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ALKANOIC
ACIDS AND
ESTERS
Introduction
Most people have used Aspirin to relieve headache pain
or to reduce a fever. Aspirin (or 2-ethanoylhydroxybenzoic
acid) is an example of a common group of organic
chemicals called esters. Aspirin is made from a weak
organic acid called salicylic acid, which can be extracted
from the leaves of the willow tree. This extraction was
performed by Hippocrates in 400 BC, and he used the
extract to ease labour pains. It was soon discovered that
the body cannot tolerate too large a dose of salicylic
acid as it causes irritation to the stomach and mouth.
The name Aspirin was coined as a trade name in
1898, when the Bayer Pharmaceutical Company patented
an analgesic that was the acetyl ester of salicylic acid.
It did not cause the same side effects as salicylic acid,
but it did reduce pain and fever.
In this chapter
10.1 Organic acids and esterification
Figure 10.1
Aspirin is an important analgesic. It reduces pain and fever.
It is an example of a group of compounds called esters.
216 THE ACIDIC ENVIRONMENT
page 217
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Remember
Before beginning this section,
you should be able to:
• identify factors that can affect
the equilibrium in a reversible
reaction
• write equations to represent
all chemical reactions.
Alkanols and alkanoic acids
Alkanols and alkanoic acids are important families of organic compounds.
Alkanols are compounds that contain the alcohol functional group
(—OH). The alkanoic acid family are weak organic acids. They contain
the carboxylic acid functional group (—COOH).
(a)
(b)
(c)
Key content
Figure 10.2 The ester called (a) ethyl acetate is manufactured from (b) ethanol and
(c) acetic acid (ethanoic acid).
Alkanols
In the Production of Materials module, we investigated the importance of a
family of molecules called alkanols. Ethanol is an important member of
this homologous series of compounds. The general formula of the alkanol
homologous series is:
CnH2n + 1OH
It is worthwhile at this stage to look back to Module 1 and review the
rules used to name these molecules. Sample problem 10.1 will help you
to revise.
Alkanols are polar molecules due to the presence of the alcohol functional group. The melting and boiling points of the alkanols are higher
than their corresponding alkanes because of dipole–dipole attractions
(Table 10.1). Dispersion forces are the only forces that bind hydrocarbon
molecules together. Methanol and ethanol have significantly higher
melting points than would be normally expected for such small molecules.
This is explained by the hydrogen bonding between the electropositive
hydrogen atoms and the electronegative oxygen atoms of alcohol groups
on neighbouring molecules. Hydrogen bonding also explains the high
water solubility of the first four members of the alkanol homologous
series. As the chain length increases, however, they become increasingly
insoluble.
Alkanols are essentially neutral molecules. The alcohol functional
group is strongly bonded to the carbon chain and there is no tendency for
the loss of hydroxide ions when alkanols dissolve in water.
H
H
H
C
H
...
Hydrogen
bonding
C
O
...
H
H
H
O
H
C
H
H
C
H
H
O
H
...
Figure 10.3
The hydrogen bonding between
short-chain alkanol molecules raises
their melting points. The hydrogen
bonding between short-chain
alkanols and water increases their
water solubility.
(n = 1, 2, 3. . .)
...
By the end of this section, you
should be able to:
• describe the differences
between the alkanol and
alkanoic acid functional groups
in carbon compounds
• identify the IUPAC
nomenclature for describing
the esters produced by
reactions of straight-chained
alkanoic acids from C1 to C8
and straight-chained primary
alkanols from C1 to C8
• explain the difference in
melting point and boiling point
caused by straight-chained
alkanoic acid and straightchained primary alkanol
structures
• identify esterification as the
reaction between an acid
and an alkanol, and describe,
using equations, examples of
esterification
• describe the purpose of
using acid in esterification for
catalysis
• explain the need for refluxing
during esterification
• outline some examples of the
occurrence, production and
uses of esters
• identify data, plan, select
equipment and perform a
first-hand investigation to
prepare an ester using reflux
• process information from
secondary sources to
identify and describe the
uses of esters as flavours and
perfumes in processed foods
and cosmetics.
H
H
O
H
Hydrogen bonding
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 217
Figure 10.4
Methanol and ethanol have melting
points that are higher than would
normally be expected for low
molecular weight molecules.
SAMPLE PROBLEM 10.1
Melting point (°C)
0
–20
–40
–60
–80
–100
–120
–140
0
2
4
6
8
Name the straight-chain alkanol with the condensed structural
formula:
CH3CH2CH2CH2CH2CH2CH2CH2OH
SOLUTION
Step 1. Count the number of carbon atoms in the chain.
Answer = 8.
Step 2. Select the stem that corresponds to an alkane with 8 carbons.
Answer = octane.
Step 3. Delete the ‘e’ and add the suffix ‘ol’.
Answer = octanol.
Step 4. Assign a locant to the carbon to which the alcohol functional
group is attached.
Answer = 1.
Step 5. Name the alkanol using the preferred IUPAC nomenclature.
Name = octan-1-ol (or 1-octanol, which is also accepted by
IUPAC as a systematic name).
Table 10.1 Melting and boiling points of alkanes and alkanols
218 THE ACIDIC ENVIRONMENT
Compound
M (g/mol)
Melting point (ºC)
Boiling point (ºC)
methane
16.0
–182.5
–161.5
methanol
32.0
–97.7
64.7
ethane
30.1
–182.8
–88.6
ethanol
46.1
–114.1
78.3
propane
44.1
–187.7
–42.1
propan-1-ol (1-propanol)
60.1
–126.2
97.2
butane
58.1
–138.3
–0.5
butan-1-ol (1-butanol)
74.1
–89.8
117.7
pentane
72.1
–129.7
36.1
pentan-1-ol (1-pentanol)
88.2
–78.9
138.0
Alkanoic acids
O
H C
O
H
Figure 10.5
Methanoic acid is commonly called
formic acid as is present in some ant
stings (formis = ‘ant’ in Latin).
Another important family or homologous series derived from alkanes are
the alkanoic acids. Alkanoic acids contain the carboxylic acid functional
group (COOH). Their general formula is:
CnH2n + 1COOH
(n = 0, 1, 2, 3. . .)
The first member of this homologous series is called formic acid.
Figure 10.5 shows the structure of formic acid (methanoic acid).
IUPAC nomenclature rules for straight-chain alkanoic acids
The straight-chain alkanoic acids are named after the corresponding
parent alkane. The carboxylic acid functional group is always at the end of
the chain, so this carbon is the locant ‘1’. The rule for naming is:
Identify the number of carbons present in the straight chain. Select the
correct stem to name the parent alkane. Remove the ‘e’ and replace it
with the suffix ‘oic acid’.
The IUPAC committee (2005) has recommended that the preferred
name for the first two members of the alkanoic acid homologous series be
their common name rather than their systematic name. Thus methanoic
acid is called formic acid and ethanoic acid is called acetic acid.
SAMPLE PROBLEM 10.2
Name the alkanoic acid whose structural formula is shown in Figure 10.6.
Figure 10.6 Use the
nomenclature rules to name this
alkanoic acid.
SOLUTION
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
O
C
O
H
Step 1. Count the number of carbons in the chain (including the
carbon of the carboxylic acid group).
Answer = 7 carbons.
Step 2. Select the stem that corresponds to an alkane with 7 carbons.
Answer = heptane.
Step 3. Delete the ‘e’ and add the suffix ‘oic acid’.
Name = heptanoic acid.
Table 10.2 lists the IUPAC preferred names of the first eight members
of the alkanoic acid homologous series.
Table 10.2. Alkanoic acids
IUPAC preferred name
Condensed structural formula
formic acid (methanoic acid)
HCOOH
acetic acid (ethanoic acid)
CH3COOH
propanoic acid
CH3CH2COOH
butanoic acid
CH3CH2CH2COOH
pentanoic acid
CH3CH2CH2CH2COOH
hexanoic acid
CH3CH2CH2CH2CH2COOH
heptanoic acid
CH3CH2CH2CH2CH2CH2COOH
octanoic acid
CH3CH2CH2CH2CH2CH2CH2COOH
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 219
Comparing the physical properties of alkanols and alkanoic
acids
Alkanols and alkanoic acids are polar molecules Table 10.3 lists the melting
and boiling points of straight-chain alkanoic acids.
Table 10.3 Melting and boiling points of alkanes, alkanols and alkanoic acids
M (g/mol)
Melting point (ºC)
Boiling point (ºC)
formic (methanoic)acid
46.0
8.4
100.8
acetic (ethanoic) acid
60.1
16.6
117.9
propanoic acid
74.1
–20.7
140.8
butanoic acid
88.1
–5.4
163.3
pentanoic acid
102.1
–34
185.5
hexanoic acid
116.2
–3
205.7
Compound
H
H
C
H
Hydrogen bonding
H
O H
O
C C H
O H O
H
Hydrogen bonding
...
C
...
Figure 10.7
The strong hydrogen bonding
between acetic acid molecules leads
to the formation of dimers.
Tables 10.2 and 10.3 show that alkanols and alkanoic acids have higher
melting and boiling points compared with their parent alkanes. It is also
apparent that the rise in melting and boiling points is greatest for the
alkanoic acid molecules. Alkanoic acids have greater molar weights than
their equivalent alkanols or alkanes and thus the dispersion forces between
their molecules are also greater. Alkanoic acids are usually slightly more
polar than alkanols and thus the dipole–dipole forces are greater. When
an alkanol and an alkanoic acid of the same molar weight are compared,
the alkanoic acid has the higher melting point or boiling point. This is
mainly due to the more extensive hydrogen bonding between alkanoic
acid molecules. As the carbon chain lengthens, the increasing dispersion
forces dominate and there is a general trend of increasing boiling point
with increasing molar weight.
The hydrogen bonding between acetic acid molecules is particularly
strong. Experiments have shown that stable pairs of acetic acid molecules
(dimers) are present in the liquid. These dimers tend to break down on
heating.
Alkanoic acids are weak acids. They can be neutralised by strong bases to
form salts and water. In the following example, butanoic acid is neutralised
by sodium hydroxide to form sodium butanoate and water.
CH3CH2CH2COOH + NaOH
NaCH3CH2CH2COO + H2O(l)
Esters
The fragrances of flowers and the smell and taste of their fruits are
produced by a variety of volatile molecules, including a group of organic
compounds called esters. Not all esters have pleasant smells, but the low
molecular weight esters are usually volatile and aromatic. Chemists have
developed a range of synthetic esters that can be used as food flavourings
as well as in perfumes.
Esters are produced by the acid-catalysed reaction between an alcohol
and a carboxylic acid. Water is a by-product of the reaction. If the alcohol
is an alkanol and the carboxylic acid is an alkanoic acid, then the ester that
is formed is called an alkyl alkanoate.
220 THE ACIDIC ENVIRONMENT
Alkyl
group
O
R
C
O
R
Alkyl group
derived from
original alkanol
Figure 10.8
The general formula of esters is often
written as RCOOR`, where R is a
hydrogen atom or an alkyl group and
R` is an alkyl group.
condensation reaction: a reaction
in which two molecules combine
together with the elimination of a
smaller molecule such as water
O
R
C
+
O
H
H
O
R
Water splits out
as the alkanoic
acid and alkanol
condense
Figure 10.9
When the alkanol and alkanoic acid
condense together, the water that is
formed comes from the OH group of the
alkanoic acid and the hydrogen from the
alcohol functional group of the alkanol.
SAMPLE PROBLEM 10.3
ALKANOL + ALKANOIC ACID p ALKYL ALKANOATE + WATER
This reaction is not an acid–base reaction. It is classified as a condensation reaction. Isotopic studies have shown that the water that is formed
comes from the ‘OH’ group of the alkanoic acid and the ‘H’ of the
alkanol functional group.
Figure 10.8 shows the general structure of alkyl alkanoate esters. All
esters contain the ester linkage or ester functional group (—COO—).
Nomenclature of esters
Alkyl alkanoate esters are named after their parent alkanols and alkanoic
acids.
The rules for naming these esters are:
1. Count the number of carbon atoms in the
alkyl group that is derived from the original
O
alkanol. Name this alkyl group by deleting
+ H
O
R
C
the ‘anol’ suffix from the alkanol and
O
R
replacing it with the suffix ‘yl’ (e.g. butanol
H
Ester
becomes butyl).
2. Identify the number of carbon atoms in the alkanoate chain that is
derived from the alkanoic acid. Name this alkanoate chain by deleting
the ‘oic acid’ suffix and replacing it with the suffix ‘oate’ (e.g. propanoic
acid becomes propanoate).
3. The name of the ester is two separate words. The first name comes
from the alkanol and the second from the alkanoic acid.
Use the nomenclature rules for esters to name the ester whose
structural formula is shown in Figure 10.10.
H
SOLUTION
H
H
H
H
C
C
C
C
H
H
H
H
O
H
H
H
O
C
C
C
H
H
H
C
H
Figure 10.10
Use the nomenclature rules to
name this ester.
Step 1. Identify the number of carbon atoms in the alkyl group
derived from the original alkanol.
This is the hydrocarbon chain that is attached to the single
bonded oxygen atom of the ester linkage.
Number of carbon atoms = 3.
Step 2. Name the alkyl group.
Name = propyl (or, 1-propyl).
Step 3. Identify the number of carbon atoms in the alkanoate chain
derived from the original alkanoic acid.
This is the chain that includes the carbon attached to the
double bonded oxygen atom (or carbonyl oxygen).
Number of carbon atoms = 5.
Step 4. Name the alkanoate group.
Name = pentanoate.
Step 5. Name the ester.
Name = propyl pentanoate (or 1-propyl pentanoate).
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 221
SYLLABUS FOCUS
21. USING INSTRUCTION TERMS CORRECTLY
When answering questions, it is important to know what the instruction terms (‘verbs’) require you to do.
Here are some examples:
‘Recall’
This instruction word requires you to present remembered facts, ideas or experiences.
Example:
Recall the common name for the compounds that react with alcohols to produce esters.
Answer:
Carboxylic acids
refluxing: the process of heating
a mixture of liquids in a flask
with an attached condenser in
order to prevent the loss of
volatile reactants or products
Esterification
Esterification is the process of making an ester. Esterification reactions are
usually quite slow and the reaction does not proceed to completion. The
rate of the reaction can be increased by:
u adding a suitable catalyst (e.g. 1–3 mL of concentrated sulfuric acid)
Apparatus open to air
u heating the reaction mixture to increase the kinetic
energy of the molecules.
The concentrated sulfuric acid that is used as
Water condenser
a catalyst also acts as a dehydrating agent. When
water is removed from the reaction equilibrium, the
equilibrium shifts to the right and increases the yield
of ester. This shift is small as only about 1–2 mole%
of the reaction mixture is catalyst. It is important to
Hose
remember that catalysts do not increase the yield
Vapours condense
of products; they only decrease the time to reach
equilibrium.
Water out
The reactants and products of the esterification
to sink
Cold water reaction are volatile, and readily vaporise on heating.
from tap
To avoid loss of material from the reaction flask, the
Hose
mixture is heated using a reflux apparatus as shown
in Figure 10.11. A water condenser is mounted above
the reaction flask (normally a round-bottom Pyrex
Condensed liquid
drops back
flask) and cold water circulates to cool the hot, rising
vapours. The vapours condense back to the liquid
Round bottom flask state and drip back into the reaction flask so that the
reaction can continue without loss of reactants or
products. This process is called refluxing.
Water bath
Because the system is open to the atmosphere
Boiling chips
Reaction mixture
there is no build up of pressure due to the producHotplate
tion of vapours. The organic liquids and vapours
are also flammable, and care must be taken to avoid
Electrical heater
fires and explosions. The reaction vessel is normally
heated by a hot-water bath (supported on an electric
hotplate) or by using a special electrical heating
Figure 10.11
mantle in which the round-bottom flask is clamped.
The reaction mixture, including a catalyst and boiling
Naked flames from a Bunsen burner are avoided to
chips, is heated under reflux. Vapours condense back to
reduce the risk of fire.
the liquid state and drip back into the reaction vessel.
222 THE ACIDIC ENVIRONMENT
10.1 PRACTICAL
ACTIVITIES
Preparation of an ester
ESTERIFICATION
If aspirin is stored in a hot,
humid environment for a long
time it starts to smell like
vinegar. This change is due to the
hydrolysis of the aspirin.
SAMPLE PROBLEM 10.4
Another feature of the reflux procedure is the use of small boiling chips
that are normally pieces of crushed ceramic. These boiling chips prevent
a process called ‘bumping’ as they provide a large surface area on which
vaporisation can occur without the risks of sudden superheating and the
explosive ejection of vapours. Refluxing may occur for hours or days until
the system reaches equilibrium. Table 10.4 shows some typical data for
the preparation of simple esters. Instead of the 1 : 1 mole ratio of alkanol
to alkanoic acid, it is common to use an excess of one reactant (usually
the alkanoic acid) in order to shift the equilibrium in the direction of the
formation of the ester. The percentage yields obtained are variable, and this
is often the result of ester being lost during the separation and purification
process.
Table 10.3 Experimental data for the preparation of esters
Alkanoic acid
Ester (% yield)
Reflux
time (h)
methanol (1 mol)
acetic acid (3 mol)
methyl acetate (67%)
5
ethanol (1 mol)
acetic acid (3 mol)
ethyl acetate (29%)
12
ethanol (1 mol)
butanoic acid (2 mol)
ethyl butanoate (69%)
14
butan-1-ol (1 mol)
formic acid (2 mol)
butyl formate (75%)
24
Alkanol
Complete the following table and draw structural formulae for each
ester.
Alkanol
Alkanoic acid
methanol
pentanoic acid
propan-2-ol (2-propanol)
(c)
SOLUTION
Ester
(a)
(b)
2-propyl hexanoate
propanoic acid
2-pentyl propanoate
(a) The alkyl group of this ester will be called ‘methyl’ and the alkanoate group will be ‘pentanoate’. The name of the ester is methyl
pentanoate.
H
H
H
H
C
C
C
C
O
H
H
C
O
H
H
H
C
H
methyl pentanoate
H
H
(b) The hexanoate group is derived from hexanoic acid.
H
H
H
H
H
H
O
C
C
C
C
C
C
H
H
H
H
H
H
2-propyl hexanoate
H
C
H
C
H
H
C
O
H
H
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 223
(c) The alkyl group is derived from an alkanol with the alcohol
functional group on C-2. This alkyl group is called
‘2-pentyl’.
H
H
O
H
C
H
Figure 10.12
C
H
H
C
C
H
C
C
O
C
H
H
C
H
H
H H
H
2-pentyl propanoate
H
(a)
Alkanol
Alkanoic acid
Ester
methanol
pentanoic acid
methyl pentanoate
propan-2-ol (2-propanol)
hexanoic acid
2-propyl hexanoate
pentan-2-ol (2-pentanol)
propanoic acid
2-pentyl propanaoate
Extraction of the ester from the reaction mixture
The ester is usually separated from the reaction mixture in a number
of steps. The physical properties of the reactants and products must be
known in order to design the most appropriate procedure.
Figure 10.13
(a) The ester may be able to be
removed from the reaction mixture
by distillation. (b) In some cases
however, the mixture must be
separated in a separating funnel
using appropriate solvents before
distillation is performed.
(b)
Example 1:
Methyl acetate — Following reflux, the reaction mixture is distilled and
the crude ester vaporises at 57ºC and forms the distillate. The excess
acetic acid (b.p. = 118ºC) and most of the methanol (b.p. = 65ºC) stays
in the distillation flask. The crude ester is washed in a separating funnel
with a little saturated salty water containing sodium hydrogen carbonate
to neutralise any acetic acid. The ester layer is removed, dried and then
re-distilled to obtain the pure ester.
Example 2:
Ethyl acetate — The reaction mixture cannot be distilled as the boiling
points of the ethanol (78ºC) and the ester (77ºC) are too close. Therefore,
following reflux, the reaction mixture is added to an equal volume of water
and saturated with salt to reduce the solubility of the ethyl acetate and
produce an ester layer. The ester layer is treated with sodium hydrogen
carbonate to neutralise any acetic acid and then saturated with salt again.
The ester layer is removed and dried before distillation to obtain the pure
ester. Much ester is lost due to its solubility in water. Fractional distillation
can be used to improve the yield.
Comparison of the boiling points of alkanols, alkanoic acids
and esters
Table 10.4 compares the boiling points of three molecules with the same
molecular weight. As they have the same molecular weights the dispersion forces between the molecules are very similar. Table 10.4 shows that
the alkanoic acid and the alkanol have significantly higher boiling points
compared with that of the ester. All three molecules are polar. The alkanol
and the alkanoic acid, however, exhibit hydrogen bonding between their
224 THE ACIDIC ENVIRONMENT
molecules, and this leads to an elevation in the boiling point. The ester
cannot form hydrogen-bonds as there are no OH functional groups in the
molecule. Consequently its boiling point is much lower.
Table 10.4 Comparison of the boiling points of an alkanol, alkanoic acid and ester with
the same molecular weight.
MF
M (g/mol)
b.p. (ºC)
propan-1-ol (1-propanol)
C3H7OH
60.1
97.2
acetic acid
CH3COOH
60.1
117.9
methyl formate
HCOOCH3
60.1
31.5
Compound
Occurrence, production and uses of esters
Esters occur widely in nature. They are also produced by various industries.
H
H
Esters in nature
O
H
C
O
C
(CH2)14
C
H
H
O
H
C
O
C
(CH2)16
C
H
H
H
H
O
H
C
O
C
H
C
H
Esters as lubricants
H
Derived from
glycerol
H
H
C
O
H
H
C
O
H
H
C
O
H
H
(CH2)12
Esters occur in all natural systems. Esters can be formed not
only from alkanoic acids, but also from other organic and
inorganic acids. Phosphate esters are esters formed from
phosphoric acid and some of these (e.g. ATP) are vitally important in energy transfer reactions in cells. Fats and oils are triesters
of glycerol and various long-chain carboxylic acids. They are
commonly known as triglycerides. Natural waxes consist of mixed
esters of long-chain alkanols and long-chain alkanoic acids (fatty
acids).
Esters are also manufactured for a wide variety of applications.
Esters have been used as jet engine lubricants for many decades
due to their low viscosity at low temperatures, as well as their
Derived from long-chain
clean high-temperature operation. Esters are excellent lubricants
fatty acids of variable
in refrigeration systems.
chain length
The use of esters as lubricants stems from their molecular polarity. Polar
ester molecules have stronger intermolecular attractions than other nonpolar oils, so they have lower rates of evaporation and higher flash points.
Polar ester molecules are attracted to metal surfaces, forming a film that
requires more energy to penetrate.
Esters are good solvents and can solubilise or disperse degradation
products; this maintains the lubrication properties. Esters are also highly
biodegradable compared with other oils.
Figure 10.14
Glycerol is a alkantriol. Its has three
alcohol functional groups. Each of
these can bond with a long-chain
carboxylic acid (or fatty acid) to form
a triester or fat.
Esters as solvents and coatings
Many natural and synthetic esters are used in wood lacquers, thinners and
a variety of coatings. Some low molecular weight esters have a high volatility
and so are useful components of some solvents used in surface finishes.
Those esters of higher molecular weight are less volatile and find use in
latex paints. Esters of acetic acid, for example, are used in aerosol sprays
and printing inks, and as solvents for personal care products and cosmetics.
Esters of propanoic acid improve solvent diffusion from coating films. They
are used in automotive refinishing and in cosmetics.
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 225
Table 10.5 shows some of the uses of various low molecular weight
esters.
Table 10.5 Uses of some important esters of acetic acid and propanoic acid
M
(g/mol)
b.p.
(ºC)
Relative
evaporation
rate
1-propyl acetate,
CH3COOC3H7
102.1
102
2.75
volatile solvent, wood
lacquers, aerosol sprays,
cosmetics and personal
care solvent
1-butyl acetate,
CH3COOC4H9
116.2
126
1(standard)
fragrance solvent,
coatings for plastics,
cosmetics and personal
care solvent
1-pentyl acetate
(amyl acetate),
CH3COOC5H11
130.2
146
0.5
coatings, cleaning
fluids, extraction solvent
for pharmaceuticals,
printing, finishing fabrics
1-butyl propanoate,
CH3CH2COOC4H9
130.2
145
0.45
appliance coatings,
automotive refinishing,
enamels, lacquers
1-pentyl propanoate,
CH3CH2COOC5H11
144.2
165
0.2
slow evaporating solvent,
appliance coatings,
automotive refinishing,
enamels, lacquers,
personal care products
Ester
O
C
H
C
C
C
—
—
C
H
Esters as plasticisers
O
—
H
O
C
—
C
H
O
H
R = alkyl or
other
group
C
Uses
R
Figure 10.15
Phthalate esters make excellent
plasticisers. They convert stiff plastic
into flexible plastic.
Phthalate esters are oily liquids that are added to hard plastics (such as
PVC) in order to soften them and increase their flexibility. These esters
are examples of plasticisers. Many products, such as medical devices
(including tubing, gloves and medical containers), shower curtains, toys
and synthetic floor coverings are made from this plasticised PVC.
There has been considerable controversy in recent years about the longterm health consequences of these phthalate esters as they are present in
so many household products. Animal studies have implicated these esters
in reproductive diseases including cancer.
Esters as alternative fuels
Esters of natural fatty acids are used as biofuels. They are commonly
called biodiesel. They are manufactured from natural oils such as soybean
and canola using a process called transesterification. In this process, the
vegetable oil is reacted with sodium hydroxide dissolved in methanol.
Esters such as methyl soyate are formed. After processing they can be
used as a biofuel in diesel engines. Bioester fuels are biodegradable. Their
higher viscosity makes them suitable engine lubricants.
Esters as flavours, fragrances and emulsifiers
The characteristic tastes and smells of fresh fruits and flowers are due to
a complex mixture of chemicals. Esters are one example of these types
of molecules. Table 10.6 lists some of the common esters that are used as
flavours in foods.
226 THE ACIDIC ENVIRONMENT
Table 10.6 Esters used as flavourings
Ester
emulsifier: a substance that can
produce a stable homogeneous
dispersion of one insoluble
liquid in another; for example,
soap emulsifies oil and water to
produce a stable dispersion or
emulsion
hydrophilic: a substance that has
a high affinity for water.
emollient: a substance that
softens and soothes the skin
H
C—
H
O
H
C
C—
O
C
H
C
H
H
C
—
C—
C
O
H
H
methyl salicylate
Figure 10.16
Oil of wintergreen has a
characteristic odour associated with
liniments and heat creams for sore
muscles.
Flavour
ethyl formate, HCOOC2H5
rum-like taste and odour; used
in artificial rum
1-butyl formate, HCOOC4H9
plum flavour
2-butyl formate, HCOOCH(CH3)(C2H5)
raspberry flavour
1-pentyl acetate, CH3COOC5H11
fruity flavour (pears and bananas)
1-octyl acetate, CH3COOC8H17
orange flavour
methyl butanoate, CH3CH2CH2COOCH3
apple flavour
ethyl butanoate, CH3CH2CH2COOC2H5
pineapple flavour
butyl butanoate, CH3CH2CH2COOC4H9
pineapple flavouring
1-pentyl butanoate, CH3CH2CH2COOC5H11
apricot flavouring
Esters can also be used as fragrances in body care products and
cosmetics. Some of the fruity esters listed in Table 10.6 are used with
various other aromatic compounds including aldehydes. Phenylmethyl
acetate is an ester with a jasmine fragrance that is used in various cosmetics.
Methyl salicylate or oil of wintergreen is used in lotions and creams to soothe
sore muscles. Esters that have a high molecular weight are also used in
cosmetics and the food industry as emulsifying agents. These emulsifiers
prevent oils and water separating into layers in cosmetics or in the food
product. Citric acid esters of monoglycerides are excellent hydrophilic
emulsifiers. They are used to stabilise mayonnaise, margarine and coffee
whitener. Some esters are used as emollients in face and body creams
and lotions. Myristyl myristate is a solid emollient ester that liquefies on
contact with the body, enhances spreadability of the cream and delivers a
velvety feel to the skin.
H
(a)
O
Glycerol forms
an ester with a
fatty acid and
citric acid.
H
C
O
C
H
C
O
H
R
R
fatty acid
H
O
H
C
Diacetyl esters of tartaric acid
are used in the food industry
in many ways.They strengthen
doughs and enhance gas
retention in the dough.This leads
to improved crust crispness in
baked goods.
O
C
O
C
H
H
C
H
C
O
C
H
O
H
O
H
O
C
H
Figure 10.17
(a) Citric acid esters of
monoglycerides are used to stabilise
cream and mayonnaise products.
(b) Myristyl myristate is an ester that
is used in skin emollients.
(b)
H
H
C
H
H
O
(CH2)12
C
O
(CH2)13
C
H
H
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 227
10.1 Questions
1. Examine the following molecular structure.
CH3CH2CH2CH2CH2CH2CHOHCH3
6. Each row of the following table shows the
names of molecules used to make esters.
Copy and complete the table.
Alkanol
Alkanoic acid
Ester
1-propyl butanoate
The preferred IUPAC name for the
molecule is
A octan-1-ol.
B octan-2-ol.
C octanoic acid.
D octan-7-ol.
2. Examine the following molecular structure.
CH3CH2CH2CH2COOCH2CH2CH2CH3
The preferred IUPAC name for the
molecule is
A nonan-4-ol.
B nonanoic acid.
C 1-butyl pentanoate.
D 1-pentyl butanoate.
3. Select the statement that is true concerning
alkanols, alkanoic acids and esters.
A Alkanols and alkanoic acids are acidic in
water solution.
B The boiling points of alkanols are higher
than the boiling points of alkanoic acids
with the same number of carbon atoms
per molecule.
C The hydrogen bonding between
ester molecules is stronger than the
hydrogen bonding between alkanoic acid
molecules.
D Acetic acid forms dimers due to the
extensive hydrogen bonding between
their molecules.
4.
Select the statement that is true about
esterification.
A Concentrated sulfuric acid is added to
the reaction mixture to act as a catalyst.
B Esterification reactions are rapid and
require little heating.
C Esterification is an example of a
neutralisation reaction.
D Catalysts are added to increase the
equilibrium yield of ester.
5. Methyl acetate is a simple ester. Name the
alkanol and the alkanoic acid from which it
is formed.
A methanoic acid and ethanol
B methanol and acetic acid
C methanol and sulfuric acid
D formic acid and ethanol
228 THE ACIDIC ENVIRONMENT
methanol
formic acid
propanoic acid
butan-2-ol
1-pentyl propanoate
hexanoic acid
7. Write condensed structural formulae
equations for the esterification reaction in
which the following alkanols and alkanoic
acids are reactants.
(a) ethanol and propanoic acid
(b) propan-1-ol and pentanoic acid
8. Explain the difference between the boiling
points of the following compounds.
Compound
Molar weight
(g/mol)
Boiling point
(ºC)
hexan-1-ol
102.2
157.5
pentanoic acid
102.1
185.5
1-propyl acetate
102.1
101.6
9. The following table provides data on the
boiling points of some alkanoic acids. The
boiling points for formic acid and acetic
acid are not listed.
Alkanoic acid
Molar weight
(g/mol)
Boiling point
(ºC)
formic acid
46
acetic acid
60
propanoic acid
74
141
butanoic acid
88
163
pentanoic acid
102
186
hexanoic acid
116
206
heptanoic acid
130
223
(a) Plot a graph of the tabulated data.
(b) Extrapolate the graph (using the data
points for the C3, C4 and C5 acids) to
estimate the boiling points of formic
acid and acetic acid.
(c) The experimental values for the boiling
points of formic acid and acetic acid
are 101ºC and 118ºC. Account for any
differences between these values and
your answer in (b).
10. State one use of each of the following
esters.
(a) 1-octyl acetate
(b) myristyl myristate
(c) 1-pentyl propanoate
11. (a) Define the term esterification.
(b) Explain why the reaction mixture
is refluxed during the esterification
process.
12. In 1895, Emil Fischer discovered that the
reaction between acetic acid and butan-1-ol
took several days to reach equilibrium
under reflux. Fischer found that if hydrogen
chloride gas was passed into the reaction
mixture, equilibrium was achieved in a few
hours.
(a) Suggest an hypothesis to account for the
behaviour of the hydrogen chloride.
(b) Explain whether or not Fischer obtained
a greater yield of product when
hydrogen chloride was used.
(c) Write condensed structural formulae
to produce an equation for this
esterification reaction.
(d) Name the ester formed.
13. In esterification reactions involving diprotic
acids such as oxalic acid, a larger amount
of concentrated sulfuric acid is added to
the reaction mixture. This action produces
an increase in yield. Account for this
observation.
14. Figure 10.18 shows the structure of Aspirin
(acetyl salicylic acid).
H
H
O
C
C
C
H
C
O H
H
C
C
O
H
—
—
—
H
C
—
C
C
O
H
Figure 10.18 Aspirin is a useful analgesic. Its systematic
name is acetyl salicylic acid.
(a) Aspirin is manufactured by the reaction
between 2-hydroxybenzoic acid and
acetic acid. Explain why Aspirin is
classified as an ester.
(b) Use condensed structural formulae to
write an equation for this esterification
reaction.
15. Explain the purpose of each of the
following procedures in the production of
esters.
(a) The reaction flask contains a few pieces
of unglazed ceramic.
(b) The reaction flask is heated with a
hot-water bath or electric heating
mantle rather than a Bunsen
burner.
(c) A water-cooled condenser is mounted
above the reaction flask.
(d) The reaction mixture is refluxed
for many hours (and sometimes
days).
16. Identify the statements listed below that are
not part of the procedure used to prepare
an ester.
(a) Add 20 mL of 2-propanol.
(b) Add 3 drops of universal indicator.
(c) Distil the reaction mixture and recover
the fractions in separate beakers.
(d) Add 30 mL of glacial acetic acid.
(e) Add 2 mL of concentrated sulfuric
acid.
(f) Add 5 mL of 2 mol/L NaOH.
(g) Use a hot-water bath to reflux the
reaction mixture.
(h) Add boiling chips.
17. (a) Ethyl butanoate is a useful ester. Name
one application of this ester in the food
industry.
(b) Justify the procedure used by a student
in a school laboratory to prepare a
sample of ethyl butanoate ester. Include
relevant equations in your answer.
18. Use the data in Table 10.5 to explain why
1-pentyl propanoate , rather than 1-propyl
acetate, is used as a solvent in automotive
paints.
19. Look at Figure 10.16.
(a) Name the acid and alkanol that react to
form the ester illustrated.
(b) State one use of this ester.
20. Propanoic acid has a molar weight of
74.1 g/mol and a boiling point of
140.8°C.
(a) Name an alkanol that has the same
molar weight as propanoic acid.
(b) Predict whether this alkanol would have
a higher or lower boiling point than
that of propanoic acid.
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 229
SUMMARY
www.jaconline.com.au/chemnsw/chemistry2
• Alkanols are alkane derivatives that contain the alcohol (OH)
functional group.
• Alkanoic acids are alkane derivatives that contain the carboxylic
acid (COOH) functional group.
MODULE 2
REVISION
• Alkyl alkanoates are esters that form when alkanols condense with
alkanoic acids with the loss of water.
• The carboxylic acid group makes alkanoic acids highly polar.
• Hydrogen bonding is an important intermolecular force between
alkanoic acid molecules. This leads to an increase in their melting
and boiling points compared with alkanes with the same number
of carbon atoms per molecule.
• Esters have no hydrogen atoms that are involved in hydrogen
bonding. Their melting and boiling points are lower than those of
the equivalent alkanoic acid.
• The reaction between alkanols and alkanoic acids is slow, so
the mixture must be heated to raise the kinetic energies of the
reactants.
• The heating of the alkanol–alkanoic-acid mixture is performed
under reflux to ensure no vapours escape. They are condensed
and return to the reaction mixture.
• A small amount of concentrated sulfuric acid is added as a
catalyst during esterification. The acid also acts as a dehydrating
agent, which helps to shift the equilibrium to the right as water is
removed. Care must be taken when handling concentrated sulfuric
acid.
• Boiling chips are added to the reaction flask during esterification
in order to prevent ‘bumping’ which can lead to dangerous
situations where hot liquids may be ejected from the apparatus.
• Esters are used in the food industry as flavours and emulsifiers.
They are also used in body-care products as fragrances in
perfumes. Some esters are used as solvents.
230 THE ACIDIC ENVIRONMENT
PRACTICAL ACTIVITIES
10.1
PRACTICAL
ACTIVITIES
PREPARATION OF AN ESTER
Aim
1. Select the alkanol and acid from the following
tabulated information. Measure out the required
volumes using clean measuring cylinders.
Experiment
Alkanoic acid
Alkanol
1
acetic acid
(24 mL)
ethanol (8 mL)
2
acetic acid
(24 mL)
propan-1-ol (8 mL)
3
acetic acid
(24 mL)
butan-1-ol (18 mL)
4
acetic acid
(24 mL)
pentan-1-ol (10 mL)
5
butanoic acid
(24 mL)
ethanol (8 mL)
To prepare an ester using the reflux technique
Materials
• alkanoic acid (choose either glacial acetic acid or
butanoic acid)
• alkanol (choose one from ethanol, propan-1-ol,
butan-1-ol or pentan1-1-ol)
• concentrated sulfuric acid (dropper bottle)
(teacher use only)
• Quickfit round-bottom boiling flask (100 mL or
250 mL)
• reflux condenser with hoses
• water bath and Bunsen burner or electric heating
mantle
• retort stand, boss heads and clamps
• boiling chips
• stirring rod
• beaker
• measuring cylinder
• Pasteur pipette
Safety
• Wear safety glasses throughout this experiment.
• Sulfuric acid is corrosive. Clean up spills
immediately. If the acid is spilled on the skin
then wash the area with large quantities of water.
Seek assistance.
• Organic chemicals are flammable. Do not allow
liquids or vapours to come into contact with
sparks or flames.
• Identify other safety precautions relevant to this
experiment by reading the method.
Method
Preparation of the ester
Refer to Figure 10.11 in the text to help you set up
the apparatus.
2. Pour the acid and the alkanol into the dry
round-bottom flask. Your teacher will then add
2 mL (40 drops) of concentrated sulfuric acid to
the flask. Now add 2 or 3 boiling chips.
3. Set up the flask and condenser for reflux as
shown. Use a heating mantle, hotplate, Bunsen
burner (with gauze) or hot-water bath to supply
the heat once the apparatus is set up. A Bunsen
burner can be used, but care should be taken
that the reflux rate is controlled. A hot-water
bath is not very suitable if the boiling points of
the reactants and product are too high.
4. Before you begin to heat the flask, ensure that
cold water is flowing from the tap to the base of
the condenser jacket and then out the top and
back into the sink.
5. Heat the flask and maintain reflux so that no
vapours are lost, and that the condensate drips
back into the reaction flask at about 2 drops
per second.
6. Reflux the mixture for at least 30 minutes
(or longer if time allows).
7. Turn off the heat and allow the apparatus to
cool. (Remove the heating mantle, hotplate or
water bath).
Isolation
You may wish to isolate the ester from the reaction
mixture. If so, then omit this section and proceed
with the optional extension.
8. Remove the boiling flask and carefully pour the
reaction mixture into a beaker containing
CHAPTER 10 ALKANOIC ACIDS AND ESTERS 231
PRACTICAL ACTIVITIES
70 mL of water. Stir with a glass rod. The
mixture contains the ester plus unreacted
alkanol and alkanoic acid. Compare the odour
of this solution with the odours of the original
reactants (see Results and analysis).
9. Clean up the equipment and workbench as
directed by your teacher. Keep the aqueous
mixture of the ester for further examination.
Results and analysis
1. Depending on the solubility of the ester you
have made, the aqueous mixture will either
remain as one phase, or separate into two layers.
(e.g. 1-butyl acetate is insoluble in water). If
separation occurs, the upper layer will be the
crude ester. It can be removed carefully using a
Pasteur pipette or by using a separation funnel.
2. Compare the odour of the ester (or the final
aqueous solution) with that of the original
alkanol and alkanoic acid. To do this you can
waft vapours towards your nose with your hand.
Describe what you smell in each case. Did the
ester have a distinctly different odour from that
of the reactants?
3. Explain how the ester could be extracted and
separated from the unreacted alkanol and
alkanoic acid.
4. Name the ester you have prepared and draw its
structural formula.
232 THE ACIDIC ENVIRONMENT
Conclusion
Write an appropriate conclusion for your
experiment.
Optional extension: Isolation of the ester
1. Add the reaction mixture to 30 mL of saturated
salt water in a beaker.
2. Pour the mixture into a separating funnel and
isolate the upper layer of crude ester.
3. Place the crude ester layer in a beaker and add
saturated sodium hydrogen carbonate to remove
any acid. Stop when fizzing ceases.
4. Isolate the ester once more using a separating
funnel.
5. Dry the ester using crystals of anhydrous calcium
chloride. Filter the mixture.
6. Set up a distillation apparatus and distil the
dried ester. Collect the distillate in a clean
beaker. The boiling points of the esters are:
ethyl acetate (77ºC)
1-propyl acetate (102ºC)
1-butyl acetate (127ºC)
1-pentyl acetate (147ºC)
ethyl butanoate (120ºC)
7. Compare the odours of the esters to those of the
original alkanol and alkanoic acid. Name the
ester you have prepared and draw its structural
formula.