Introduce a carbonyl side chain while distinguishing electrophilic ring substitution from nucleophilic carbonyl reactions.
Use an acyl chloride and anhydrous AlCl₃
Benzene reacts with an acyl chloride in the presence of anhydrous aluminium chloride, with suitable warming/reflux conditions, to form an aromatic ketone. AlCl₃ accepts an electron pair and helps generate an acylium electrophile, RCO⁺. Water would react with the acyl chloride and interfere with the Lewis acid, so anhydrous conditions matter.
With propanoyl chloride the whole CH₃CH₂CO group is introduced. The product is C₆H₅COCH₂CH₃, 1-phenylpropan-1-one. The carbonyl carbon is attached directly to the benzene ring; do not instead attach the ethyl end of the acyl group.
Attack the positively charged acyl carbon
The benzene π pair attacks the electrophilic carbon of RCO⁺. Draw the same type of non-aromatic positive intermediate as in nitration, this time with H and –C(=O)R attached to the attacked ring carbon. Removing H⁺ and returning the C–H pair into the ring restores aromaticity.
AlCl₄⁻ can accept the released proton, giving HCl and regenerating AlCl₃ in the syllabus catalytic cycle. Real acylation mixtures can bind AlCl₃ strongly to the carbonyl product, so this cycle is a simplified mechanistic model, not a claim that every industrial work-up uses only a tiny catalyst loading.
June 2022 Paper 2 Q08.1–08.3 applied acylation reasoning to a supplied unfamiliar acylating system. Use the actual electrophile given in such a problem; do not replace it automatically with CH₃CO⁺ from a memorised example.
Diagram placeholder
Propanoylation of benzene to add
Labels to include:
- CH₃CH₂–C⁺=O electrophile and AlCl₄⁻
- Ring π-pair arrow to acyl carbon
- Ring intermediate with H and –C(=O)CH₂CH₃ at attacked carbon
- Partial horseshoe and + charge
- C–H bond arrow back into ring
- C₆H₅C(=O)CH₂CH₃ product
- AlCl₃ regeneration and HCl
The attack arrow points to electrophilic carbon, not oxygen. Preserve the carbonyl group in the electrophile and final ketone, and show the connection between ring carbon and carbonyl carbon.
Acylation describes a change, not one universal mechanism
For an unfamiliar structure, circle the bond or group that changes before choosing a mechanism. A benzene ring elsewhere in the molecule does not make every reaction electrophilic substitution.
| Reaction | Attacking species/site | Mechanism |
|---|---|---|
| Benzene + acyl chloride / AlCl₃ | Ring π electrons attack acylium C | Electrophilic substitution |
| Alcohol + acyl chloride | O lone pair attacks carbonyl C | Nucleophilic addition–elimination |
| Amine + acyl chloride | N lone pair attacks carbonyl C | Nucleophilic addition–elimination |
| Aromatic ketone + NaBH₄ | H⁻ attacks carbonyl C | Nucleophilic addition |
The carbonyl side chain is a useful next reaction site
NaBH₄ reduces C₆H₅COCH₂CH₃ to C₆H₅CH(OH)CH₂CH₃. The new alcohol carbon has phenyl, ethyl, H and OH groups and can be chiral. Under achiral reduction conditions, equal attack on the planar carbonyl faces can give a racemate.
KCN followed by dilute acid instead adds a carbon as a hydroxynitrile: C₆H₅C(OH)(CN)CH₂CH₃. The ring remains aromatic in both transformations. Track carbon atoms and functional groups separately when combining aromatic and aliphatic chemistry.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. What catalyst and condition are essential in the standard acyl chloride Friedel–Crafts example?Show answer
Anhydrous AlCl₃. Water would consume acyl chloride and interfere with the Lewis acid; suitable warming is used for the stated preparation.
Q2. What ketone forms from benzene and ethanoyl chloride?Show answer
C₆H₅COCH₃, phenylethanone (also called acetophenone). The carbonyl carbon bonds directly to the ring.
Q3. Why is benzene acylation electrophilic substitution while ethanol acylation is addition–elimination?Show answer
Benzene donates ring π electrons to an acyl electrophile and ultimately replaces ring H. Ethanol donates an oxygen lone pair to carbonyl carbon, then a leaving group is expelled as C=O reforms.
Q4. Write the AlCl₃ regeneration equation in the simplified mechanism.Show answer
AlCl₄⁻ + H⁺ → AlCl₃ + HCl.
Q5. Does NaBH₄ reduction of C₆H₅COCH₂CH₃ remove benzene’s aromatic ring?Show answer
No. It reduces the side-chain carbonyl to C₆H₅CH(OH)CH₂CH₃. The ring remains aromatic, and the alcohol carbon now has four different groups.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 organic chemistry specification — 3.3.10 coverage and required skills.
- Chemrevise: Aromatic chemistry — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 2 mark scheme — Q03.2 p14 and report Q03.2 p3: electrophilic substitution, arrow to nitronium nitrogen and C–H bond return into the ring.
- AQA June 2023 Paper 2 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
- AQA June 2022 Paper 2 mark scheme — Q08.1–08.3 pp28–29 and report Q08 p6: supplied acylating reagent, AlCl₃, electrophile and ring intermediate.
- AQA June 2022 Paper 2 examiner report — Read with the matching question context described in the mark-scheme source.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.