ALCOHOLS
The alcohols form a homologous series with the general formula CnH2n+1OH. They
contain the functional group –OH, which is called a hydroxyl group. It is the
presence of this functional group which gives alcohols their characteristic
properties. The first members of the series are:
H
H
C
OH
H
H
methanol
H
H
C
C
H
H
OH
H
ethanol
H
H
H
C
C
C
H
H
H
OH
propan-1-ol
ETHANOL C2H5OH
Ethanol can be produced in two ways:
 by the fermentation of sugars
 by the hydration of ethene.
1. Fermentation of Sugars
Fermentation is used to produce ethanol as an industrial chemical. The raw
material, usually glucose, is dissolved in water to make a solution containing about
15% by mass of glucose. Yeast is then added, and the temperature is maintained at
about 25oC for 3 to 5 days. Yeast is a living organism which contains the enzyme,
zymase. Enzymes are biological catalysts. Under these conditions, the sugar
reacts to form ethanol and carbon dioxide.
C6H12O6
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
2C2H5OH + 2CO2
1
The carbon dioxide is allowed to escape and air is prevented
from entering the fermentation vessel, often by using a water
trap.
When the fermentation is complete, the yeast is filtered off, and
the ethanol is separated from the reaction mixture by fractional
distillation.
Fermentation is also used:
 to produce ethanol in beer and wine
 to produce the bubbles of CO2 which make bread dough
rise
fermentation
mixture
N.B. The simple laboratory test for carbon dioxide is that it turns limewater milky.
2. Hydration of Ethene
Ethene undergoes an addition reaction with steam at high temperature (~300 oC)
and pressure (60-70 atm) in the presence of a strong acid catalyst (phosphoric
acid), to form ethanol.
C2H4 + H2O
ethene
C2H5OH
ethanol
COMPARISON OF THE TWO PROCESSES
Fermentation
Disadvantages:
 a slow reaction (3 to 5 days)
 the product is a mixture containing only about 10% ethanol, from
which the ethanol is separated by distillation
 the process is a batch process
Advantages
 the raw material (glucose) is renewable
 the process is low-tech: it needs only inexpensive equipment and
unskilled labour
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
2
Hydration
Advantages:
 a fast reaction (seconds)
 the product is pure
 the process is a continuous process
Disadvantages
 the raw material (ethene) is obtained from crude oil, which is nonrenewable
 the process is high-tech: it requires expensive equipment and semiskilled labour
ETHANOL AS A FUEL
Combustion
Ethanol is a fuel and burns to produce carbon dioxide and water.
C2H5OH
+ 3O2
2CO2 + 3H2O
Ethanol is mixed with petrol in biofuels. The impact on the environment of burning
a biofuel is less than burning a fossil fuel as the carbon dioxide produced during
combustion is equal to that which was taken in by the plant during photosynthesis.
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
3
Carboxylic acids
The carboxylic acids form a homologous series with the general formula CnH2nO2.
They contain the functional group –COOH, which is called an acid group. It is the
presence of this functional group which gives carboxylic acids their characteristic
properties. The first members of the series are:
H
H
C
H
C
OH
C
OH
Ethanoic acid
H
O
H
H
Methanoic acid
H
O
O
C
C
H
H
C
OH
Propanoic acid
Carboxylic acids react in the same way as the mineral acids you are familiar with
i.e. they react with metals to produce salts and hydrogen gas.
2CH3COOH + Mg
Mg(CH3COO) 2 +
H2
They are weak acids and so have fewer H+ ions in solutions and hence reactions are
not as vigorous as strong acids of equal concentration. Weak acids typically have a
pH of around 3-4.
Carboxylic acids are made by oxidising alcohols. This happens in the air and
explains why wine which has been left without a cork in it for some time, tastes
sour. The ethanol has been oxidised to ethanoic acid which is the acid present in
vinegar.
On an industrial scale, alcohols are oxidised to carboxylic acids much more quickly
by using oxidising agents.
C2H5OH +
[O]
CH3COOH
Here [O] is used to show oxygen gained from an oxidising agent.
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
4
Esters
Carboxylic acids are also used to make useful compounds called esters. Esters are
made by reacting carboxylic acids with alcohols using a conc. sulphuric acid
catalyst. These esters are used as food flavourings (many fruity sweets contain
esters) and as solvents. The characteristic smell you get when you remove the lid
on nail varnish is an ester.
Naming esters is as simple as a,b,c… (alcohol before carboxylic acid).
H
H
H
C
C
H
H
O
C
+ HO
OH
Propanoic acid
water
H
H
C
C
H
H
H
H
ethanol
H
H
C
C
H
H
O
C
O
H
H
C
C
H
H
+ H 2O
H
ethyl propanoate
This is a condensation reaction since a small molecule, in this case water, is also
produced.
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
5
Topic 17b Organic chemistry
Summery questions
1
2
3
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
6
4
5
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
7
6
7
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
8
8
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
9
(d)
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
10
9
Which of the following will react together to form propyl ethanoate?
A)
1 and 3
1
2
3
4
B) 2 and 3
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
C)
11
2 and 4
D)
1 and 4
10
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
12
IGCSE TOPIC 10.17b: ORGANIC CHEMISTRY
13