Esters

What are esters and what functional group they contain?

Esters

• Alcohols and carboxylic acids react to make esters in esterification reactions

• Esters are compounds with the functional group R-COO-R

  

• Esters are volatile compounds (organic compounds that easily evaporate at room temperature) and are sweet-smelling oily liquids used in food flavourings and perfumes

What is produced when ethanol and ethanoic acid react and what catalyst is needed?

Making Esters

• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form the ester, ethyl ethanoate:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Diagram showing the formation of ethyl ethanoate

During this esterification reaction, a molecule of water is also produced

How do you name esters?

Naming Esters

• An ester is made from an alcohol and carboxylic acid

• The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’

• The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’

  • E.g. The ester formed from pentanol and butanoic acid is called pentyl butanoate

Diagram showing the origin of each carbon chain in ester

• Some examples of common esters:

Examples of Esters Table

How do you prepare a sample of an ester?

Practical: Preparation of ethyl ethanoate

Aim:

To prepare a small sample of ethyl ethanoate

Diagram:

The preparation of ethyl ethanoate

Method:

1. A mixture of ethanoic acid, ethanol and concentrated sulfuric acid is gently heated by either a water bath or an electric heater (ethanol is flammable, so a Bunsen can’t be used!)

2. The ester is then distilled off as soon as it is formed and collected in a separate beaker by condensation

3. As esters have low boiling points (they are volatile), they are the first to evaporate from the reaction mixture. Removing them from the mixture by distillation prevents the reverse reaction from occurring

Result:

• The distillate can be smelt to check whether an ester has been produced

• Small quantities of ethanoic acid, sulfuric acid and ethanol can also be collected in the process

• To remove acidic impurities, sodium carbonate solution can be added, until the mixture stops fizzing (no more carbon dioxide is evolved)

• To remove ethanol, calcium chloride solution can be added