Prepare halogenoalkanes from alkanes and alkenes
Compare UV-initiated alkane substitution with room-temperature addition of halogens or hydrogen halides to alkenes.
Section 15 · Halogen compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which condition is required for alkane substitution by Cl₂ or Br₂?
What you will learn
- Recall alkane halogenation conditions
- Recall alkene addition routes
- Distinguish substitution from addition products
Alkanes undergo radical substitution
Chlorine or bromine can replace a hydrogen in an alkane under ultraviolet light. Ethane chlorination gives chloroethane and hydrogen chloride as a representative first substitution: C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
This is a free-radical chain reaction. Further substitution can occur, so the equation shows one step rather than a guaranteed single pure product.
Alkenes undergo addition
A halogen X₂ or gaseous hydrogen halide HX can add across C=C at room temperature. For example, ethene + HBr → bromoethane and ethene + Br₂ → 1,2-dibromoethane.
Addition consumes the π bond; substitution replaces a hydrogen. Use the reagent and structural change to classify the route.
Pause and explain
How does ethene form bromoethane?
Worked example
Give one reaction and the conditions for preparing bromoethane from ethane, and a second route from ethene.
Show the worked solution
- From ethane, replace one H with Br using Br₂ under ultraviolet light; HBr is also formed.
- From ethene, add HBr across C=C at room temperature.
- Both routes can form bromoethane, but their reaction types differ.
Answer Ethane + Br₂ → bromoethane + HBr under ultraviolet light (free-radical substitution); ethene + HBr(g) → bromoethane at room temperature (electrophilic addition).
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write one balanced equation for monobromination of ethane and state the condition.
Give me a hint
Replace one hydrogen and show the displaced hydrogen as HBr.
Compare my reasoning
- Ethane loses one H and gains Br.
- Br₂ supplies both the substituting Br and the HBr Br.
- Check carbon, hydrogen and bromine atoms.
C₂H₆ + Br₂ → C₂H₅Br + HBr, under ultraviolet light.
Look for these in your work
- Substitution shown
- HBr included
- Condition stated
Classify ethene + Br₂ → 1,2-dibromoethane and state the condition from this syllabus outcome.
Give me a hint
Check whether bromine replaces H or adds across the double bond.
Compare my reasoning
- One Br attaches to each carbon of C=C.
- No alkene hydrogen is replaced in the addition equation.
- The reaction is electrophilic addition at room temperature.
Electrophilic addition of Br₂ to ethene at room temperature.
Look for these in your work
- Addition identified
- Connectivity correct
- Condition stated
A student claims the two bromoethane routes use the same mechanism because they give the same product. Explain why this is not justified.
Give me a hint
Compare the starting hydrocarbon, conditions and bond changes.
Compare my reasoning
- Ethane route replaces C–H with C–Br under UV.
- Ethene route consumes C=C by adding HBr at room temperature.
- The same product can be formed by different reaction pathways.
The routes have different reactants, conditions and reaction types; matching products do not imply matching mechanisms.
Look for these in your work
- First route compared
- Second route compared
- Conclusion justified
Common mistakes
- Using UV light as the condition for alkene addition
- Calling alkane substitution an addition reaction
- Omitting HBr from the alkane substitution equation
What can you explain now?
Which condition is required for alkane substitution by Cl₂ or Br₂?
Compare with the explanation
Ultraviolet light
UV light initiates the free-radical chain reaction.
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Make it stick
What conditions distinguish halogen substitution of an alkane from addition to an alkene?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.