Convert an alcohol into a halogenoalkane
Recall the HX, KCl/concentrated acid, phosphorus chloride and thionyl chloride routes.
Section 16 · Hydroxy compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which change occurs when an alcohol becomes a halogenoalkane by substitution?
What you will learn
- Recall alcohol-to-halide reagent choices
- State complete reagent combinations
- Describe substitution at C–OH
Replace –OH with a halogen
Alcohols form halogenoalkanes by substitution using HX(g), or a chloride route with KCl and concentrated H₂SO₄ or H₃PO₄. For example, ethanol + HCl → chloroethane + water.
The syllabus also lists PCl₃ with heat, PCl₅, or SOCl₂ to replace –OH with Cl. These are alternative reagents; choose one complete route and name its conditions where stated.
Track atoms and avoid incomplete reagent names
The C–O bond is replaced by C–X while the carbon skeleton is retained. For an HCl route, the alcohol oxygen leaves in water.
KCl alone is not the listed combination: pair it with concentrated sulfuric or phosphoric acid. Do not confuse this substitution with dehydration, which removes water to form C=C.
Pause and explain
Which is a complete listed chloride route from an alcohol?
Worked example
Give two syllabus routes for converting propan-1-ol into 1-chloropropane.
Show the worked solution
- Use one hydrogen chloride route.
- A second option is KCl with concentrated acid.
- Other listed chlorinating reagents include PCl₃/heat, PCl₅ and SOCl₂.
Answer Use HCl(g), or KCl with concentrated H₂SO₄/H₃PO₄; PCl₃/heat, PCl₅ or SOCl₂ are alternatives.
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.
Name three distinct reagent choices for converting ethanol to chloroethane.
Give me a hint
List alternative routes rather than mixing them.
Compare my reasoning
- One is HCl(g).
- One is KCl with concentrated H₂SO₄ or H₃PO₄.
- PCl₃/heat, PCl₅ and SOCl₂ are other listed options.
For example: HCl(g); KCl with concentrated H₂SO₄; and SOCl₂ (each a separate route).
Look for these in your work
- Three options named
- KCl route complete
- Alternatives kept separate
A student treats propan-2-ol with concentrated H₂SO₄ and claims 2-chloropropane is formed. Correct the reagent set.
Give me a hint
Check whether the mixture contains a chlorine source.
Compare my reasoning
- Concentrated H₂SO₄ alone contains no chlorine.
- Use HCl, KCl plus concentrated acid, PCl₅ or SOCl₂.
- The intended transformation is substitution of –OH by Cl.
Include a chlorine source, such as HCl(g), KCl with concentrated acid, PCl₅ or SOCl₂.
Look for these in your work
- Missing atom source found
- Valid reagent supplied
- Reaction type stated
How is alcohol substitution different from alcohol dehydration?
Give me a hint
Compare the group leaving and product bond.
Compare my reasoning
- Substitution replaces –OH with X and forms a halogenoalkane.
- Dehydration removes water and forms C=C.
- The reagents and products therefore differ.
Substitution gives R–X; dehydration eliminates water to form an alkene.
Look for these in your work
- Both transformations named
- Products distinguished
- Bond change explained
Common mistakes
- Using KCl without concentrated acid
- Calling the route elimination
- Combining all alternative chlorinating reagents in one mixture
What can you explain now?
Which change occurs when an alcohol becomes a halogenoalkane by substitution?
Compare with the explanation
C–OH is replaced by C–X
The carbon skeleton remains while the hydroxy group is replaced.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which reagents convert an alcohol into a halogenoalkane, and which are alternatives?
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.