EstersOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is the ester functional group?
Esters are a family of organic compounds containing the functional group −COO−. Esters are formed when a carboxylic acid reacts with an alcohol, in the presence of an acid catalyst, in a reaction called esterification.
Section 2
How is ethyl ethanoate made?
The ester ethyl ethanoate is produced from the reaction between ethanol and ethanoic acid:
C2H5OH + CH3COOH → CH3COOC2H5 + H2O
(ethanol + ethanoic acid → ethyl ethanoate + water)
This reaction needs an acid catalyst and is a good example of esterification: the −OH of the alcohol and the −COOH of the acid combine, releasing water and forming the ester's −COO− link.
Ester names follow the pattern 'alkyl alkanoate' — the first part comes from the alcohol, the second from the acid. Ethyl (from ethanol) ethanoate (from ethanoic acid) is the classic example examiners use.
Section 3
What are the properties and uses of esters?
Esters are:
- Volatile compounds — they evaporate easily at room temperature
- Known for their distinctive, often pleasant smells
Because of these properties, esters are widely used as:
- Flavourings in the food industry
- Perfumes and fragrances
Many fruity smells and flavours in foods and drinks are due to naturally occurring esters, and synthetic esters are made to recreate these smells in perfumes and flavourings.
Section 4
How can you recognise an ester?
An ester can be recognised from its name or structural formula:
- By name: ester names typically end in '-oate' and are written as two words, e.g. ethyl ethanoate, methyl propanoate
- By structural formula: look for the −COO− link between two carbon-containing groups, e.g. CH3COOC2H5
This distinguishes esters from carboxylic acids (−COOH) and alcohols (−OH), which each have only one oxygen-containing functional group attached differently.
Don't confuse the ester link −COO− with the carboxylic acid group −COOH — esters have no acidic hydrogen on the functional group itself, since it links two carbon chains rather than ending in −OH.
Must Know
- Esters contain the functional group −COO−
- Esters are formed from a carboxylic acid + an alcohol, using an acid catalyst (esterification)
- Ethyl ethanoate is formed from ethanol + ethanoic acid, releasing water
- Esters are volatile with distinctive smells
- Esters are used as flavourings and perfumes
- Ester names end in '-oate' and are written as two words (e.g. ethyl ethanoate)
That's the notes covered.
Carry on to the next subtopic.