A Level Chemistry: Carboxylic Acids and Acyl Chlorides — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Carboxylic Acids and Acyl Chlorides revision notes

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Section A

1. Explain why ethanoic acid is acidic but ethanol is not. [3]

2. State the reagent used to convert a carboxylic acid to an acyl chloride. [1]

3. Give a simple chemical test to distinguish a carboxylic acid from a phenol. [2]

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Section B

4. The acid dissociation constants of three acids are given.

Acid	K_a / mol dm⁻³
ethanoic	1.7 × 10⁻⁵
chloroethanoic	1.3 × 10⁻³
trichloroethanoic	2.3 × 10⁻¹

(a) State which is the strongest acid, with a reason from the data. [2]

(b) Explain the trend in terms of molecular structure. [3]

5. Ethanoyl chloride reacts vigorously with water, alcohols and amines.

(a) Write equations for the reaction with (i) water, (ii) ethanol, (iii) methylamine. [3]

(b) State the observation common to all three. [1]

(c) Name the mechanism. [1]

(d) Explain why acyl chlorides are more reactive towards nucleophiles than carboxylic acids. [3]

6. Ethyl ethanoate can be prepared from ethanoic acid and ethanol, or from ethanoyl chloride and ethanol.

(a) State the conditions for the first route and one disadvantage. [3]

(b) Explain why the second route gives a much higher yield. [3]

(c) Phenol reacts only slowly and reversibly with ethanoic acid directly, but reacts readily with ethanoyl chloride. Suggest why this makes acyl chlorides synthetically important. [2]

7. Rank carboxylic acids, phenols and alcohols in order of decreasing acidity, and explain the difference using delocalisation. [3]

8. State three reagents that can convert a carboxylic acid into an acyl chloride. [2]

9. Write the equation for benzoyl chloride reacting with phenol, and name the organic product. [2]

10. Explain why chlorine-substituted carboxylic acids become more acidic as more chlorine atoms are added, and as those atoms are placed closer to the carboxyl group. [2]

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Answers

1. Ethanoic acid loses H⁺ to form the ethanoate ion [1], in which the negative charge is delocalised over both oxygen atoms [1], stabilising it. The alkoxide ion from ethanol has no such delocalisation, so it is far less stable and ethanol does not lose H⁺ readily [1].

2. SOCl₂ (thionyl chloride) [1]. (PCl₅ also accepted.)

3. Add sodium carbonate [1]; the carboxylic acid gives effervescence of carbon dioxide, the phenol does not [1].

4. (a) Trichloroethanoic acid [1], because it has the largest K_a [1].

(b) Chlorine atoms are strongly electron-withdrawing [1]. They pull electron density away from the carboxylate ion, spreading and stabilising the negative charge [1]. The more chlorine atoms, the greater the stabilisation, so the acid dissociates more readily [1].

5. (a) (i) CH₃COCl + H₂O → CH₃COOH + HCl [1] (ii) CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl [1] (iii) CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl [1]

(b) Steamy white fumes of hydrogen chloride [1].

(c) Nucleophilic addition–elimination [1].

(d) In a carboxylic acid, a lone pair on the –OH oxygen delocalises into the C=O group, partly cancelling the δ+ charge on the carbonyl carbon [1]. Chlorine’s lone pairs, in larger 3p orbitals, overlap poorly with carbon’s 2p orbital and delocalise far less, so in an acyl chloride the carbonyl carbon, bonded to two electronegative atoms (O and Cl), keeps a greater δ+ charge and is attacked more readily by nucleophiles [1]. Chloride is also a much better leaving group than hydroxide, so it is lost readily in the elimination step once the nucleophile has added [1].

6. (a) Concentrated sulfuric acid catalyst, heat under reflux [1] [1]. Disadvantage: the reaction is reversible, so the equilibrium yield is limited [1].

(b) The reaction of an acyl chloride with an alcohol is irreversible [1], fast and proceeds at room temperature [1], so it goes essentially to completion rather than reaching an equilibrium [1].

(c) Phenols are weakly nucleophilic, so direct esterification with a carboxylic acid is slow and reversible and gives only a poor yield [1], whereas acyl chlorides provide a fast, reliable route to phenyl esters — extending the range of compounds that can be made in practice [1].

7. Carboxylic acids > phenols > alcohols [1]. In a carboxylate ion the negative charge is delocalised equally over both oxygen atoms, a more effective, symmetrical delocalisation than a phenoxide ion’s charge spreading unevenly around a large aromatic ring [1]; an alkoxide ion (from an alcohol) has no delocalisation available at all [1]. Both C–O bonds in the carboxylate become equivalent, each with partial double-bond character.

8. Any three: PCl₃ (plus heat), PCl₅, or SOCl₂ (thionyl chloride) [1] [1]. These are the same chlorinating-reagent trio used for converting an alcohol’s –OH group to –Cl, now applied to a carboxylic acid’s –OH.

9. C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ + HCl [1] — the product is phenyl benzoate, an aryl ester [1].

10. Chlorine is strongly electronegative and withdraws electron density inductively through the carbon chain, further stabilising the negative charge on the carboxylate ion [1]. More chlorine atoms, or chlorine atoms closer to the carboxyl group, strengthen this inductive effect, so the compound becomes more acidic [1].

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Where marks are usually lost

• Explaining acidity without delocalisation of the carboxylate.
• Getting the electron-withdrawing effect backwards.
• Forgetting HCl as a product of every acyl chloride reaction.
• Saying acyl chlorides are more reactive “because they are more polar” without explaining the leaving group.
• Not stating that esterification is reversible when explaining low yield.
• Placing phenol above carboxylic acids in the acidity order — phenol sits between the carboxylic acid and the alcohol, with the alcohol being the weakest of the three.
• Forgetting that a phenoxide ion’s delocalisation is spread unevenly around the ring, unlike a carboxylate’s equal spread over two oxygens.