Build a full comparison of aryl and alkyl halides
Use connectivity, carbon hybridisation and bond delocalisation together to explain the halogenoarene–halogenoalkane reactivity difference.
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
What three ideas make a complete explanation of the reactivity difference?
What you will learn
- Classify chlorobenzene and chloroethane
- Link ring attachment to C–Cl bond character
- Present a complete comparative explanation with a qualified conclusion
Make the comparison explicit
Name the compounds and locate chlorine: chlorobenzene has ring-bound Cl; chloroethane has Cl bonded to an aliphatic carbon. This prevents the comparison from silently changing to a side-chain isomer.
Then compare the bonding: chlorine's lone pair delocalises into the aromatic π system, giving chlorobenzene's C–Cl bond partial double-bond character. The bond is shorter and stronger than in chloroethane.
Connect bond strength to substitution
A nucleophile can replace Cl only if the C–Cl bond breaks. The stronger aryl C–Cl bond in chlorobenzene resists that step, so chlorobenzene is less reactive toward nucleophilic substitution than chloroethane.
A full explanation has three connected parts: delocalisation, stronger/shorter C–Cl bond, and lower substitution reactivity. Avoid unsupported absolutes such as ‘never reacts’.
Pause and explain
What is the correct qualified conclusion?
Worked example
Write a concise comparative explanation of the reactivity of chlorobenzene and chloroethane.
Show the worked solution
- State that chlorobenzene is less reactive toward nucleophilic substitution.
- Explain the chlorine lone pair's delocalisation into the ring and partial double-bond character.
- Link the stronger C–Cl bond to more difficult bond breaking.
Answer Chlorobenzene is less reactive because a chlorine lone pair delocalises into the ring, strengthening and shortening the aryl C–Cl bond; it is harder to break than chloroethane's C–Cl bond.
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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.
Mark this explanation complete or incomplete: ‘Chlorobenzene is less reactive because its C–Cl bond is stronger.’ What mechanism-level reason is missing?
Give me a hint
Add the electronic cause of the bond strength.
Compare my reasoning
- State the chlorine lone pair overlaps with the aromatic π system.
- This gives the bond partial double-bond character.
- The stronger bond is less easily broken during substitution.
It is incomplete: explain that chlorine's lone pair delocalises into the ring, giving the C–Cl bond partial double-bond character and strengthening it.
Look for these in your work
- Gap identified
- Delocalisation added
- Bond effect connected
Why is the phrase ‘chlorobenzene is less reactive’ more accurate than ‘chlorobenzene does not react’?
Give me a hint
Keep the claim within a comparative syllabus outcome.
Compare my reasoning
- The outcome compares chlorobenzene with chloroethane.
- It describes lower reactivity toward nucleophilic substitution.
- It does not claim that reaction is impossible under all conditions.
The syllabus establishes a relative difference in nucleophilic-substitution reactivity; it does not support an absolute claim that chlorobenzene can never react.
Look for these in your work
- Comparison stated
- Reaction type specified
- Absolute avoided
A structure is C₆H₅CH₂Cl, not C₆H₅Cl. Can the chlorobenzene reactivity explanation be applied unchanged? Explain.
Give me a hint
First classify where chlorine is attached.
Compare my reasoning
- C₆H₅CH₂Cl has side-chain Cl.
- It is not chlorobenzene with aryl C–Cl bonding.
- The specific delocalised aryl-bond explanation cannot be transferred unchanged.
No. C₆H₅CH₂Cl has side-chain chlorine; chlorobenzene has direct ring–Cl bonding and the aryl C–Cl delocalisation explanation applies to the latter.
Look for these in your work
- Connectivity identified
- Compound classification correct
- Explanation boundary stated
Common mistakes
- Listing a bond difference without linking it to reaction rate
- Saying a nucleophile breaks the benzene ring
- Leaving out which compound is more reactive
- Using ‘impossible’ instead of the relative statement less reactive
What can you explain now?
What three ideas make a complete explanation of the reactivity difference?
Compare with the explanation
Delocalisation, stronger C–Cl bond, lower substitution reactivity
These link electronic structure to the observed comparison.
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Make it stick
How do you make a full, accurate and appropriately qualified aryl-versus-alkyl halide comparison?
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.