Compare acyl-chloride, alkyl-chloride and aryl-chloride hydrolysis
Explain why acyl chlorides hydrolyse most readily, alkyl chlorides more slowly and halogenoarenes least readily under comparable conditions.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 33. Focus on the named compounds, reagents and conditions in each outcome; apply the specified reaction type rather than extending it to unlisted cases. Written practice is self-checked
Before you begin
- Recall carboxylic acids, esters, phenols, alcohols, halogenoalkanes and electrophilic/nucleophilic reaction ideas from earlier topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which chloride class hydrolyses most readily?
What you will learn
- Order the three chloride classes by ease of hydrolysis
- Explain the reactivity of an acyl chloride
- Connect low aryl-chloride reactivity to delocalisation and bond strength
Acyl chlorides hydrolyse readily
Acyl chlorides react rapidly with water at room temperature. The carbonyl polarises the C–Cl bond and makes the acyl carbon strongly susceptible to nucleophilic attack; chloride is a good leaving group and addition–elimination follows.
A halogenoalkane hydrolyses by nucleophilic substitution at a saturated carbon and is less reactive under comparable conditions.
Aryl chlorides are least reactive
In a halogenoarene such as chlorobenzene, a chlorine lone pair can delocalise into the aromatic π system. This gives the aryl C–Cl bond partial double-bond character; it is shorter and stronger than the C–Cl bond in a typical halogenoalkane.
The qualitative order is acyl chloride > halogenoalkane > halogenoarene in ease of hydrolysis. Compare the bonds and mechanisms; do not claim aryl chlorides can never react under any conditions.
Pause and explain
Why is chlorobenzene less reactive than chloroethane toward hydrolysis?
Worked example
Order ethanoyl chloride, chloroethane and chlorobenzene by ease of hydrolysis.
Show the worked solution
- Acyl chloride has an electrophilic carbonyl carbon and a good leaving group.
- Chloroethane undergoes substitution at an sp³ carbon.
- Chlorobenzene has a stronger aryl C–Cl bond with partial double-bond character.
Answer Ethanoyl chloride > chloroethane > chlorobenzene.
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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 qualitative order of hydrolysis ease for CH₃COCl, CH₃CH₂Cl and C₆H₅Cl.
Give me a hint
Classify each as acyl, alkyl or aryl chloride.
Compare my reasoning
- CH₃COCl is an acyl chloride.
- CH₃CH₂Cl is a halogenoalkane.
- C₆H₅Cl is a halogenoarene.
CH₃COCl > CH₃CH₂Cl > C₆H₅Cl.
Look for these in your work
- Classes identified
- Order correct
- Hydrolysis comparison
Explain why an acyl chloride is more reactive toward water than a halogenoalkane.
Give me a hint
Compare the electrophilic centre and the leaving-group pathway.
Compare my reasoning
- The acyl carbon is strongly electrophilic because of the carbonyl.
- Water can add to this carbon.
- Chloride leaves as the carbonyl reforms.
The polar carbonyl makes the acyl carbon highly susceptible to nucleophilic attack; addition–elimination expels chloride readily, so hydrolysis is rapid.
Look for these in your work
- Carbonyl effect
- Attack site
- Leaving-group step
A student says chlorobenzene hydrolyses slowly only because it is insoluble in water. Give the bond-based explanation asked by the syllabus.
Give me a hint
Focus on chlorine lone-pair interaction with the aromatic ring.
Compare my reasoning
- A chlorine lone pair delocalises into the π system.
- The aryl C–Cl bond gains partial double-bond character.
- The bond is shorter and stronger, making hydrolysis less easy.
Delocalisation gives the aryl C–Cl bond partial double-bond character, making it shorter and stronger than an alkyl C–Cl bond; this explains its lower reactivity.
Look for these in your work
- Delocalisation
- Bond character
- Relative strength
Common mistakes
- Claiming all C–Cl bonds have the same reactivity
- Ignoring aryl bond delocalisation
- Saying chlorobenzene can never react
- Calling acyl-chloride hydrolysis a substitution at an sp³ carbon
What can you explain now?
Which chloride class hydrolyses most readily?
Compare with the explanation
Acyl chloride
The carbonyl carbon is highly susceptible to nucleophilic attack.
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Make it stick
What structural feature explains the order acyl chloride > alkyl chloride > halogenoarene for hydrolysis?
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.