What are alkanes and how are they classified in AS-Level Organic Chemistry?

Alkanes are a class of hydrocarbons that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. In AS-Level Organic Chemistry, alkanes are classified as saturated hydrocarbons because they contain the maximum number of hydrogen atoms per carbon atom. They follow the general formula CnH2n+2, where 'n' is the number of carbon atoms.

How do the physical properties of alkanes change with increasing molecular size?

As the molecular size of alkanes increases, their boiling and melting points generally increase due to stronger van der Waals forces between the molecules. This is because larger alkanes have more surface area, leading to greater intermolecular attractions. Additionally, alkanes are non-polar and insoluble in water but soluble in organic solvents.

What is the significance of isomerism in alkanes at the Cambridge International A Levels?

Isomerism in alkanes is significant because it introduces the concept of structural isomers, which are compounds with the same molecular formula but different structural arrangements. This is an important topic in the Cambridge International A Levels as it helps students understand how different structures can lead to different chemical and physical properties, even though the molecular formula remains the same.

How do alkanes react with halogens, and what is the mechanism involved?

Alkanes react with halogens in a substitution reaction known as halogenation, which typically occurs under the presence of UV light or heat. The mechanism involved is free radical substitution, which includes initiation, propagation, and termination steps. This reaction is important in AS-Level Organic Chemistry as it illustrates the reactivity of alkanes and the formation of haloalkanes.

Why are alkanes considered to be relatively unreactive compared to other hydrocarbons?

Alkanes are considered relatively unreactive due to the strength of the C-C and C-H bonds and the lack of polarity in these bonds. This makes them less likely to participate in reactions compared to unsaturated hydrocarbons like alkenes and alkynes, which have double or triple bonds that are more reactive. This stability is a key topic in understanding the chemical behavior of alkanes in AS-Level Organic Chemistry.

Why is "Omitting the radical dots in the mechanism." a common mistake in Alkanes?

Why it happens: Treating radicals as normal molecules. How to avoid it: Show the unpaired electron (•) on every radical. Source: 9701 Examiner Reports 2022-2024

Why is "Mislabelling propagation as termination (or vice versa)." a common mistake in Alkanes?

Why it happens: Not tracking radical creation/destruction. How to avoid it: Propagation regenerates a radical; termination only removes radicals.

Why is "Not stating UV light for initiation." a common mistake in Alkanes?

Why it happens: Forgetting the condition. How to avoid it: Initiation requires UV light to cause homolytic fission.

Why is "Assuming chlorination gives a single pure product." a common mistake in Alkanes?

Why it happens: Stopping after the first substitution. How to avoid it: Further substitution gives a mixture (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄).

Why is "Giving CO₂ as a product of incomplete combustion." a common mistake in Alkanes?

Why it happens: Defaulting to complete combustion. How to avoid it: Incomplete combustion → CO and/or soot (limited oxygen).

Hydrocarbons(AS-Level Organic Chemistry)Cambridge International A Level Chemistry 9701

Alkanes

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Alkanes — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

How alkanes are made (hydrogenation, cracking), their combustion, the free-radical substitution mechanism with halogens, why alkanes are so unreactive, and the environmental impact of burning them.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

14.1.1 — Recall how alkanes are produced: (a) addition of hydrogen to an alkene (H₂, Pt/Ni, heat); (b) cracking of a longer-chain alkane (heat, Al₂O₃).
14.1.2 — Describe (a) complete and incomplete combustion; (b) free-radical substitution by Cl₂ or Br₂ in UV light.
14.1.3 — Describe the mechanism of free-radical substitution (initiation, propagation, termination).
14.1.4 — Suggest how cracking gives more useful alkanes and alkenes of lower Mr from heavier fractions.
14.1.5 — Understand the general unreactivity of alkanes (towards polar reagents) in terms of C–H bond strength and lack of polarity.
14.1.6 — Recognise the environmental consequences of CO, NOₓ and unburnt hydrocarbons from combustion, and their catalytic removal.

Making alkanes and combustion

Hydrogenate alkenes or crack long chains; alkanes burn completely (CO₂) or incompletely (CO/C).

Making alkanes:

Hydrogenation of an alkene: $\text{CH}_2\text{=CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3$ (H₂(g), Ni or Pt catalyst, heat).
Cracking a longer-chain alkane (heat with Al₂O₃): e.g. $\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$ .

Combustion:

Complete (plenty of O₂): $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$ — releases lots of energy.
Incomplete (limited O₂): produces carbon monoxide (CO) or carbon (soot): $2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$ . CO is toxic (binds to haemoglobin).

Hydrogenation (H₂, Ni/Pt, heat) and cracking make alkanes.
Complete combustion → CO₂ + H₂O.
Incomplete → CO (toxic) or C (soot) + H₂O.

Free-radical substitution mechanism

UV light drives a chain reaction: initiation makes radicals, propagation continues the chain, termination ends it.

Alkanes react with chlorine (or bromine) in UV light by free-radical substitution — a hydrogen atom is replaced by a halogen. The mechanism has three stages (using CH₄ + Cl₂):

1. Initiation — UV light breaks the Cl–Cl bond by homolytic fission, forming two chlorine radicals: $\text{Cl}_2 \xrightarrow{\text{UV}} 2\text{Cl}\bullet$

2. Propagation — radicals react and regenerate radicals, continuing the chain: $\text{Cl}\bullet + \text{CH}_4 \rightarrow \bullet\text{CH}_3 + \text{HCl}$ $\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\bullet$

3. Termination — two radicals combine, removing radicals and ending the chain: $\text{Cl}\bullet + \text{Cl}\bullet \rightarrow \text{Cl}_2$ $\bullet\text{CH}_3 + \text{Cl}\bullet \rightarrow \text{CH}_3\text{Cl}$ $\bullet\text{CH}_3 + \bullet\text{CH}_3 \rightarrow \text{C}_2\text{H}_6$

Initiation creates radicals, propagation keeps the chain going (radicals regenerated), termination destroys radicals.

UV light; homolytic fission makes radicals.
Initiation: Cl₂ → 2Cl•.
Propagation (×2): radicals regenerated.
Termination: two radicals combine.

Unreactivity, cracking and the environment

Strong non-polar bonds make alkanes inert; cracking upgrades them; combustion pollutes.

Why alkanes are unreactive. C–C and C–H bonds are strong and have almost no polarity (C and H have similar electronegativities). With no δ+ or δ− centres, alkanes are not attacked by polar reagents (nucleophiles or electrophiles) — they react only via radicals or combustion.

Cracking. Long-chain alkanes from heavy crude-oil fractions are broken into smaller, more useful molecules — shorter alkanes (fuels like petrol) and alkenes (feedstock for polymers). This matches supply to demand.

Environmental consequences of combustion:

Carbon monoxide (CO) — toxic, from incomplete combustion.
Oxides of nitrogen (NOₓ) — formed at the high temperature of engines; cause acid rain and smog.
Unburnt hydrocarbons — contribute to photochemical smog. These are removed by catalytic converters (e.g. $2\text{NO} + 2\text{CO} \rightarrow \text{N}_2 + 2\text{CO}_2$ ; see Topic 12).

Unreactive: strong, non-polar C–C/C–H bonds → no attack by polar reagents.
Cracking → smaller alkanes + alkenes (more useful).
Pollutants: CO, NOₓ, unburnt hydrocarbons → catalytic converters.

How it’s examined

The free-radical substitution mechanism (initiation/propagation/termination, with the • shown) is a guaranteed Paper 2 question, along with combustion equations and the unreactivity explanation. Examiner reports flag missing radical dots, mislabelling the steps, and explaining unreactivity without mentioning lack of polarity.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 14.1; 9701 Examiner Reports 2022-2024; 9701/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Alkanes

Step-by-step solutions to past-paper-style questions on alkanes, written exactly the way a tutor would explain them at the board.

1Combustion equations (3 marks)
Getting started• combustion
Question
Write equations for the complete and incomplete combustion of propane, C₃H₈.
Step-by-step solution
1. Step 1
  Complete (excess O₂):
  $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$
2. Step 2
  Incomplete (limited O₂) → CO:
  $2\text{C}_3\text{H}_8 + 7\text{O}_2 \rightarrow 6\text{CO} + 8\text{H}_2\text{O}$
Answer
Complete: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O; incomplete: 2C₃H₈ + 7O₂ → 6CO + 8H₂O.
Examiner tip
Incomplete combustion (limited O₂) gives CO (and/or C soot), not CO₂.
2Why alkanes are unreactive (2 marks)
Getting started• unreactivity
Question
Explain why alkanes do not react with polar reagents such as nucleophiles.
Step-by-step solution
1. Step 1
  C–H and C–C bonds are strong and essentially non-polar (C and H similar electronegativity).
2. Step 2
  No δ+ or δ− centres, so polar reagents are not attracted → no reaction.
Answer
Strong, non-polar bonds with no δ+/δ− sites → not attacked by polar reagents.
3Free-radical substitution mechanism (5 marks)
Building confidence• mechanism
Question
Write the mechanism for the reaction of methane with chlorine in UV light to form chloromethane, labelling each stage.
Step-by-step solution
1. Step 1
  Initiation (UV, homolytic fission).
  $\text{Cl}_2 \rightarrow 2\text{Cl}\bullet$
2. Step 2
  Propagation step 1.
  $\text{Cl}\bullet + \text{CH}_4 \rightarrow \bullet\text{CH}_3 + \text{HCl}$
3. Step 3
  Propagation step 2.
  $\bullet\text{CH}_3 + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\bullet$
4. Step 4
  Termination (any radical–radical combination).
  $\bullet\text{CH}_3 + \text{Cl}\bullet \rightarrow \text{CH}_3\text{Cl}$
Answer
Initiation Cl₂→2Cl•; two propagation steps; termination (e.g. •CH₃ + Cl• → CH₃Cl).
Examiner tip
Show the radical dots and label the stages. UV light must be stated for initiation.
4Cracking of alkanes (3 marks)
Building confidence• cracking
Question
Write an equation for cracking decane (C₁₀H₂₂) into octane and one other product, and state why cracking is carried out.
Step-by-step solution
1. Step 1
  Long chain → shorter alkane + an alkene (atoms balance).
  $\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$
2. Step 2
  Why: converts less useful long chains into more valuable shorter alkanes (fuels) and alkenes (for polymers).
Answer
C₁₀H₂₂ → C₈H₁₈ + C₂H₄; cracking turns long chains into useful fuels and alkenes.
Examiner tip
Check the equation balances for both C and H; an alkene is usually a product.
5Why chlorination gives a mixture (4 marks)
Stretch• mechanism, limitation
Question
Explain why the reaction of methane with chlorine in UV light does not give pure chloromethane. (4)
Step-by-step solution
1. Step 1
  Further substitution: CH₃Cl still has C–H bonds, so it reacts again.
  $\text{CH}_3\text{Cl} \rightarrow \text{CH}_2\text{Cl}_2 \rightarrow \text{CHCl}_3 \rightarrow \text{CCl}_4$
2. Step 2
  A mixture of chlorinated products forms (mono-, di-, tri-, tetra-).
3. Step 3
  Termination also gives by-products such as ethane (•CH₃ + •CH₃ → C₂H₆).
4. Step 4
  So the yield of any single product is low — a key limitation of the method.
Answer
Continued substitution gives CH₂Cl₂/CHCl₃/CCl₄ (plus ethane from termination) → a mixture, low yield of CH₃Cl.
Examiner tip
The product still has C–H bonds, so it substitutes further — this is why mechanism control is poor.
6Pollutants from burning alkanes (4 marks)
Stretch• pollution
Question
Discuss the pollutants formed when alkane fuels burn in an engine and their environmental effects. (4)
Step-by-step solution
1. Step 1
  CO (incomplete combustion) — toxic; binds to haemoglobin.
2. Step 2
  CO₂ — a greenhouse gas contributing to global warming.
3. Step 3
  Oxides of nitrogen (NOₓ) — form at high engine temperature (N₂ + O₂); cause acid rain and smog.
4. Step 4
  Unburnt hydrocarbons and carbon (soot) — contribute to smog and respiratory problems.
Answer
CO (toxic), CO₂ (greenhouse), NOₓ (acid rain/smog), unburnt HC and soot — all with environmental/health effects.
Examiner tip
Pair each pollutant with its specific effect; NOₓ comes from N₂ + O₂ at high temperature, not from the fuel.

Key Formulae — Alkanes

The formulae you need to memorise for alkanes on the Cambridge International A Level 9701 paper, with every variable defined in plain English and a note on when to use it.

General formula of alkanes
$C_nH_{2n+2}$
$n$
number of carbon atoms
When to use
Deducing an alkane's formula; saturated, so the maximum number of hydrogens.

Key Definitions and Keywords — Alkanes

Definitions to memorise and the exact keywords mark schemes credit for alkanes answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Free-radical substitution
Examiner keyword
A substitution mechanism (initiation, propagation, termination) in which radicals replace a hydrogen atom, e.g. alkane + halogen in UV light.
Initiation / propagation / termination
Examiner keyword
Initiation makes radicals (homolytic fission, UV); propagation regenerates radicals; termination combines radicals to remove them.
Free radical
Examiner keyword
A species with an unpaired electron (shown by •), very reactive.
Complete / incomplete combustion
Examiner keyword
Complete (excess O₂) gives CO₂ + H₂O; incomplete (limited O₂) gives CO and/or carbon (soot) + H₂O.
Cracking
Examiner keyword
Breaking long-chain alkanes into smaller, more useful alkanes and alkenes (using heat/catalyst).

Common Mistakes and Misconceptions — Alkanes

The traps other students keep falling into on alkanes questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Omitting the radical dots in the mechanism.
9701 Examiner Reports 2022-2024
Why it happens
Treating radicals as normal molecules.
How to avoid it
Show the unpaired electron (•) on every radical.
✕Mislabelling propagation as termination (or vice versa).
Why it happens
Not tracking radical creation/destruction.
How to avoid it
Propagation regenerates a radical; termination only removes radicals.
✕Not stating UV light for initiation.
Why it happens
Forgetting the condition.
How to avoid it
Initiation requires UV light to cause homolytic fission.
✕Assuming chlorination gives a single pure product.
Why it happens
Stopping after the first substitution.
How to avoid it
Further substitution gives a mixture (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄).
✕Giving CO₂ as a product of incomplete combustion.
Why it happens
Defaulting to complete combustion.
How to avoid it
Incomplete combustion → CO and/or soot (limited oxygen).

Practice questions

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Alkanes — frequently asked questions

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Alkanes

Short Study Notes in the form of Slides

Short Study Notes — Alkanes

Detailed Notes

What you’ll learn

How it’s examined

1Combustion equations (3 marks)

2Why alkanes are unreactive (2 marks)

3Free-radical substitution mechanism (5 marks)

4Cracking of alkanes (3 marks)

5Why chlorination gives a mixture (4 marks)

6Pollutants from burning alkanes (4 marks)

General formula of alkanes

Free-radical substitution

Initiation / propagation / termination

Free radical

Complete / incomplete combustion

Cracking

✕Omitting the radical dots in the mechanism.

✕Mislabelling propagation as termination (or vice versa).

✕Not stating UV light for initiation.

✕Assuming chlorination gives a single pure product.

✕Giving CO₂ as a product of incomplete combustion.

Practice questions

Past paper style quiz

Assess your understanding

Alkanes — frequently asked questions