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

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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

---

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]

---

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]

---

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) The hydroxyl oxygen’s lone pair delocalises into the carbonyl carbon more effectively than chlorine’s lone pair does, so in an acyl chloride the carbonyl carbon carries a greater δ+ charge and is more strongly attacked by nucleophiles [1]. Chloride is also a much better leaving group than hydroxide, so it departs more readily once the nucleophile has attacked [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].

---

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.