Reactions of Alkenes & Alcohols (AQA GCSE Chemistry): Revision Notes
Carboxylic acids
What are carboxylic acids?
Carboxylic acids are organic compounds that you can make by oxidising alcohols. They have a special functional group called -COOH. This group is what makes them behave like acids.
A functional group is a specific arrangement of atoms that determines how a molecule will react. The -COOH group is responsible for all the characteristic properties and reactions of carboxylic acids.
Structure and naming
All carboxylic acids contain the -COOH group. This is made up of:
- A carbon atom
- Two oxygen atoms
- One hydrogen atom
Names always end in "-anoic acid":
- Methanoic acid = (simplest carboxylic acid)
- Ethanoic acid = (found in vinegar)
- Propanoic acid =
- Butanoic acid =
You can spot carboxylic acids from their names or formulas - just look for the -COOH part!
Naming Rule: To name a carboxylic acid, count the carbon atoms in the longest chain (including the carbon in -COOH), then add "-anoic acid" to the end. This naming pattern is consistent for all carboxylic acids.
Properties of carboxylic acids
They dissolve in water to make acidic solutions. This happens because carboxylic acids release hydrogen ions (H⁺) when they mix with water. These hydrogen ions make the solution acidic.
The smaller carboxylic acids (like methanoic and ethanoic acid) are very soluble in water, but as the carbon chain gets longer, solubility decreases. This is because the hydrocarbon chain becomes more dominant than the polar -COOH group.
Reactions of carboxylic acids
With carbonates
Carboxylic acids react with carbonates to make carbon dioxide gas. You'll see fizzing when this happens.
Worked Example: Reaction with Calcium Carbonate
When you add calcium carbonate to ethanoic acid:
Word equation: Ethanoic acid + Calcium carbonate → Calcium ethanoate + Carbon dioxide + Water
Chemical equation:
Observation: You'll see fizzing as CO₂ gas is produced.
With alcohols
When carboxylic acids meet alcohols, they make esters. You need an acid catalyst to help this reaction work.
Worked Example: Esterification Reaction
Ethanoic acid + Ethanol → Ethyl ethanoate + Water
Chemical equation:
Conditions needed: Concentrated sulfuric acid catalyst and gentle heating
Testing for carboxylic acids
You can test if something is a carboxylic acid in two ways:
- Universal indicator test - The mixture should turn red or orange, showing it's acidic
- Calcium carbonate test - Add some calcium carbonate powder and look for fizzing. The gas given off should make limewater go cloudy
Practical Testing Tips:
- For the carbonate test, use a small amount of solid calcium carbonate or sodium carbonate
- The fizzing should be immediate if a carboxylic acid is present
- To test the gas produced, bubble it through limewater - it will turn milky white if CO₂ is present
Key Points to Remember:
- Carboxylic acids have the functional group -COOH
- Their names always end in "-anoic acid"
- They make acidic solutions when mixed with water
- They react with carbonates to produce carbon dioxide gas (fizzing)
- They can react with alcohols to make esters (needs acid catalyst)
- You can test them using universal indicator or calcium carbonate