Propanoic acid is a carboxylic acid. Describe the chemical properties of propanoic acid and explain how you could distinguish it from a carbonyl compound such as propanal. Include the name of a suitable reagent for the test and the expected observations for each compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Propanoic acid (CH₃CH₂COOH) and propanal (CH₃CH₂CHO) are both organic compounds with similar molecular formulae but different functional groups.
Model answer (5 marks)
Propanoic acid is a weak acid that partially dissociates in water to give H⁺ ions. It reacts with carbonate salts (e.g. Na₂CO₃ or CaCO₃) to form CO₂ gas, giving effervescence. Propanal, a carbonyl compound, does not react with carbonates and therefore shows no effervescence.
To distinguish the two, a suitable reagent is Fehling’s solution (or alternatively Tollens’ reagent). When propanal is added to Fehling’s solution the aldehyde is oxidised to propanoic acid, giving a brick‑red precipitate of Cu₂O. Propanoic acid gives no change – the solution remains blue, indicating a negative result.
To distinguish the two, a suitable reagent is Fehling’s solution (or alternatively Tollens’ reagent). When propanal is added to Fehling’s solution the aldehyde is oxidised to propanoic acid, giving a brick‑red precipitate of Cu₂O. Propanoic acid gives no change – the solution remains blue, indicating a negative result.
Examiner tips
- Mention the weak‑acid nature and CO₂ evolution for the acid; state that the carbonyl does not give effervescence; name Fehling’s or Tollens’ reagent and describe the red precipitate or silver mirror for the aldehyde and no change for the acid.
Common mistakes
- Saying the acid reacts with carbonates but not specifying CO₂ effervescence; confusing the aldehyde test with a non‑reduction test; using the wrong reagent name (e.g. citing a reducing agent that does not distinguish the groups).
Mark scheme (5 marks)
- Propanoic acid is a weak acid / partially dissociates in water / produces H⁺ ions in solution
- Propanoic acid reacts with carbonates / named carbonate (e.g. sodium carbonate / calcium carbonate) to produce carbon dioxide gas / effervescence
- Propanal (carbonyl compound) does NOT react with carbonates / does not produce effervescence / no carbon dioxide produced
- Named suitable reagent to distinguish aldehyde from carboxylic acid: Fehling's solution OR Tollens' reagent / acidified potassium dichromate / Benedict's solution
- Correct contrasting observations for the named reagent: propanal gives a positive result (e.g. Fehling's → brick-red precipitate / Tollens' → silver mirror) whereas propanoic acid gives no change / negative result
Key terms in this question
carboxylic acid · carbonyl compound
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
More Carbonyls and acids questions
- Methanal (HCHO) and ethanal (CH₃CHO) are both aldehydes. Propanone (CH₃COCH₃) is…
- Methanoic acid (HCOOH) is the simplest carboxylic acid. It is found in ant venom…
- Butanedioic acid is a dicarboxylic acid found in unripe fruit. It reacts with so…
- Ethanoic acid reacts with ethanol to form an ester and water. Describe the prope…