Prepare chlorobenzene from benzene
Use chlorine and an aluminium halide catalyst to substitute a ring hydrogen and make chlorobenzene.
Section 31 · Halogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 31. The syllabus examples are benzene to chlorobenzene and methylbenzene to the 2- and 4-ring-substituted products. Keep ring-bound chlorine distinct from side-chain –CH₂Cl. Explain the reactivity comparison using the C–Cl bond and aromatic delocalisation. Written practice is self-checked
Before you begin
- Use aromatic substitution from Topic 30, structural formulas and the difference between an sp² aromatic carbon and an sp³ alkyl carbon.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which product has chlorine directly attached to the benzene ring?
What you will learn
- Recall the arene chlorination reagents and catalyst
- Write the substitution product and hydrogen chloride by-product
- Identify chlorobenzene as a halogenoarene
Chlorine substitutes on the aromatic ring
Benzene reacts with Cl₂ in the presence of AlCl₃ or AlBr₃ to form chlorobenzene. One ring hydrogen is replaced by chlorine, and HCl is formed overall.
The chlorine atom is bonded directly to an aromatic carbon: C₆H₅Cl. This is chlorobenzene, a halogenoarene. It is not benzyl chloride, which has a –CH₂Cl side chain.
Keep reagent and catalyst roles distinct
Cl₂ supplies chlorine; AlCl₃ or AlBr₃ acts as the Lewis-acid catalyst that activates the halogen for electrophilic substitution.
The overall equation C₆H₆ + Cl₂ → C₆H₅Cl + HCl makes the substitution and atom balance visible. It is not addition across the ring because the aromatic system is restored in the product.
Pause and explain
What is formed alongside chlorobenzene in the overall substitution equation?
Worked example
Give the product and catalyst for chlorinating benzene at the ring.
Show the worked solution
- Use Cl₂ with AlCl₃ or AlBr₃.
- Replace one ring H with Cl.
- The organic product is chlorobenzene and HCl is the by-product.
Answer Benzene + Cl₂ → chlorobenzene + HCl, using AlCl₃ or AlBr₃ catalyst.
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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.
Write a balanced equation for benzene forming chlorobenzene.
Give me a hint
Replace one ring hydrogen and account for both chlorine atoms.
Compare my reasoning
- Benzene is C₆H₆.
- Chlorobenzene is C₆H₅Cl.
- The remaining H and Cl form HCl.
C₆H₆ + Cl₂ → C₆H₅Cl + HCl.
Look for these in your work
- Ring substitution represented
- Chlorine atoms balanced
- By-product correct
State two suitable catalysts for the chlorination reaction in the syllabus.
Give me a hint
Recall the aluminium halide options.
Compare my reasoning
- Use AlCl₃ or AlBr₃.
- Either acts as a Lewis-acid catalyst.
- Cl₂ remains the halogen reagent.
AlCl₃ or AlBr₃.
Look for these in your work
- First catalyst named
- Second catalyst named
- Reagent separated from catalyst
A product is described as C₆H₅CH₂Cl. Is it chlorobenzene? Explain from its connectivity.
Give me a hint
Trace whether Cl is bonded to the ring carbon or the side-chain carbon.
Compare my reasoning
- C₆H₅CH₂Cl has a CH₂ group between the ring and Cl.
- Therefore chlorine is on the side chain.
- Chlorobenzene is C₆H₅Cl with direct ring–Cl bonding.
No. C₆H₅CH₂Cl is chloromethylbenzene (benzyl chloride); chlorobenzene is C₆H₅Cl with chlorine directly bonded to the aromatic ring.
Look for these in your work
- Side-chain connectivity identified
- Product distinguished
- Chlorobenzene formula stated
Common mistakes
- Placing Cl on a one-carbon side chain
- Calling the product chloromethylbenzene
- Forgetting the catalyst
- Writing a permanent addition product
What can you explain now?
Which product has chlorine directly attached to the benzene ring?
Compare with the explanation
Chlorobenzene
Its formula C₆H₅Cl has ring-bound chlorine.
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Make it stick
How do the formula and catalyst condition identify chlorobenzene formation from benzene?
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