10. ESTERS –
structure, chemistry & uses and how
to make an ester
Brown's GCSE/IGCSE/O Level KS4 science CHEMISTRY Revision Notes
GCSE level chemistry revision
All my GCSE level oil and organic chemistry revision
All my advanced A level organic chemistry notes
Advanced A Level notes on carboxylic acids & esters
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Carboxylic acids - structure, chemistry and
More on USES OF ESTERS
- Carboxylic acids are used to manufacture esters by reacting them with alcohols
- Carboxylic acids react with alcohols
members of another homologous series called esters.
sulphuric acid acts as a catalyst in this reaction.
- General word equation for esterification:
- carboxylic acid + alcohol == acid catalyst ==> ester + water
- ethanoic acid + ethanol
ethyl ethanoate + water
- sometimes more simply written as
- CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
- The reaction is reversible and
the mixture reaches equilibrium, about 2/3rds
of the carboxylic acid and alcohol have been converted to the ester.
- Without a strong acid catalyst
e.g. conc. sulfuric acid, the reaction is very slow and the mixture is
heated to further increase the rate of reaction (see details of the
- Structures of other esters
made from ethanoic acid:
methyl ethanoate using
methanol, and ethanoic acid
ethanoate from using propanol (propan–1–ol, n–propyl alcohol)
and ethanoic acid.
- Note the arrangement of the atoms at the
ester linkage, the functional group -COOC-.
- The first part of an ester's name is
derived from the alcohol e.g. methyl from methanol, ethyl from ethanol
and propyl from propanol etc.
- The second part of the name comes from the
carboxylic acid and ends in ...anoate e.g. methanoate from methanoic acid,
ethanoate from ethanoic acid and propanoate from propanoic acid etc.
- The procedure for preparing
- This is illustrated in the diagrams below and a detailed
description of the method for preparing ethyl ethanoate is described.
- Ethyl ethanoate and water are both
colourless, but to help in following the procedure via the diagrams, I've
coloured the ester
yellow and the reaction mixture and aqueous solutions a pale grey.
- This is a detailed description that will do for Advanced A level chemistry
Stage 1 in making an ester
STAGE 1 Making the ester: In the
round-bottomed flask the alcohol (ethanol) is mixed with the carboxylic
acid (ethanoic acid) and a small amount of concentrated sulfuric acid
(catalyst) is added too. Anti-bumping granules are added to ensure a smooth
boiling action. The mixture is carefully heated to get the mixture
gently boiling and refluxing.
- Stage 1 is a technique called 'heating
under reflux', and ensures the reaction occurs the fastest at
highest possible reaction temperature, the boiling point of the mixture.
However, to prevent vapour loss by boiling/evaporation, particularly of
the desired product - the ester, the vapourised
liquids are condensed back into the reaction flask recycling everything.
- The diagram shows a bunsen
burner being used to supply the heat ('my days'), these days its more
likely, and safer, to use an electrical heater that the round bottomed
flask fits in snugly.
Stage 2 in making an ester
STAGE 2 Fractional distillation: The colourless ester liquid is
separated from the reaction mixture by fractional
distillation which is fully explained on the
Elements, Compounds, Mixtures
Notes (the example described is separating an ethanol/water mixture, but the same principal applies in separating the ester from
some of the
water, unreacted alcohol and acid and the sulphuric acid catalyst. Again
the mixture gently heated and boiled, but this time you want the vapour of
the lowest boiling component (ester) to separate out in the fractionating
column and pass through into the condenser. This happens when the
temperature at the top of the column reaches the boiling point of the ester.
The ester and small quantities of carboxylic acid, sulfuric acid and alcohol
can be collected from the condenser in a suitable glass vessel. Preferably a
quick-fit one that connects to the condenser, BUT it must not be a
completely sealed system otherwise pressure would build up, hence the vent
to the sink. You should realise at this point in the preparation that the
ester (ethyl ethanoate) is very impure.
Stage 3 in making an ester
STAGE 3 Removing acidic impurities: The rest of the procedure is
all about purifying the initial ester distillate from the fractional
distillation. The condensate (liquid distillate) from the fractional
distillation apparatus is transferred to a separating funnel (tap
funnel). Sodium carbonate solution is added to neutralise any acids and the
stopper replaced. The separating funnel is shaken to ensure complete removal
of the acid, but carbon dioxide is formed, so every so often you invert the
funnel, open the tap and allow the gas to escape. When there doesn't seem to
be any more effervescence or gas pressure, the mixture is allowed to settle.
When the two layers have fully separated, the stopper is removed, and the
lower aqueous layer is careful run off, don't lose any of the ester in the
process! When doing the run-off the stopper must be removed. The acidic
impurities and any salts formed have now been removed in the aqueous sodium
carbonate solution, therefore there should be no carboxylic acid or sulfuric
acid catalyst left in the ester layer.
Stage 4 in making an ester
STAGE 4 Removing the ethanol impurity: However, despite removing
some impurities there will still be some traces of the alcohol left in the
ester layer. Concentrated calcium chloride solution is added to the still
impure ester in the separating funnel and the mixture shaken again. The
aqueous calcium chloride will remove any remaining unreacted alcohol
(ethanol). Again, the lower aqueous layer is tapped off to leave only the
ester layer which will still contain some water.
Stage 5 in making an ester
STAGE 5 Drying the product: By now the only impurity left is
water. So, to dry the ester, it is run off (tapped off) from the separating
funnel into a small conical flask and some granules of anhydrous calcium
chloride added. The conical flask is stoppered and the mixture shaken, and
the calcium chloride absorbs any remaining moisture in the ester. The pure
ester can than be filtered off.
- You can make butyl ethanoate and other
the same reaction and procedure.
- ethanoic acid +
butan–1–ol ===> butyl ethanoate + water
- At GCSE level butan-1-ol might be
just written as 'butanol'.
The yield of ester
- Its an equilibrium, and
starting with the pure acid plus pure alcohol, you heat the mixture in
and you get about 2/3rds
conversion* to the ester, and the preparation reaction is
catalysed by a
few drops of concentrated sulphuric acid.
This means a theoretical maximum reaction
yield of about 67%. - in reality, a lot less due to losses in the various
- For more on % yields and 'atom
Calculations section 14.
very simple method of making an ester
- This is a nice class experiment
for GCSE/IGCSE or advanced level students.
- You can mix equal volumes of small
quantities of a carboxylic acid and an alcohol with an even smaller volume
of concentrated sulfuric acid.
- The mixture is gently warmed in beaker of warm
water for 5-10 minutes.
- The mixture is then poured into a beaker of
sodium hydrogencarbonate solution.
- The sodium hydrogencarbonate neutralises the acid
catalyst and any unreacted carboxylic acid.
- You should get some drops of ester left on
the surface which can be carefully smelled to appreciate the aroma of the
- You can do this is as a nice class
experiment with ethanoic acid and a variety of alcohols and noting what they
think the esters smells like (likely to be 'fruity') alongside appreciating
its molecular structure too!
HYDROLYSIS of esters
- Esters are usually sweet/pleasant
smelling colourless liquids and occur widely–naturally in plants.
- Esters occur widely in
- Esters are usually sweet/pleasant smelling liquids and widely used as fragrances
(components in perfumes) and
- Perfumes can natural, obtained from plant
sources, or artificial, since esters are readily synthesised in the
substances are used in many cosmetics but many mixtures contain
synthetic organic compounds.
- Many esters have pleasant sweet or fruity
smells and the colourless liquids are quite volatile, that's why fruits have
strong pleasing odours or aromas.
- The pleasure of most flavours and fragrances
from fruits is due to esters, the vapours from esters definitely entice the
receptors in your nose to feel good!
- How and why do we smell perfumes? (or any other substance)
- Anything that we smell must have come from
substances evaporating, no matter how little of it evaporates, the nose is
quite sensitive to low concentrations of many chemicals in air.
- Therefore, in order to smell a substance,
that substance must be to some extent be a volatile material.
- If a substance isn't volatile, you are
highly unlikely to smell it i.e. detect it with your nose.
- The most volatile materials are those that
most easily evaporate e.g. like petrol, how easily a liquid evaporates is
referred to as its volatility.
- The intermolecular forces between molecules
are relatively weak in liquids that are volatile, so the particles don't
need to much kinetic energy to escape from the surface of your skin.
- Because of random collisions, the particles in a liquid have a variety
of speeds and kinetic energies.
- Evaporation occurs all the time from
volatile liquids, but it is the higher kinetic energy particles that can
overcome the attractive forces between the molecules in the bulk of the
liquid and escape from the surface into the surrounding air.
- It is these higher kinetic energy escaped molecules that diffuse through
the air to reach the receptor cells in the nostril to trigger the sense of
- That is why perfume molecules must be quite volatile to work, but they
must be not too volatile or their effect won't last very long.
- On heating particles gain kinetic energy
and move faster and are more are able to overcome the intermolecular forces
between the molecules, therefore theoretically, perfumes should smell
stronger in a warmer room.
- Because they are volatile and pleasing to
the nostril, it makes esters ideal
for cosmetic perfumes and cosmetic fragrances in general, but esters are
also used in air fresheners e.g. flowery smells like jasmine.
- Because fruit sources are limited, many
esters are now synthesised in large quantities so the flavourings and
derived taste and aromas in fruit drinks, sweets and cakes etc. may be from
manufactured esters simulating strawberry, pineapple, pear, apple, grape,
orange, banana when used as food and drink additives etc.
- Esters are used in pharmaceutical and
household products e.g. ointments, washing–up liquids to give the
medications or cleaning products a pleasant odour.
- Examples of plant ester sources:
- Lavender oil essence is distilled
from the lavender plant
- Examples of flavouring esters:
- Pear drop sweet essence is an ester.
- Factors affecting perfume
design e.g. using esters:
- Designing a perfume – several issues
to address by way of design factors.
- You can't just use any ester, no matter
how beautiful it smells.
- The chemicals in cosmetic perfumes must
have a particular set of properties including ...
- the chemical components (they)
must evaporate easily, otherwise the molecules will never
reach your nose, but different evaporation rates are needed by
different components to give a prolonged effect,
- they must not react with water
in your sweat forming compounds that might not smell nice, like
carboxylic acids, which could be irritating to the skin too,
- at the same time, they must not
be soluble in water or they would be easily washed away,
- they must be non-toxic and
not be absorbed by the skin to cause irritation or poisoning, but
you do want them to be absorbed by the skin, BUT harmlessly,
- they must not irritate the skin,
since you are applying the perfume to you skin of the neck or wrists
as well as part of your clothing.
- The perfume needs to be a mixture of
compounds to give a prolonged perfumery effect.
- The perfumer chemist has to design the
mixture to give a particular fragrance which includes ...
- the top note – the first fragrant
molecule to be released,
- and the low note, the last molecule to be
- Cosmetic companies are always developing
new products to comply with our aesthetic desires!
- BUT, every new product must be thoroughly
tested before it is ready for the consumer.
- Unfortunately, this sometimes involves using
animals e.g. to monitor their skin response to the new cosmetic formulation.
- Opinions can be strongly divided and
divisive as to the merit and ethics (morality) of using animal testing for
new cosmetic formulations (and of course using animals to test new medicines
from the pharmaceutical industry).
- One view is that animal testing is worth it
to avoid possible adverse effects on consumers, so any discomfort or worse,
suffered by the animals to prevent us suffering in the same way, a sort of
'health and safety' issue argument.
- The opposing view argues that it is entirely
wrong to use animals in tests. 'Animal rights' people argue its unfair to
use defenceless animals who cannot speak for themselves AND the results of
animal tests are not necessarily conclusive and so unnecessary animal
- So, because of concerns about animal welfare
in conducting tests of cosmetics on animals, the European Union (EU) has
banned almost all of these animal test procedures.
Why does a substance dissolve in one
liquid solvent but not another?
- Esters are used as solvents
- e.g. nail
varnish remover (the solvent propanone/acetone is also used), but also in paints, glues and ink formulations as a medium
compatible with the other ingredients.
- Some larger ester molecules are used in
plastic formulations as plasticisers which are added to make the
polymer more flexible.
- They aren't totally free of health issues
but esters have replaced more harmful aromatic hydrocarbon solvents like
benzene (a carcinogen – a cancer promoting chemical) and methylbenzene (old
name toluene, also carcinogenic) in paint and varnishing products.
- BUT take care ...
- (i) although imparting a pleasant odour,
ester fumes can irritate mucous membranes in the nose and mouth,
- (ii) because they are volatile and
combustible, the fumes are highly flammable and easily ignited by a
naked flame. The vapour is heavier than air and will not disperse quickly,
- (iii) some people may be allergic to ester
fumes, or indeed their use as food additives.
Other natural esters - triglycerides
- There are three particle interactions going
on if you mix one substance with another e.g. a liquid solvent that
may or may not dissolve a solid.
- The three possible attractions are (i) solid ... solid, (ii)
solid ... liquid and (iii) liquid ...
- The relative strength of these attractive
intermolecular forces decides whether e.g. a solid will dissolve in a
- For example, nail varnish will not dissolve
in water, but will dissolve in organic solvents like an ester, alcohol or
- Nail varnish is insoluble in water
because the intermolecular forces between the nail varnish molecules
themselves, and between the water molecules themselves are much stronger
than the attraction between water and the nail varnish molecules, so the
nail varnish cannot possibly dissolve in water. Forces (i) and (iii)
override force (ii)
- However, nail varnish will dissolve in
organic solvents like butyl ethanoate or ethyl ethanoate (esters, old
names butyl acetate and ethyl acetate), ethanol ('alcohol')
and propanone (old name acetone) solvents. Here the organic solvent
intermolecular attraction to the nail varnish molecules can override the
nail varnish ... nail varnish and the solvent ... solvent intermolecular
forces and the nail varnish will dissolve. In this case attractive force
(ii) overrides both attractive forces (i) and (iii).
- Since different solvents are different
molecular affinities for different substances, the solubility of a solute in
a solvent can vary quite considerably from one solvent to another.
- The question of which solvent you choose
to use to dissolve a substance depends on two main factors ..
- (a) How soluble is the substance in
- (b) How safe is to use the solvent?
e.g. in terms of inhaling vapour or spillage on the skin (gloves!), is it
harmful?, irritating?, even toxic?, and is it highly flammable, so more
dangerous to use.
- Chlorinated organic solvents e.g.
trichloromethane ('chloroform') tend to be harmful, alcohols and esters are
safer but are more flammable.
is repeated in alcohols
- Esters from the 'triol' alcohol
, which has three C-O-H groups, is the alcohol plants and animals use
to make oils and fats - which are esters we use in food and soaps.
- Animals and
plants combine glycerol and long chain fatty acids to make triglyceride
esters - fats from animals and oils from plants.
- For more details on fatty acids, oil and fat
margarine and soaps
- Polymers - polyesters like Terylene (diagram
- The diagram above shows part of structure of
Terylene, a very useful polymer used for making plastic objects and also
manufactured as fibres for use in fabrics for the clothing industry.
- You don't have to know any detailed
molecular structure at GCSE/IGCSE level, but I have highlighted the -COOC-
ester linkage, which is the same functional group structure as in the
'little' esters described on the page above.
- The most common use of polyester today is
called PET (for short!) and is used to make the plastic bottles for storing
liquids in like soft drinks, PET is very useful because it is transparent,
shatterproof and cheap!
- Fine polyester fibres can be made into a
variety of articles of clothing which are lighter and cheaper than
traditional materials like wool.
- Plastic bottles made from polyester can
recycled and turned into fibres again and reused in clothing.
- For more see section
11. Condensation polymers including
Multiple Choice Quizzes and Worksheets
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