SO₂ + O₂ → 2SO₃ ; SO₃ + H₂O → H₂SO₄

Acid rain damages buildings (particularly limestone and marble), acidifies lakes (harming fish), and damages vegetation.

Nitrogen oxides (NOₓ): At the high temperatures inside an engine, atmospheric nitrogen reacts with oxygen to form NO and NO₂:

N₂ + O₂ → 2NO ; 2NO + O₂ → 2NO₂

NOₓ contributes to acid rain, photochemical smog, and respiratory illness.

Unburned hydrocarbons and particulates (soot, PM2.5): Contribute to smog, respiratory disease, and climate forcing. Soot also deposits on lungs and surfaces.

1.5 Catalytic Converters

Modern petrol cars contain a catalytic converter in the exhaust system to reduce emissions. A honeycomb of ceramic coated with platinum, palladium, and rhodium catalyses reactions including:

• 2CO + 2NO → 2CO₂ + N₂
• 2CO + O₂ → 2CO₂
• C₈H₁₈ + 25/2 O₂ → 8CO₂ + 9H₂O (converts unburned hydrocarbons)

The net effect is to convert the three main pollutants (CO, NOₓ, unburned hydrocarbons) into less harmful CO₂, N₂, and H₂O. Note that CO₂ is still a greenhouse gas and is not removed.

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2. Free Radical Substitution with Halogens

Alkanes react with chlorine (or bromine) gas in the presence of ultraviolet light to produce a haloalkane and hydrogen halide. This is a substitution reaction — one H atom on the alkane is replaced by one halogen atom — proceeding through a free radical chain mechanism.

Overall reaction (methane + chlorine):

CH₄ + Cl₂ → CH₃Cl + HCl

Conditions: UV light (to break the Cl–Cl bond homolytically). No light → no reaction at room temperature.

2.1 The Three Steps of the Mechanism

graph LR
  A[Initiation<br/>UV breaks Cl-Cl] --> B[Propagation<br/>radicals react, more radicals made]
  B --> C[Termination<br/>two radicals combine]

2.2 Initiation

Ultraviolet light provides enough energy to homolytically break the Cl–Cl bond:

Cl₂ →(UV)→ 2 Cl•

This is the only step in which radicals are created from non-radical species. Remember to use half-arrows (each electron of the Cl–Cl bond goes to a different Cl atom).

2.3 Propagation

In propagation, a radical reacts with a non-radical to give a new radical. The number of radicals is conserved: one in, one out. There are two propagation steps:

Propagation Step 1:

Cl• + CH₄ → HCl + •CH₃