Use acyl-group connectivity and acid–base stoichiometry to predict reactions with water, alcohols, phenols, ammonia and amines.
Replace the acid OH, not the carbonyl oxygen
An acyl chloride contains RCOCl. A carboxylic acid reacts with SOCl₂ to form the corresponding acyl chloride, with SO₂ and HCl as other products. The carbonyl remains intact and the acid OH is replaced by Cl.
For propanoic acid, the product is CH₃CH₂COCl, propanoyl chloride. This is different from a chloro-substituted carboxylic acid: chlorine is directly attached to the carbonyl carbon, not to an alkyl carbon elsewhere in the chain.
Acyl chlorides are much more reactive acylating agents than carboxylic acids in the taught reactions. Their reactions with water account for their sensitivity to moisture. The specification requires these transformations; it does not require you to invent an unrequested acyl-substitution mechanism.
Water, alcohol and phenol give related products
Water converts an acyl chloride back to a carboxylic acid and HCl. An alcohol gives an ester and HCl; unlike acid–alcohol esterification, the by-product is not water.
Phenol also reacts with an acyl chloride to form an ester. In the product the phenyl group is attached through oxygen: RCOOC₆H₅. Do not draw RCOC₆H₅, which would be a ketone produced by a different reaction, Friedel–Crafts acylation.
Compare ethanoyl chloride reacting with phenol and with benzene under halogen-carrier conditions. Phenol acylation at oxygen gives phenyl ethanoate, CH₃COOC₆H₅. Benzene ring acylation gives C₆H₅COCH₃. The presence or absence of the linking oxygen is a decisive connectivity difference.
Why excess ammonia or amine is needed
Ammonia reacts with an acyl chloride to make a primary amide, RCONH₂. One ammonia molecule supplies the nitrogen group and a second accepts the HCl produced. With excess ammonia the balanced overall equation is RCOCl + 2NH₃ → RCONH₂ + NH₄Cl.
A primary amine R′NH₂ gives a secondary amide, RCONHR′. A second amine molecule accepts HCl and forms an alkylammonium chloride. Do not attach NH₃⁺ to the carbonyl carbon as the neutral amide product: count N bonds and distinguish the amide from the separate ammonium salt.
If a question writes HCl as a separate product in a simplified equation, its immediate acid–base reaction with excess ammonia or amine still explains the overall stoichiometry. Follow the requested form and make the conditions clear.
Worked route: acid to an N-substituted amide
To make CH₃CH₂CONHCH₃ from propanoic acid, first use SOCl₂ to form propanoyl chloride. Then react with excess methylamine, CH₃NH₂. The propanoyl fragment remains CH₃CH₂CO– and the incoming amine supplies –NHCH₃.
The organic product contains four carbons in total: three from the acid and one from methylamine. Its amide nitrogen has bonds to the carbonyl carbon, H and CH₃. A separate methylammonium chloride salt is formed from the excess amine.
A useful final check is to classify each reagent by the atom that becomes bonded to the acyl carbon: water/alcohol/phenol supply oxygen; ammonia/amine supply nitrogen. Then balance H and Cl and check whether an acid–base by-product must be included.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.
Q1. Give all products when ethanoyl chloride reacts with water.Show answer
Ethanoic acid and HCl: CH₃COCl + H₂O → CH₃COOH + HCl.
Q2. Explain the structural difference between phenyl ethanoate and the product of benzene Friedel–Crafts acylation with ethanoyl chloride.Show answer
Phenyl ethanoate is CH₃COO–C₆H₅, with an oxygen joining the acyl group to the ring. Friedel–Crafts acylation gives CH₃CO–C₆H₅, a ketone with direct carbon–carbon attachment.
Q3. Balance ethanoyl chloride with excess ammonia.Show answer
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl. One NH₃ forms the amide and another neutralises HCl.
Q4. Predict the amide from benzoyl chloride, C₆H₅COCl, and excess ethylamine.Show answer
C₆H₅CONHCH₂CH₃, a secondary amide. Ethylammonium chloride is the salt by-product.
Q5. A student makes CH₃CHClCOOH when asked to use SOCl₂ on propanoic acid. What went wrong?Show answer
They substituted chlorine on the carbon chain. SOCl₂ replaces the carboxylic acid OH to give CH₃CH₂COCl, retaining the carbonyl and forming an acyl chloride.
Sources
Sources and examiner guidance (reviewed 6 October 2026)
- OCR A H432 specification — version 3.1 — 6.1.3, printed pp. 56–57; outcomes and additional guidance, with relevant Module 1 practical skills.
- Chemrevise — OCR A 6.1.3 — Pages 1–4; secondary coverage cross-check. Lesson explanations, data and questions are original Finesse material.
- OCR H432/02 mark scheme — June 2025 — Q18(b–c), Q21(e); printed pp. 19,32. Question-specific evidence, not universal marking rules.
- OCR H432/02 examiner report — June 2025 — Q18(b–c), Q21(e); printed pp. 31–32,47–48. Read with the corresponding question context.
- OCR H432/02 question paper — June 2025 — Q18(b–c), Q21(e); context for the assessment references, not reproduced questions.
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