Convert alcohols into halogenoalkanes
Recall the stated hydrogen-halide, potassium-halide/acid and phosphorus- or sulfur-halide routes.
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
What structural change makes a halogenoalkane from an alcohol?
What you will learn
- Recall alcohol substitution using HX
- Recognise KCl with concentrated acid as an HX source
- Select PCl₃, PCl₅ or SOCl₂ conditions
Replace the alcohol hydroxy group
Alcohols can be converted to halogenoalkanes by substitution: the C–O bond is replaced by C–X. Hydrogen halides HX(g) are a stated option; for example, ethanol + HBr → bromoethane + H₂O.
A chloride can be formed using KCl with concentrated H₂SO₄ or H₃PO₄, which supplies HCl in the reaction mixture. State the named reagent combination and do not confuse it with aqueous KCl alone.
Other halogenating agents are in scope
The syllabus also lists PCl₃ with heat, PCl₅, and SOCl₂ as reagents for replacing alcohol –OH with Cl. These are alternative methods, not reagents that all must be combined in one reaction.
For a route question, specify the alcohol, target halogenoalkane and one valid reagent set. Include water or other coproducts only when asked for a balanced equation; focus on the stated substitution and conditions.
Pause and explain
Which listed route can form chloroethane from ethanol?
Worked example
Give two different reagent options for converting ethanol to chloroethane.
Show the worked solution
- One route uses concentrated HCl as the hydrogen halide reagent.
- Another uses KCl with concentrated H₂SO₄ or H₃PO₄ to generate HCl in the mixture.
- Alternatively, a phosphorus or sulfur chloride can replace –OH with Cl.
Answer Use HCl(g), KCl with concentrated H₂SO₄ or H₃PO₄, PCl₃ with heat, PCl₅, or SOCl₂ to convert ethanol to chloroethane by substitution.
Try a topic test
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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.
Name two syllabus reagent routes to convert propan-1-ol to 1-chloropropane.
Give me a hint
Choose two independent alcohol-to-halide reagents from the syllabus list.
Compare my reasoning
- A hydrogen-halide route can use HCl.
- KCl with concentrated H₂SO₄ or H₃PO₄ can supply HCl.
- PCl₃ with heat, PCl₅ or SOCl₂ are other options.
For example, use HCl(g), or KCl with concentrated H₂SO₄; PCl₃/heat, PCl₅ and SOCl₂ are also listed routes.
Look for these in your work
- Two valid routes named
- Conditions/reagent pair kept together
- Substitution recognized
Explain why KCl alone is not the complete route described for converting an alcohol to a chloroalkane.
Give me a hint
Identify how the hydrogen halide is generated in the stated combination.
Compare my reasoning
- The syllabus specifies KCl together with concentrated H₂SO₄ or H₃PO₄.
- The concentrated acid helps generate HCl in the reaction mixture.
- Name the full reagent combination rather than KCl alone.
Use KCl with concentrated H₂SO₄ or H₃PO₄, not KCl alone, for the stated chloride route.
Look for these in your work
- Missing reagent identified
- Role explained
- Full pair stated
A student uses concentrated H₂SO₄ alone and writes that the product must be a chloroalkane. Correct the route.
Give me a hint
Check whether a chlorine source is included.
Compare my reasoning
- The alcohol contains no chlorine atom.
- The listed route pairs concentrated acid with KCl to supply chloride/HCl.
- Alternatively, use an explicit chlorinating agent such as PCl₅ or SOCl₂.
Include a chlorine source: KCl with concentrated acid, HCl, PCl₃, PCl₅ or SOCl₂.
Look for these in your work
- Atom source checked
- Complete reagent route given
- Alternative route supplied
Common mistakes
- Using aqueous chloride salt alone as the listed reagent combination
- Calling the alcohol conversion elimination
- Treating the alternative reagents as a single combined mixture
What can you explain now?
What structural change makes a halogenoalkane from an alcohol?
Compare with the explanation
Replace the C–OH group with C–X
The carbon–oxygen bond is replaced by a carbon–halogen bond.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which alcohol-to-halide routes are available, and what role does KCl play with concentrated acid?
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.