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Acids & Esters (A2 Only)

Chemistry: AQA A Level, topic 6.3.1

Carboxylic Acids

Carboxylic acids have some useful reactions, among them neutralisation reactions and esterification.

Structure

Structure

  • Above is a molecule containing -COOH, the carboxylic acid functional group. This example is called propanoic acid.
Acidic properties

Acidic properties

  • Carboxylic acids are weak acids.
  • This means they only partially ionise in water.
  • They will react with carbonates to produce carbon dioxide.
  • This can be used as a test for carboxylic acids.
Esterification

Esterification

  • Carboxylic acids will react with alcohols.
  • This reaction is called esterification. It produces an ester.
  • Usually, you would use sulfuric acid as a catalyst for this reaction.
  • You don't need the mechanism for this, but the ester group is shown on the next slide.
Structure of ester

Structure of ester

  • The ester formed from the reaction between an alcohol and a carboxylic acid will contain the ester functional group, R-COO-R, as shown above.

Esters

Esters are very useful compounds and also occur in nature.

Uses of esters

Uses of esters

  • Examples of uses of esters are:
  • Plasticisers.
  • Solvents (ethyl ethanoate).
  • Perfumes.
  • Food flavourings (ethyl butrate, pineapple).
Biodiesel

Biodiesel

  • Biodiesel is made up of methyl esters of long chain carboxylic acids.
  • The long-chained carboxylic acids are reacted with methanol to make the esters.
  • Biodiesel is made by reacting vegetable oils with methanol in the presence of a catalyst.

Natural Esters

The ester functional group is widely used in nature.

Fats and oils

Fats and oils

  • Vegetable oils and animal fats are esters of propane-1,2,3-triol.
  • Biologists would call this triol, glycerol.
  • At room temperature, vegetable fats are liquids (oils) but animal fats are usually solid.
  • This is because vegetable oils are usually unsaturated and have weaker van der Waals forces.
Soaps

Soaps

  • You can hydrolyse natural esters to give soaps and glycerol.
  • Glycerol is the common name for propane-1,2,3-triol.
  • Soaps are just salts of long chain carboxylic acids.

Test yourself on Acids & Esters (A2 Only)

  • Carboxylic acid reacts with carbonates to produce CO2. What can we use this for?

  • What must a carboxylic acid react with to produce an ester?

  • What are esters particularly useful for?

  • Which of these is glycerol?

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