Use Friedel–Crafts acylation
Use ethanoyl chloride and aluminium chloride with heat to attach an acyl group to benzene.
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 group is transferred in the specified acylation?
What you will learn
- Recall CH₃COCl, AlCl₃ and heat as the specified conditions
- Predict the aryl ketone product
- Distinguish acylation from alkylation
Transfer the acyl group
Friedel–Crafts acylation of benzene uses CH₃COCl and AlCl₃ with heat. The ethanoyl group, CH₃CO–, replaces a ring hydrogen, giving phenylethanone (also commonly called acetophenone).
The carbonyl stays between the aromatic ring and the methyl group: C₆H₅COCH₃. Do not confuse this product with methylbenzene, which would result from transferring CH₃ without the carbonyl.
Balance the overall substitution
The simplified equation is C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl, with AlCl₃ and heat. The ring loses H; chloride combines with it to form HCl.
This is the listed syllabus example. Distinguish the acyl chloride reagent from the acyl group transferred to the ring.
Pause and explain
What is the organic product formula for the specified benzene acylation?
Worked example
Name the product of benzene reacting with CH₃COCl and AlCl₃ on heating.
Show the worked solution
- CH₃COCl provides the CH₃CO– acyl group.
- That group replaces a ring hydrogen.
- The product is phenylethanone, C₆H₅COCH₃.
Answer Phenylethanone (acetophenone) forms, with HCl as by-product.
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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 organic product and by-product for the specified acylation.
Give me a hint
Replace one ring hydrogen with CH₃CO–.
Compare my reasoning
- The benzene ring becomes C₆H₅–.
- Attach the acyl carbonyl: –COCH₃.
- Use the displaced H and Cl to form HCl.
C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl.
Look for these in your work
- Acyl group retained
- Ring hydrogen displaced
- Equation balanced
Explain the structural difference between the products of Friedel–Crafts alkylation and acylation.
Give me a hint
Compare the group attached to the ring.
Compare my reasoning
- Alkylation attaches CH₃ directly to the ring.
- Acylation attaches CH₃CO– through its carbonyl carbon.
- The acylation product therefore contains C=O.
Alkylation gives methylbenzene with direct ring–CH₃ bonding; acylation gives phenylethanone with ring–COCH₃ and a carbonyl group.
Look for these in your work
- Alkylation group stated
- Acylation group stated
- Carbonyl difference stated
A learner predicts phenylethanol from benzene and CH₃COCl/AlCl₃. Locate the error.
Give me a hint
Check whether the acylation conditions contain a reducing reagent.
Compare my reasoning
- The transferred group is CH₃CO–.
- No reducing agent is stated to convert C=O to CH₂OH.
- The product is phenylethanone.
The conditions introduce CH₃CO– without reducing the carbonyl; the product is phenylethanone, not phenylethanol.
Look for these in your work
- Acyl group tracked
- Unsupported reduction identified
- Product corrected
Common mistakes
- Dropping the carbonyl and giving methylbenzene
- Leaving Cl in the organic product
- Confusing acylation with esterification
- Omitting heat or AlCl₃
What can you explain now?
Which group is transferred in the specified acylation?
Compare with the explanation
CH₃CO–
Ethanoyl chloride transfers an acyl group containing the carbonyl.
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Make it stick
How can the carbonyl group help you distinguish the acylation product from the alkylation product?
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.