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AQA A Level Chemistry

Reactions of Alkanes

Alkanes are relatively unreactive because C-C and C-H bonds are strong and essentially non-polar. Their key organic reaction in this section is free radical substitution with halogens, which occurs through a chain mechanism involving initiation, propagation and termination steps.

Exam Paper
Paper 2
7405/2
Specification Points
3.3.2.4
1 point covered
Sub-topics
5 lessons
Revision notes available
Exam Board
AQA
AQA 7405

Revision Notes

5 revision notes, in the order the specification teaches them.

A radical pulling a hydrogen from a methane molecule
1

Free Radical Substitution

Revise the overall reaction type, why radicals are formed and how halogenation replaces a hydrogen atom in an alkane.

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Light splitting a halogen molecule into two radicals
2

Initiation

Learn how ultraviolet light causes homolytic fission of a halogen molecule to form two reactive halogen radicals.

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A radical used and regenerated in a cycle
3

Propagation

Review the two repeating steps that produce the organic product and regenerate a radical so the chain reaction continues.

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Two radicals joining to end the chain
4

Termination

Revise how two radicals combine to form a stable molecule, removing radicals and stopping that part of the chain.

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More and more hydrogen atoms replaced by halogen
5

Further Substitution

Learn how substitution can continue to form mixtures of products when more hydrogen atoms are replaced by halogen atoms.

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Summary Map

Download, print and revise the reactions of this topic at a glance.

Summary maps

Download, print and revise the reactions of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Alkanes Map

AQA A-level Chemistry · 3.3.2 Alkanes

Every reaction on one A4 page: reagents, conditions and reaction type for each arrow, colour coded, with the key tests.

Download the map4 reactions · A4 landscape · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Alkanes Summary Map

AQA A-level Chemistry · 3.3.2 Alkanes

The whole section on one A4 page: crude oil, thermal and catalytic cracking, combustion, pollutants, converters and flue-gas desulfurisation, with the free radical mechanism in a table.

Download the map6 key ideas + the mechanism · A4 portrait · print ready

© 2026 Online Learning System. For personal study: printing is allowed; editing, copying and redistribution are not.

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Revision Game

Match reagents to products, free, with your board's reagents and conditions.

Illustration of memory game cards, two turned face up as a matching pairFree

Revision Game: Alkanes

Turn the cards over two at a time: match reagents to what they make, and find how each product is made.

A free memory game on the reactions in this topic, with your board's reagents, conditions and spec points.

  • Two ways to playMatch each set of reagents to what the reaction makes, or see a product and find the reactant and reagents that make it.
  • Drawn the way the exam draws themEvery structure is drawn in ChemDraw, with the reagents and conditions your exam board asks for.
  • It countsRounds earn XP and keep your streak going, and every correct match shows in your KASP analytics, apart from your marks.

Study Cards

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Specification

Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.

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Self-assessment · 26 points

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3.3.2.1

  1. i

    know that alkanes are saturated hydrocarbons

    Revise
  2. ii

    know that petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation

    Revise

3.3.2.2

  1. i

    know that cracking involves breaking C-C bonds in alkanes

    Revise
  2. ii a

    know that thermal cracking takes place at high pressure

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  3. ii b

    know that thermal cracking takes place at high temperature

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  4. ii c

    know that thermal cracking produces a high percentage of alkenes

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  5. iii a

    know that catalytic cracking takes place at slight pressure

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  6. iii b

    know that catalytic cracking takes place at high temperature

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  7. iii c

    know that catalytic cracking takes place in the presence of a zeolite catalyst

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  8. iii d

    know that catalytic cracking is used mainly to produce motor fuels

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  9. iii e

    know that catalytic cracking is used mainly to produce aromatic hydrocarbons

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  10. iv

    explain the economic reasons for cracking alkanes

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3.3.2.3

  1. i

    know that alkanes are used as fuels and that combustion can be complete or incomplete

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  2. ii a

    know that internal combustion engines produce NOx

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  3. ii b

    know that internal combustion engines produce CO

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  4. ii c

    know that internal combustion engines produce carbon

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  5. ii d

    know that internal combustion engines produce unburned hydrocarbons

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  6. ii e

    know that catalytic converters remove NOx

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  7. ii f

    know that catalytic converters remove CO

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  8. ii g

    know that catalytic converters remove unburned hydrocarbons

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  9. iii

    know that combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution

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  10. iv

    explain why sulfur dioxide can be removed from flue gases using calcium oxide or calcium carbonate

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3.3.2.4

  1. i

    describe the reaction of methane with chlorine

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  2. ii a

    explain the initiation step in the chlorination of methane as a free-radical substitution mechanism

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  3. ii b

    explain the propagation steps in the chlorination of methane as a free-radical substitution mechanism

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  4. ii c

    explain the termination steps in the chlorination of methane as a free-radical substitution mechanism

    Revise