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 carboxylic acids.
The many uses of esters
However unlike the alcohol and carboxylic acid that makes them up many esters
have very pleasant fruity smells and aromas e.g. oranges, bananas, apricots, plums and pineapples as well as many
other fruity odours are all due to presence of esters.
Esters have a wide range of highly recognisable aromas and flavours; not just fruity one; for example the ester ethyl methanoate is a simple ester made from the alcohol ethanol and the carboxylic acid methanoic acid that gives the smell of rum while the ester benzyl ethanoate has a floral smell reminiscent of jasmine and pears, but perhaps the best know ester is ethyl ethanoate which can be made very easily in the lab from the alcohol ethanol and the carboxylic acid ethanoic acid, ethyl ethanoate smells of pear drops; this ester is commonly used as a solvent in nail-varnish remover and some glues.
Esters also have many other important uses and you will find them in a wide variety of products from perfumes, scents, air fresheners, paints, inks and dyes where they are used as solvents, to cakes, sweets and ice-creams where they are used as artificial flavouring where they are used to produce a wide variety of sweet smelling odours.
Fragrances and Food Flavourings
The wide range of aromas and scents from ester are used in:
Perfumes, fragrances and air fresheners where their pleasant smells and aroma are taken advantage of. Esters are famous for their pleasant, fruity, and floral smells and being volatile compounds means they easily turn into a gas at room temperature. This allows the molecules to enter our nose and be detected by our sense of smell.
In food flavourings: The same fruity smells are used to create artificial flavourings for sweets, ice creams, and other foods. For example the ester octyl ethanoate gives the flavour of oranges and the ester pentyl ethanoate smells and tastes like bananas.
Cosmetics and lotions
Esters are also commonly used in many cosmetics and lotions where some of the larger ester molecules are used as thickening agents to increase the viscosity of lotions and creams; however they also perform other functions such as:
Emollients: Esters are a key ingredient in many moisturising creams and lotions where they form a protective, non-greasy layer on the skin that traps moisture, keeping the skin soft and hydrated.
Emulsifiers: Many lotions and creams are often mixtures of oil and water. Esters can act as emulsifiers, helping these two immiscible liquids to stay mixed together and not separating out into two separate layers.
Solvents: Esters are commonly used as solvents where they can dissolve other ingredients, like fragrances or active compounds in perfumes; to ensure they are evenly distributed in the product.
Cleaning solutions
Esters are also used in cleaning agents because of their ability to dissolve grease and oils. This makes them a dual-purpose ingredient in many household products that aim to both clean and leave a fresh scent behind.
Making esters in the lab
Esters are very easy to make in the lab. Simply mix equal amounts of a carboxylic acid and an
alcohol in a
test-tube as shown in the image opposite, a few drops of concentrated sulfuric acid is also added to act as a catalyst.
The test-tube is then placed in a beaker of hot water. The carboxylic acid and the alcohol will react quickly in a condensation reaction or esterification reaction as follows:
Esters are volatile substances; that is they have low boiling points and evaporate easily and after a few minutes you
can smell the ester as the vapour leaves the top of the test-tube. If the reaction mixture is then poured into a beaker of water the ester will float and top of the water since the two liquids are immiscible and most esters are less dense than water, the of the immiscibility of the two liquids will also concentrate the ester at the surface of the water and this will further allow you to smell the ester formed.
Making esters this way is a condensation or esterification reaction; during a condensation reaction two molecules react and join together to make one larger molecule and a small molecule such as water is released. In this case the carboxylic acid and alcohol molecules react to make
the ester and
the small molecule water is also produced.
Esters: Quick questions
Making the ester ethyl ethanoate
The ester ethyl ethanoate is made by reacting ethanoic acid and the alcohol ethanol in the presence of a concentrated sulfuric acidcatalyst.
Ethyl ethanoate can be used as nail varnish remover; it also smells of pear drops. An equation for this condensation reaction is shown below, hopefully you can see that the ethanoic acid loses a -OH and the ethanol molecule loses a -H atom, these two groups then join to form a molecule of water:
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.
Naming esters
Esters are named by taking the alcohol name and naming the alkyl group present in it e.g. methanol would be methyl,
ethanol would be ethyl, propanol would be propyl. The name of the carboxylic acid is changed by altering the ending -ic is replaced by -ate. Ethyl ethanoate
for example is a common ester made from the alcohol ethanol and the carboxylic acid ethanoic acid, other common esters include:
Esters in fruits
The tastes and smells from fruits are due in part 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. Underneath the name of the ester in the image below is the name of the alcohol and the carboxylic acid that could be used to make it.
Self-check
Use the flashcards below to review your understanding on the uses, names and preparation of esters.
What two compounds react to make an ester?
A carboxylic acid and an alcohol.
What is the name of the reaction in which esters are made?
An esterification or a condensation reaction.
What is the role of concentrated sulfuric acid in esterification reactions?
The concentrated sulfuric acid acts as a catalyst.
Why are esters used in perfumes?
Because they are volatile with pleasant aromas.
What is a condensation reaction?
Reaction where 2 small molecules combine and a small molecule is released.
What does it mean when we say esters are volatile?
They evaporate easily at room temperature.
Which ester smells like pear drops?
Ethyl ethanoate.
Name two uses of esters in the food industry
Artificial flavourings and sweetners.
Name the ester formed from ethanoic acid and ethanol.
Ethyl ethanoate
Why are esters used in cleaning fluids?
Esters are able to dissolve grease and oils.
Key Points
Esters are made by reacting an alcohol
and a carboxylic acid in the presence of a concentrated sulfuric acidcatalyst.
The reaction is an esterification reaction or a condensation reaction; that is one whereby two small molecules join together to make a larger one and release a small molecule such as water. The reaction is a reversible one.
Esters are pleasant smelling compounds and are often used in perfumes, cosmetics, lotions and creams, air fresheners and also as flavourings in many foodstuffs. They are also used as solvents and plasticisers.
Small esters molecules will be particularly volatile and they will likely have low boiling points, meaning they can easily turn into a gas at room temperature. This is why their smells are easily detected.
