Esters
Chemistry: Cambridge International A Level, topic 3.6.2
Esters
Esters are very useful compounds and also occur in nature.
Uses of esters
- Examples of uses of esters are:
- Plasticisers.
- Solvents (ethyl ethanoate).
- Perfumes.
- Food flavourings (ethyl butrate, pineapple).
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.
Hydrolysis of Esters in Polyesters
An ester is a type of organic compound containing a carbon atom which has a double bond to one oxygen and a single bond to another oxygen. Polyesters are molecules containing multiple ester groups.
Hydrolysis of esters
- Hydrolysis reactions involve reacting a molecule with water.
- Hydrolysis using pure water is too slow so is never used. The reaction can be acid or base catalysed instead.
- Dilute acids and bases are used for the reaction.
Hydrolysis using dilute alkali
- Hydrolysing esters is usually done with dilute alkali.
- This is because it is not a reversible reaction and the products of the reaction are easier to separate.
- The ester should be mixed with an excess of dilute alkali like sodium hydroxide solution and heated under reflux.
- The products of the reaction are a carboxylate salt and an alcohol.
Hydrolysis using dilute acid
- The ester should be mixed with dilute acid and heated under reflux.
- The products of the reaction are a carboxylic acid and an alcohol.
- The reaction is reversible but can be pushed towards the products by using an excess of dilute acid.
Hydrolysis of polyesters
- Polyesters are attacked readily by alkalines but much more slowly by acids.
- A polyester will react with an alkali to cut the polymer chain and produce a alcohol and a carboxylate salt as with esters.
- A polyester will also react with an acid to cut the polymer chain and produce a alcohol and a carboxylic acid but this process is very slow.
Test yourself on Esters
What are esters particularly useful for?
What products are made when an ester is hydrolysed in dilute alkali?
Which of these statements about the hydrolysis of esters are true?