Methanol and butanoic acid can react together to form an ester. Name the ester formed and describe the conditions needed to carry out this reaction in the laboratory. Explain why butanoic acid is described as a weak acid and state one use of esters.

WJEC A-Level Chemistry (Wales) — 4.5 Carboxylic acids and their derivatives · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

The ester formed is methyl butanoate.

To prepare it, a mixture of methanol and butanoic acid is refluxed in the presence of a strong acid catalyst, usually concentrated sulphuric acid. The heat and the catalyst drive the esterification.

Butanoic acid is a weak acid because it does not fully dissociate in water; only a small fraction of the molecules ionise to give β³²´ and β³²µ.

Esters are widely used as flavourings, perfumes and solvents.

Examiner tips

  • Use the exact name "methyl butanoate". Mention both the acid catalyst and the need for reflux/heat. Explain partial ionisation to justify weak acidity. Give one common ester application such as flavouring or perfume.

Common mistakes

  • Calling the ester "butyl methyl ether". Omitting the acid catalyst or the reflux condition. Saying butanoic acid is a strong acid. Listing an unrelated use such as a polymer.

Mark scheme (5 marks)

  1. The ester formed is methyl butanoate
  2. An acid catalyst is required (concentrated sulfuric acid)
  3. The reaction requires heat / is carried out under reflux / mixture is heated
  4. Butanoic acid does not fully / completely dissociate (in water) / only partially ionises
  5. Esters are used as flavourings / perfumes / solvents

Key terms in this question

ester · weak acid

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