A-Level — chemistryMedium4 min read

Alkanes and Combustion

Master alkane nomenclature, structural isomerism, free radical substitution, and combustion at A-Level Chemistry.

Tutor AI Team2026-02-25T12:50:14.136Z

# Alkanes and Combustion

Alkanes are the simplest organic compounds and form the backbone of organic chemistry nomenclature. At A-Level, you need to understand IUPAC naming, structural and chain isomerism, and the mechanism of free radical substitution.

1. IUPAC Nomenclature

The Rules

Find the longest continuous carbon chain — this gives the parent name
Number the chain so that substituents get the lowest possible numbers
Name and number each substituent (branch)
Use prefixes: di-, tri-, tetra- for multiple identical substituents
List substituents in alphabetical order

Naming Groups

Carbons	Parent Name	Substituent
1	Methane	Methyl (CH₃−)
2	Ethane	Ethyl (C₂H₅−)
3	Propane	Propyl (C₃H₇−)
4	Butane	Butyl

Example

2,3-dimethylpentane: a 5-carbon chain with methyl groups on C2 and C3.

2. Isomerism

Structural Isomers

Same molecular formula, different structural arrangement.

Types:

Chain isomerism: different carbon chain arrangements (e.g. butane vs methylpropane)
Position isomerism: same chain, different positions of functional group (e.g. propan-1-ol vs propan-2-ol)
Functional group isomerism: same formula, different functional group (e.g. ethanol vs methoxymethane)

Stereoisomers

E/Z (geometric) isomerism: different spatial arrangements around a C=C double bond (requires two different groups on each C of the double bond)
Optical isomerism: mirror image molecules (chiral centre with 4 different groups)

3. Free Radical Substitution

Alkanes react with halogens (Cl₂, Br₂) in the presence of UV light via a free radical mechanism.

The Three Stages

1. Initiation — UV light breaks the halogen molecule homolytically: $\text{Cl}_2 \xrightarrow{\text{UV}} 2\text{Cl}\cdot$

(Homolytic fission: bond breaks evenly, one electron to each atom, forming radicals)

2. Propagation — chain reaction produces products and regenerates radicals: $\text{Cl}\cdot + \text{CH}_4 \rightarrow \text{CH}_3\cdot + \text{HCl}$ $\text{CH}_3\cdot + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\cdot$

(Each step produces a new radical, continuing the chain)

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

Limitations of Free Radical Substitution

Produces a mixture of products (mono-, di-, tri-substituted, plus other by-products)
Difficult to control — further substitution occurs (CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄)
Multiple possible termination products (including ethane from two methyl radicals)

4. Combustion

Complete Combustion

$C_nH_{2n+2} + \frac{3n+1}{2}\text{O}_2 \rightarrow n\text{CO}_2 + (n+1)\text{H}_2\text{O}$

Incomplete Combustion

Insufficient oxygen → CO, C (soot), H₂O

Environmental Impact

CO₂ → greenhouse effect → climate change
CO → toxic
NOₓ → acid rain, photochemical smog
Unburned hydrocarbons → smog
SO₂ (from sulfur impurities) → acid rain

Worked Example: Naming

Problem

Question: Name CH₃CH(CH₃)CH₂CH₃.

Solution

Longest chain = 4 carbons (butane). Methyl group on C2. Name: 2-methylbutane.

Worked Example: Free Radical Mechanism

Problem

Question: Write the mechanism for the reaction of ethane with bromine in UV light.

Initiation: $\text{Br}_2 \xrightarrow{\text{UV}} 2\text{Br}\cdot$

Propagation: $\text{Br}\cdot + \text{C}_2\text{H}_6 \rightarrow \text{C}_2\text{H}_5\cdot + \text{HBr}$ $\text{C}_2\text{H}_5\cdot + \text{Br}_2 \rightarrow \text{C}_2\text{H}_5\text{Br} + \text{Br}\cdot$

Termination: $2\text{Br}\cdot \rightarrow \text{Br}_2$ , etc.

Solution

6. Practice Questions

1. Name: CH₃CH₂CH(CH₃)CH(CH₃)CH₃
1. Draw all structural isomers of C₅H₁₂.
1. Write the mechanism for the reaction of methane with chlorine, naming each stage.
1. Why does free radical substitution produce a mixture of products?
1. Write a balanced equation for the complete combustion of octane.

Want to check your answers and get step-by-step solutions?

7. Exam Tips

Use curly arrows with half-headed arrows (fishhooks) for radical mechanisms
Know the three stages: initiation, propagation, termination
UV light is needed for initiation — not heat or a catalyst
Include the dot (•) when writing radicals
In naming, always find the longest chain first

Summary

Alkanes: saturated hydrocarbons, $C_nH_{2n+2}$ , IUPAC naming
Structural isomerism: chain, position, functional group
Free radical substitution: UV + halogen → initiation, propagation, termination
Products are mixed (multiple substitution products)
Complete combustion → CO₂ + H₂O; incomplete → CO + C + H₂O

Tags:

#a-level#chemistry#organic#alkanes

Ready to Ace Your A-Level chemistry?

Get instant step-by-step solutions to any problem. Snap a photo and learn with Tutor AI — your personal exam prep companion.