Predict ring or side-chain halogenation
Use catalyst and light conditions to decide whether halogen replaces a ring hydrogen or a side-chain hydrogen in 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 conditions favour halogenation on the aromatic ring?
What you will learn
- Predict ring halogenation with AlCl₃ or AlBr₃
- Predict side-chain halogenation under ultraviolet light
- Connect conditions to electrophilic or radical substitution
A Lewis-acid catalyst directs reaction to the ring
For halogenation of the aromatic ring, use Cl₂ or Br₂ with AlCl₃ or AlBr₃. The halogen replaces a hydrogen directly on the ring by electrophilic substitution.
In methylbenzene, ring substitution gives a halogenated aromatic ring while keeping CH₃ attached. For example, bromination under AlBr₃ conditions yields ring-brominated methylbenzene isomers.
Ultraviolet light can direct substitution to the side chain
Under UV light, halogen radicals can replace a hydrogen in methylbenzene's side chain. Chlorination at the benzylic position can give C₆H₅CH₂Cl and HCl.
The quick distinction is conditions and location: Lewis-acid catalyst means ring substitution; UV light means side-chain radical substitution. Do not infer both products from one condition set unless the question asks about competing pathways.
Pause and explain
What does UV light commonly indicate for chlorination of methylbenzene?
Worked example
Methylbenzene reacts separately with Br₂/AlBr₃ and with Cl₂ under UV light. Where does halogenation occur in each case?
Show the worked solution
- Br₂/AlBr₃ promotes electrophilic substitution on the aromatic ring.
- Cl₂/UV light promotes radical substitution at the side chain.
- The products therefore place halogen in different positions.
Answer Br₂/AlBr₃ gives ring bromination; Cl₂ under UV light gives side-chain chlorination, such as C₆H₅CH₂Cl.
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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.
Predict whether ring or side-chain substitution is favoured for methylbenzene + Cl₂ under UV light.
Give me a hint
Use the light condition to identify the reaction pathway.
Compare my reasoning
- UV light initiates radical chemistry.
- A side-chain hydrogen is replaced.
- The benzylic product can be C₆H₅CH₂Cl.
Side-chain chlorination is favoured, producing chloromethylbenzene (benzyl chloride) and HCl.
Look for these in your work
- Condition interpreted
- Reaction location identified
- Representative product named
Predict where Br enters when methylbenzene reacts with Br₂ and AlBr₃.
Give me a hint
Distinguish Lewis-acid activation from photochemical radical conditions.
Compare my reasoning
- AlBr₃ activates bromine for electrophilic substitution.
- The ring hydrogen is replaced.
- The methyl side chain remains attached.
Bromine substitutes on the aromatic ring; ring-brominated methylbenzene forms.
Look for these in your work
- Catalyst interpreted
- Ring site chosen
- Side chain retained
A student predicts the same product from Br₂/AlBr₃ and Cl₂/UV for methylbenzene. Explain why conditions change the position of substitution.
Give me a hint
State the different mechanism family and the hydrogen removed in each case.
Compare my reasoning
- AlBr₃ supports electrophilic substitution on the aromatic ring.
- UV light supports radical substitution at the benzylic side chain.
- The products place the halogen at different carbons.
The catalyst condition promotes ring substitution, while UV light promotes side-chain radical substitution; therefore the halogen attaches at different positions.
Look for these in your work
- Both conditions distinguished
- Mechanism families stated
- Product location linked
Common mistakes
- Using UV light to predict ring bromination
- Using AlBr₃ to predict benzylic radical substitution
- Confusing side-chain bromomethylbenzene (benzyl bromide) with ring-brominated methylbenzene
- Ignoring which hydrogen is replaced
What can you explain now?
Which conditions favour halogenation on the aromatic ring?
Compare with the explanation
Cl₂ or Br₂ with AlCl₃ or AlBr₃
The Lewis-acid catalyst promotes electrophilic ring substitution.
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Make it stick
Which conditions tell you whether methylbenzene is halogenated on the ring or on its side chain?
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