Aspirin (2-ethanoylhydroxybenzoic acid) is manufactured by reacting 2-hydroxybenzoic acid with ethanoic anhydride rather than with ethanoic acid. Explain why ethanoic anhydride is preferred over ethanoic acid for this esterification reaction, and state the name of the other organic product formed when ethanoic anhydride reacts with the hydroxyl group of 2-hydroxybenzoic acid.

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).

Aspirin is one of the world's most widely used medicines. Its synthesis involves an acylation reaction in which an —OH group reacts with an acylating agent to form an ester linkage.

Model answer (5 marks)

1. Ethanoic anhydride is a more reactive acylating agent than ethanoic acid.
2. In ethanoic acid the hydroxyl group attached to the carboxyl carbon reduces the electrophilicity of the carbonyl carbon, making it a poorer acylating agent.
3. The esterification of an alcohol with ethanoic acid is reversible and limited by equilibrium, whereas with ethanoic anhydride the reaction is essentially irreversible.
4. Ethanoic anhydride is safer and less hazardous to handle than acyl chlorides such as ethanoyl chloride, making it preferable on an industrial scale.
5. The other organic product formed is ethanoic acid.

Examiner tips

  • Use the command word ‘explain’ – give a clear, concise reason for each point. Include the key terms: ‘more reactive’, ‘electrophilic’, ‘reversible’, ‘irreversible’, ‘safer’, ‘ethanoic acid’.
  • Show the points in order and keep each sentence short – examiners look for concise, accurate statements.

Common mistakes

  • Confusing ethanoic acid with acetic acid or using the wrong product name. Failing to mention the equilibrium/irreversibility difference. Using vague terms like ‘better’ without explaining why.

Mark scheme (5 marks)

  1. Ethanoic anhydride is a more reactive acylating agent than ethanoic acid
  2. Ethanoic acid is a poor acylating agent because the —OH group on the carboxyl group makes it less electrophilic / the carbonyl carbon is less susceptible to nucleophilic attack
  3. The reaction of ethanoic acid with an alcohol is reversible / an equilibrium, whereas the reaction of ethanoic anhydride is essentially irreversible
  4. Ethanoic anhydride is safer / less hazardous to handle than acyl chlorides (e.g. ethanoyl chloride), which also makes it preferable on an industrial scale
  5. The other organic product is ethanoic acid

Key terms in this question

ethanoic anhydride · esterification

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