Compare chloroethane and chlorobenzene reactivity
Explain why chlorobenzene is less reactive than chloroethane toward nucleophilic substitution by comparing their C–Cl bonds.
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 structural feature strengthens the C–Cl bond in chlorobenzene?
What you will learn
- Distinguish a halogenoalkane from a halogenoarene by connectivity
- Compare C–Cl bond environments in chloroethane and chlorobenzene
- Explain chlorobenzene's lower reactivity toward nucleophilic substitution
The chlorine atom has a different bonding environment
In chloroethane, chlorine is bonded to an sp³ carbon in an aliphatic chain. In chlorobenzene, chlorine is bonded directly to an sp² carbon in the aromatic ring.
A lone pair on chlorine can overlap with the benzene π system. This delocalisation gives the aryl C–Cl bond some partial double-bond character, making it shorter and stronger than the C–Cl bond in chloroethane.
A stronger aryl C–Cl bond is harder to break
Nucleophilic substitution requires the C–Cl bond to break as the nucleophile replaces chlorine. Because the bond in chlorobenzene is strengthened by partial double-bond character and its carbon is sp² hybridised, chlorobenzene is much less reactive than chloroethane in this comparison.
Do not say chlorobenzene cannot react under any conditions. The syllabus outcome asks for the relative reactivity and its explanation; state that it is less reactive toward nucleophilic substitution under comparable conditions.
Pause and explain
Which compound is less reactive toward nucleophilic substitution?
Worked example
Explain why chloroethane undergoes nucleophilic substitution more readily than chlorobenzene.
Show the worked solution
- Chloroethane has a C–Cl bond to an sp³ carbon.
- In chlorobenzene the Cl lone pair overlaps with the ring π system, giving the C–Cl bond partial double-bond character.
- The stronger aryl C–Cl bond is harder to break, so chlorobenzene is less reactive.
Answer Chlorobenzene is less reactive because delocalisation gives its aryl C–Cl bond partial double-bond character, making it shorter and stronger than the C–Cl bond in chloroethane.
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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.
Compare the carbon atom bonded to chlorine in chloroethane and chlorobenzene.
Give me a hint
Identify whether each carbon belongs to an alkyl chain or aromatic ring.
Compare my reasoning
- In chloroethane, C–Cl is on an sp³ aliphatic carbon.
- In chlorobenzene, C–Cl is directly on an sp² aromatic carbon.
- The aromatic bond can interact with the ring π system.
Chloroethane has an aliphatic sp³ C–Cl bond; chlorobenzene has an aryl sp² C–Cl bond that can gain partial double-bond character through delocalisation.
Look for these in your work
- First bonding environment stated
- Second bonding environment stated
- Electronic interaction noted
Complete the reasoning: chlorine's lone pair overlaps with the aromatic π system, so the aryl C–Cl bond has …
Give me a hint
State the effect on bond order, strength and reactivity.
Compare my reasoning
- It gains partial double-bond character.
- It becomes shorter and stronger.
- It is harder for a nucleophile to displace Cl.
The bond has partial double-bond character, is shorter and stronger, and is less readily broken in nucleophilic substitution.
Look for these in your work
- Bond character stated
- Strength effect stated
- Reactivity consequence stated
A learner explains chlorobenzene's lower reactivity by saying that chlorine is farther from the nucleophile. Give the syllabus-supported structural explanation instead.
Give me a hint
Focus on the C–Cl bond, hybridisation and delocalisation.
Compare my reasoning
- Chlorine is attached directly to an sp² ring carbon.
- Its lone pair overlaps with the aromatic π system.
- This strengthens the C–Cl bond and makes substitution less facile.
The chlorine lone pair delocalises into the ring, giving the aryl C–Cl bond partial double-bond character; its shorter, stronger bond is harder to break than chloroethane's.
Look for these in your work
- Bonded carbon described
- Delocalisation included
- Relative reactivity explained
Common mistakes
- Saying the C–Cl bond in chlorobenzene is weaker
- Ignoring the chlorine lone-pair/ring π overlap
- Claiming chlorobenzene has chlorine on a side chain
- Saying less reactive means chemically impossible to react
What can you explain now?
What structural feature strengthens the C–Cl bond in chlorobenzene?
Compare with the explanation
Delocalisation of a chlorine lone pair into the aromatic π system
Overlap gives the aryl C–Cl bond partial double-bond character.
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Make it stick
How does the chlorine lone pair change the aryl C–Cl bond and its reactivity?
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