# 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.

> WJEC A-Level Chemistry (Wales) — 3.4 Carbonyls and acids · Describe · 5 marks

> Propanoic acid (CH₃CH₂COOH) and propanal (CH₃CH₂CHO) are both organic compounds with similar molecular formulae but different functional groups.

## Mark scheme (5 marks)

1. Propanoic acid is a weak acid / partially dissociates in water / produces H⁺ ions in solution
2. Propanoic acid reacts with carbonates / named carbonate (e.g. sodium carbonate / calcium carbonate) to produce carbon dioxide gas / effervescence
3. Propanal (carbonyl compound) does NOT react with carbonates / does not produce effervescence / no carbon dioxide produced
4. Named suitable reagent to distinguish aldehyde from carboxylic acid: Fehling's solution OR Tollens' reagent / acidified potassium dichromate / Benedict's solution
5. 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

- [carboxylic acid](https://www.gradenine.co.uk/glossary/carboxylic-acid)
- [carbonyl compound](https://www.gradenine.co.uk/glossary/carbonyl-compound)

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Source: [GradeNine](https://www.gradenine.co.uk/q/propanoic-acid-is-a-carboxylic-acid-9c671f81) · Published by Druglandscape Ltd.