Chemsitry only
Esters
If you made esters in your science lesson then it is a lesson you will definitely remember!
Esters are wonderfully smelly substances made by a condensation reaction between an alcohol and a
carboxylic acid. Personally I find the smell of short chain
alcohols pleasant but as the chain
length grows, especially if it branches the smell is actually quite horrible! Similarly for
carboxylic
acids the smell gets very unpleasant as the chain length grows, a mix between really bad body odour
for short chain carboxylic acids to a "goaty" smell for medium chain length acids.
However esters
have a very fruity smell e.g. oranges, bananas, apricots, plums and pineapples as well as many
other fruity odours are all due to presence of esters.
Making Esters
Esters are very easy to make. Equal amounts of the carboxylic acid and the
alcohol are placed in a
test-tube as shown, a few drops of concentrated sulfuric acid is added to act as a catalyst.
The test-tube is then placed in a beaker of hot water. The carboxylic acid and the alcohol then react in a condenation reaction or esterification reaction as follows:
carboxylic acid(aq) + alcohol(aq) ⇌ ester(l) + hydrogen oxide(l)
Esters are volatile and after a few minutes you
can smell the ester as the vapour leaves the top of the test-tube.
Making esters this way is a condensation reaction, during a condensation reaction a small molecule such as water is released. In this case the acid and alcohol react to make
the ester and
the small molecule water is also produced.
The ester ethyl ethanoate is made by reacting ethanoic acid and ethanol in the presence of a sulfuric acid catalyst.
Ethyl ethanoate can be used as nail varnish remover, it also smells of pear drops!. An equation for this reaction is shown below.
ethanoic acid(aq) + ethanol(aq) ⇌ ethyl ethanoate(l) + water(l)
CH 3COOH(aq) + CH3CH2OH(aq) ⇌ CH3COOCH2CH3(l) + H2O(l)
The diagram below shows an outline of the reactive functional groups in the alcohol and
carboxylic acid that take part
in the reaction.
Naming esters
If you study the image above you will see that the ester group (-COO-) is made when the --OH group is
removed from the acid and the -H is removed from the
alcohol. The acid and alcohol molecules
then join together to form the ester. The -OH and -H which are removed join
to form water.
Esters are named by taking the alcohol and naming the alkyl group present e.g. methanol would be methyl,
ethanol would be ethyl, propanol would be propyl. The acid ending -ic is replaced by --ate. Ethyl ethanoate
is a common ester made from ethanol and ethanoic acid, other common esters include:
| odour |
formula |
name |
acid and alcohol used |
| pears |
CH3COOCH2CH2CH3 |
propyl ethanoate |
ethanoic acid and propanol |
| apples |
CH3CH2CH2COOCH3 |
methyl butanoate |
butanoic acid and methanol |
Key Points
- Esters are made by reacting an alcohol
and a carboxylic acid in the presence of a sulfuric acid catalyst.
The reaction is a condensation reaction, that is it produces a small molecule such as water.
- Esters are pleasant smelling compounds and are often used in candles, foodstuffs and air freshners. They are also used as solvents and plasticisers.
Practice questions
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