Chemistry only
If you made esters in your science lesson then it is a lesson you will definitely remember!
Esters are wonderfully smelly substances made in 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 odours
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. Esters have many important uses and you will find them in a wide variety of products from paints, inks and dyes where they are used as solvents, to cakes, sweets and ice-creams where they are used as artificial flavouring and they are used to produce many sweet smelling odours.
Esters are also widely used in perfumes, fragrances and air fresheners where their pleasant smells and aroma are taken advantage of. They are also commonly used in many cosmetics and lotions as thickening agents.
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 condensation reaction or esterification reaction as follows:
The ester ethyl ethanoate is made by reacting ethanoic acid and the alcohol 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.
The diagram below shows an outline of the reactive functional groups in the alcohol and carboxylic acid that take part in this esterification reaction. You can see from the diagram that the carboxylic acid molecule loses an -OH group and the alcohol molecule loses a hydrogen atom. The -OH from the carboxylic acid and the -H from the alcohol then join to form a molecule of water. What's left of the carboxylic acid molecule and the alcohol molecule then join to form the ester molecule.
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 alcohol ethanol and ethanoic acid, other common esters include:
The tastes and smells from fruits are due to esters and a combination of many other substances. However the esters shown in the image below are largely responsible for the smell and tastes of the fruits shown. These esters can of course be made in the lab and used to add these favours to sweets and other food stuffs.
