esters

Chemsitry only

Esters

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 esters.



Making 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 folows:

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, the acid and alcohol react to make the ester and 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
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.

how to make an ester

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 is 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

Practice questions

Check your understanding - Questions on esters

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