Halogenate an aromatic ring
Use chlorine or bromine with an aluminium halide catalyst to substitute a ring hydrogen in benzene or methylbenzene.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 30. Follow the listed reagents, catalysts, temperatures and reaction scope for benzene and methylbenzene. Directing effects are limited to –NH₂, –OH, –R, –NO₂, –COOH and –COR. Written practice is self-checked
Before you begin
- Use the Topic 29 model of benzene as a planar aromatic ring with a delocalised π system, and recognise electrophiles, nucleophiles and organic functional groups.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which catalyst is suitable for brominating benzene?
What you will learn
- Recall aromatic halogenation reagents and catalyst
- Predict a halogenoarene product from benzene or methylbenzene
- Distinguish ring substitution from side-chain halogenation
The catalyst enables ring substitution
Benzene and methylbenzene react with chlorine or bromine in the presence of AlCl₃ or AlBr₃ to form halogenoarenes. The halogen replaces a hydrogen attached directly to the aromatic ring.
For benzene, chlorination gives chlorobenzene and bromination gives bromobenzene. With methylbenzene, ring substitution can give positional isomers; do not assume a single product unless the question specifies one.
Keep the reaction conditions attached to the reaction
A Lewis-acid catalyst such as AlCl₃ or AlBr₃ activates the halogen for electrophilic substitution. These catalyst conditions distinguish ring halogenation from side-chain substitution under ultraviolet light.
Write the ring product and the hydrogen halide by-product when balancing an equation. For example, C₆H₆ + Cl₂ → C₆H₅Cl + HCl.
Pause and explain
What happens to the benzene ring hydrogen during chlorination with AlCl₃?
Worked example
Give the product and conditions for brominating benzene at the ring.
Show the worked solution
- Use Br₂ and an AlBr₃ catalyst.
- A ring hydrogen is replaced by bromine.
- The products are bromobenzene and HBr.
Answer Benzene reacts with Br₂ in the presence of AlBr₃ to form bromobenzene and HBr.
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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 the overall equation for chlorination of benzene with the appropriate catalyst condition.
Give me a hint
Replace one ring H with Cl and balance the hydrogen as HCl.
Compare my reasoning
- Use Cl₂ with AlCl₃ as catalyst.
- The aromatic product is C₆H₅Cl.
- Place the removed H with the other Cl to form HCl.
C₆H₆ + Cl₂ → C₆H₅Cl + HCl, with AlCl₃ catalyst.
Look for these in your work
- Reagent and catalyst correct
- Ring product correct
- Atoms balanced
What product family results when methylbenzene is brominated with AlBr₃ at the ring?
Give me a hint
Follow the bond to the aromatic ring rather than the methyl group.
Compare my reasoning
- Br replaces a ring hydrogen.
- The benzene ring remains aromatic.
- The product is a ring-brominated methylbenzene isomer.
A ring-brominated methylbenzene forms, such as 2- or 4-bromotoluene; the exact positional isomer depends on where substitution occurs.
Look for these in your work
- Ring site identified
- Product family named
- Isomer limitation noted
A proposed equation for benzene bromination gives cyclohexene as the product. Explain why that does not match the syllabus reaction.
Give me a hint
Check catalyst, bond change and aromaticity.
Compare my reasoning
- The stated reaction uses Br₂ with AlBr₃.
- It replaces one ring H with Br.
- It forms bromobenzene while the aromatic ring is restored.
The syllabus reaction is electrophilic substitution with Br₂/AlBr₃, forming bromobenzene and HBr; cyclohexene is not the halogenoarene product.
Look for these in your work
- Conditions checked
- Substitution identified
- Correct product family given
Common mistakes
- Using UV light as the condition for aromatic ring halogenation
- Forgetting the AlCl₃ or AlBr₃ catalyst
- Putting halogen on the methyl side chain when the question says ring substitution
- Forgetting the HCl or HBr by-product
What can you explain now?
Which catalyst is suitable for brominating benzene?
Compare with the explanation
AlBr₃
The syllabus lists AlBr₃ for bromination.
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Make it stick
Which reagent and catalyst produce a halogenoarene by substituting a ring hydrogen?
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.