Explain why an acyl chloride, such as ethanoyl chloride, is a more reactive acylating agent than a carboxylic acid anhydride, such as ethanoic anhydride, when reacting with a nucleophile such as water.

Edexcel A-Level Chemistry (9CH0) — 17.3 Carboxylic acids and derivatives · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Acyl chlorides and carboxylic acid anhydrides are both used as acylating agents in organic synthesis. Chemists often choose between them based on reactivity and the by-products formed.

Model answer (5 marks)

1. The C–Cl bond in ethanoyl chloride is more polar than the C–O bond in ethanoic anhydride, making the carbonyl carbon more electron‑poor.
2. Consequently the carbonyl carbon of the acyl chloride is a stronger electrophile and is attacked more readily by the nucleophile.
3. The C–Cl bond is weaker than the C–O bond in the anhydride, so it breaks more easily during the reaction.
4. The by‑product of the acyl chloride hydrolysis is HCl, a strong acid that fully dissociates, whereas the anhydride gives ethanoic acid, a weak acid.
5. In practice acyl chlorides react vigorously (even with cold water) whereas anhydrides react more slowly, demonstrating the greater reactivity of acyl chlorides.

Examiner tips

  • Use the word ‘polar’ and ‘electrophile’ to show understanding of bond polarity. Mention the weaker C–Cl bond to justify easier bond cleavage. State the by‑product and its acidity to explain the difference in reaction conditions. Give a clear example of the reaction rate difference to illustrate reactivity.

Common mistakes

  • Confusing the anhydride with a simple carboxylic acid; not recognising the anhydride’s C–O bond. Forgetting to mention the strength of the C–Cl bond or the acidity of HCl. Using vague terms like ‘more reactive’ without explaining the underlying electronic reasons.

Mark scheme (5 marks)

  1. The C–Cl bond in an acyl chloride is more polar than the C–O bond in an anhydride
  2. The carbonyl carbon in an acyl chloride is therefore a stronger electrophile
  3. The C–Cl bond is weaker than the C–O bond in an anhydride, so it is easier to break during the reaction
  4. The by-product of an acyl chloride reacting with water is HCl, which is a strong acid and fully dissociates, whereas an anhydride produces a carboxylic acid (ethanoic acid), which is a weak acid
  5. Acyl chlorides react vigorously (even at room temperature / with cold water), whereas anhydrides react more slowly, illustrating the greater reactivity of acyl chlorides

Key terms in this question

acylating agent

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