Vinegar contains ethanoic acid, a member of the carboxylic acids homologous series. Describe the key features of carboxylic acids, including their functional group, typical chemical properties, and name the other three members of the homologous series to which ethanoic acid belongs.

OCR A-Level Chemistry A (H432) — 6.1 Aromatic compounds, carbonyls and acids · Describe · 5 marks · View as Markdown

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

Vinegar is a dilute solution of ethanoic acid, widely used in food preservation. Ethanoic acid is formed when ethanol is oxidised. It belongs to a homologous series of organic compounds that share a common functional group and display typical acidic behaviour.

Model answer (5 marks)

Carboxylic acids contain the functional group –COOH.
They are acidic: they donate H⁺ ions in solution and react with bases, carbonates and metals.
The homologous series starts with methanoic acid, followed by ethanoic acid, then propanoic acid and butanoic acid.

Examiner tips

  • Use the exact term –COOH for the functional group. Show the acidic behaviour with a short example. Name the three other members in order.
  • common_mistakes
  • :
  • Writing the wrong functional group (e.g. –COO⁻). Omitting the acidic property. Listing the wrong members of the series.

Mark scheme (5 marks)

  1. Carboxylic acids contain the functional group –COOH
  2. Carboxylic acids have typical acidic properties (e.g. react with bases/carbonates/metals, or produce H⁺ ions in solution)
  3. Methanoic acid is the first member of the homologous series
  4. Propanoic acid is the third member of the homologous series
  5. Butanoic acid is the fourth member of the homologous series

Key terms in this question

carboxylic acids · functional group · homologous series · ethanoic acid

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