Propanoic acid is a carboxylic acid used as a food preservative. Describe the key properties of carboxylic acids, including their functional group, homologous series membership, and typical chemical behaviour.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid (CH₃CH₂COOH) is added to bread and other baked goods to extend their shelf life by inhibiting the growth of mould.
Model answer (5 marks)
Carboxylic acids contain the functional group –COOH.
They form a homologous series; each member differs by a –CH₂– unit and shares the same general formula R–COOH.
Examples of members are methanoic, ethanoic, propanoic and butanoic acids.
They are acidic: they react with carbonates, metals and alkalis to give CO₂, water and a salt, and they dissolve in water to give an acidic solution.
They can be prepared by oxidation of primary alcohols.
They form a homologous series; each member differs by a –CH₂– unit and shares the same general formula R–COOH.
Examples of members are methanoic, ethanoic, propanoic and butanoic acids.
They are acidic: they react with carbonates, metals and alkalis to give CO₂, water and a salt, and they dissolve in water to give an acidic solution.
They can be prepared by oxidation of primary alcohols.
Examiner tips
- Use the exact term –COOH for the functional group. Show the series concept with a –CH₂– difference. Name at least two acids. Mention a typical acidic reaction. Include a synthetic route such as oxidation of alcohols.
Common mistakes
- Writing the functional group as –COO– instead of –COOH. Forgetting to state the –CH₂– difference in the series. Listing only one acid or a non‑carboxylic acid. Omitting the acidic behaviour or giving a non‑acidic reaction.
Mark scheme (5 marks)
- Carboxylic acids contain the functional group –COOH
- Carboxylic acids form a homologous series, with members differing by –CH₂– / sharing the same general formula or functional group
- Names at least two correct members of the homologous series of carboxylic acids (methanoic, ethanoic, propanoic, butanoic)
- Carboxylic acids have typical acidic properties, e.g. they react with carbonates / metals / alkalis OR they dissolve in water to produce an acidic solution
- Carboxylic acids can be formed by the oxidation of alcohols
Key terms in this question
carboxylic acids · functional group · homologous series
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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