Predict ring-chlorinated methylbenzene products
Recognise the two distinct ring-substitution positions formed when methylbenzene is chlorinated under catalyst conditions.
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 carbon bears chlorine in 4-chloromethylbenzene as the Topic 31 ring-substitution example?
What you will learn
- Identify ring substitution of methylbenzene with Cl₂ and an aluminium halide catalyst
- Name the 2- and 4-ring-substituted products
- Distinguish those products from side-chain chlorination
The methyl group creates distinct ring positions
With Cl₂ and AlCl₃ or AlBr₃, methylbenzene undergoes electrophilic substitution on the aromatic ring. The syllabus names 2-chloromethylbenzene and 4-chloromethylbenzene as examples of the products.
Here the locants 2 and 4 identify where chlorine is attached to the ring relative to the methyl group at carbon 1. In less ambiguous wording, these are 2-chlorotoluene and 4-chlorotoluene. The chlorine is ring-bound, not in a –CH₂Cl side chain.
Use conditions to resolve the naming ambiguity
When a question says ring chlorination or gives AlCl₃/AlBr₃, attach Cl directly to a ring carbon. When it gives UV light, side-chain radical substitution can give C₆H₅CH₂Cl.
Both products contain one chlorine atom, but their connectivity differs. Draw or state the bond location whenever the name could be read ambiguously.
Pause and explain
Which conditions indicate ring chlorination of methylbenzene?
Worked example
Methylbenzene reacts with Cl₂ and AlCl₃. Name the two distinct ring-substitution products in the syllabus example.
Show the worked solution
- The reaction occurs on the aromatic ring.
- The methyl group is at position 1.
- Chlorine can be at position 2 or position 4.
Answer 2-chloromethylbenzene and 4-chloromethylbenzene (2- and 4-chlorotoluene), with chlorine attached directly to the ring.
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.
A student draws 2-chloromethylbenzene with –CH₂Cl at position 2. Correct the structure for the syllabus ring-substitution product.
Give me a hint
Read the locant as the position of Cl on the ring, with methyl at position 1.
Compare my reasoning
- Keep CH₃ attached to ring carbon 1.
- Attach Cl directly to adjacent ring carbon 2.
- There should be no CH₂ group between ring and Cl.
Draw methyl at ring carbon 1 and chlorine directly on ring carbon 2; the product is the ring-substituted 2-chloromethylbenzene (2-chlorotoluene).
Look for these in your work
- Methyl position fixed
- Ring chlorine position correct
- Connectivity distinguished
What two ring-position products are named for chlorinating methylbenzene with AlCl₃?
Give me a hint
Use the syllabus example and preserve ring-bound Cl.
Compare my reasoning
- The methyl group sets position 1.
- The two distinct cited positions are 2 and 4.
- Name each with ring-bound chlorine.
2-chloromethylbenzene and 4-chloromethylbenzene (2- and 4-chlorotoluene).
Look for these in your work
- First position correct
- Second position correct
- Products identified as ring-substituted
Compare C₆H₄ClCH₃ and C₆H₅CH₂Cl by chlorine location.
Give me a hint
Follow the atom sequence from the benzene ring to chlorine.
Compare my reasoning
- In C₆H₄ClCH₃, Cl is attached directly to the ring.
- In C₆H₅CH₂Cl, Cl is attached to a side-chain carbon.
- The formulas encode different connectivities and compounds.
C₆H₄ClCH₃ represents ring-chlorinated methylbenzene; C₆H₅CH₂Cl is chloromethylbenzene with side-chain chlorine.
Look for these in your work
- Ring-bound Cl located
- Side-chain Cl located
- Isomers distinguished
Common mistakes
- Reading 2-chloromethylbenzene as a –CH₂Cl side chain
- Giving only one distinct ring position
- Using UV light when the question specifies catalyst conditions
- Ignoring that ring substituents change product position
What can you explain now?
Which carbon bears chlorine in 4-chloromethylbenzene as the Topic 31 ring-substitution example?
Compare with the explanation
Ring carbon 4
The locant 4 is the position of ring-bound chlorine relative to methyl at 1.
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Make it stick
How do you distinguish the ring-chlorinated products of methylbenzene from chloromethylbenzene?
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.