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Chapter 29 Alkanols
29.1 Introducing alkanols
29.2 Manufacture of ethanol
29.3 Uses of alkanols
29.4 Properties of alkanols
29.5 Alcohol power
29.6 Problems associated with alcoholic drinks
29.7 How much is one unit of alcohol?
CONTENTS OF CHAPTER 29
2
29.1 INTRODUCING ALKANOLS
29.1 INTRODUCING ALKANOLS
WHAT ARE ALKANOLS?
Alkanols (or alcohols) are a homologous series of organic compou
nds with the general formula CnH2n+1OH or simply ROH. Their mol
ecules consist of a functional group — the hydroxyl group (—OH)
joined to an alkyl group (R—).
The two simplest alkanols are methanol and ethanol.
4
Figure 29.1
The two simplest alkanols.
ETHANOL — THE MOST IMPORTANT ALKANOL
Ethanol is the most important alkanol.
29.1 INTRODUCING ALKANOLS
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29.2 MANUFACTURE OF ETHANOL
29.2 MANUFACTURE OF ETHANOL
In general, industrial ethanol is formed from ethene, a petroleum p
roduct. Ethanol in alcoholic drinks is made from agricultural produ
cts by fermentation.
ETHANOL FROM CATALYTIC HYDRATION OF ETH
ENE
HEATERREACTOR(300oC, 65 atm, catalyst)
CONDENSERa mixture of ethanol and water
ethene
steam
unreacted ethene and steam recirculated
Figure 29.2 A flow diagram for ethanol manufacture by the catalytic hydration of ethene.
6
The process produces a mixture of ethanol and water. Ethanol
can be separated out by fractional distillation.
ETHANOL FROM FERMENTATION OF STARCH OR
SUGAR
FERMENTATION is the slow breakdown of large organic
molecules (e.g. starch) to smaller molecules (e.g. ethanol) by
micro-organisms (e.g. yeast).
29.2 MANUFACTURE OF ETHANOL
7
Figure 29.4 Some raw materials for fermentation.
29.2 MANUFACTURE OF ETHANOL
Grape Wheat Potato
Apple Barley Rice Maize
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(a) (b)
Figure 29.5
(a) Yeast is a living plant. It can supply
enzymes for fermentation.
29.2 MANUFACTURE OF ETHANOL
(b) Yeast under microscope. Note the
oval-shaped cells.
9
29.2 MANUFACTURE OF ETHANOL
diastase
2 —C6H10O5 — (s) + nH2O(l) nC12H22O11(aq)
starch maltose
maltase
C12H22O11(aq) + H2O(l) 2C6H12O6(aq)
maltose water glucose
zymase
C6H12O6(aq) 2C2H5OH(aq) + 2CO2(g)
glucose ethanol carbon dioxide
To produce beer, use barley (or maize) as the starch source.
[ ]n
10
(a) (b)
Figure 29.6
(a) Brewing beer by the traditional met
hod of fermentation.
29.2 MANUFACTURE OF ETHANOL
(b) Brewing beer in a local brewery.
11
The result of fermentation is a dilute (roughly 10%) solution of
ethanol in water. The concentration of alcohol cannot be greater
than 15%. At concentrations higher than this, the yeast dies and
fermentation stops. We can increase the concentration of ethanol
in the product by distillation.
29.2 MANUFACTURE OF ETHANOL
12
solution of glucose with yeast
limewater
short rubber tubing
Figure 29.7 Fermentation of glucose in the laboratory.
29.2 MANUFACTURE OF ETHANOL
13
A29.1
(a) The mixture in the flask smells of ethanol.
(b) The limewater can test for the carbon dioxide given out in the
fermentation process. Besides, the arrangement serves as
an air-lock to prevent oxidation of ethanol to ethanoic acid by
air.
29.2 MANUFACTURE OF ETHANOL
15
29.3 USES OF ALKANOLS
29.3 USES OF ALKANOLS
ALKANOLS AS FUELS
Methanol and ethanol burn readily in air with a clean flame. In a fe
w countries, ethanol (in a pure form or mixed with petrol) is used a
s a motor vehicle fuel.
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Figure 29.8
Ethanol burns with a clean
flame in a good supply of air.
29.3 USES OF ALKANOLS
Figure 29.9
Ethanol is used as a motor vehicle fuel
in Brazil.
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Figure 29.10
Methylated spirit is the most
widely used industrial alcohol.
Methylated spirit is the most widely used industrial alcohol. Its
main use is as a fuel.
29.3 USES OF ALKANOLS
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ALKANOLS AS SOLVENTS
Alkanols are very good solvents. Ethanol is used as a solvent for
cosmetics, perfumes and tincture of iodine. Methylated spirit is an
industrial solvent for paints and dyes.
29.3 USES OF ALKANOLS
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ETHANOL IN ALCOHOLIC DRINKS
All alcoholic drinks contain ethanol to a greater or lesser extent.
Table 29.2
Starch/sugar source and
approximate percentage
volume of ethanol in some
alcoholic drinks.
29.3 USES OF ALKANOLS
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Figure 29.13
Beer and wine are
produced by
fermentation alone.
Brandy has to be
obtained by
distillation of wine.
29.3 USES OF ALKANOLS
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PRODUCTION OF VINEGAR
Vinegar is a dilute solution (~6%) of ethanoic acid. To make
vinegar, add suitable micro-organisms to a dilute solution of
ethanol. Then expose the mixture to air for one or two weeks. The
following reaction occurs:
[O]
CH3CH2OH CH3COOH from air
29.3 USES OF ALKANOLS
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Figure 29.14
Ordinary vinegar is
colourless. It may be brown
due to a colouring matter
added.
29.3 USES OF ALKANOLS
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PRODUCTION OF ESTERS
Esters are usually pleasant-smelling liquid compounds. Each
ester has its own characteristic sweet fruity smell. Many synthetic
esters are used in cosmetics and artificial flavourings (added to
food and drinks).
Esters are also good solvents for organic compounds.
29.3 USES OF ALKANOLS
27
29.3 USES OF ALKANOLS
Figure 29.17 Synthetic esters are often added to food and drinks as flavourings.
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29.4 PROPERTIES OF ALKANOLS
29.4 PROPERTIES OF ALKANOLS
WHAT INFLUENCE PROPERTIES?
An alkanol molecule can be represented as:
As the size of the alkyl group increases, there is a gradual
change in physical properties.
30
Chemical properties of an alkanol are mainly due to the hydro
xyl group.
PHYSICAL PROPERTIES OF ALKANOLS
Simple alkanols are liquids at room conditions. Melting point, boili
ng point and density increase steadily as the number of carbon at
oms increases.
Solubility
The lower alkanols (methanol, ethanol, propan-l-ol and propa
n-2-ol) are miscible with water in all proportions. But as the numbe
r of carbon atoms increases, the solubility rapidly decreases.
29.4 PROPERTIES OF ALKANOLS
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insoluble hexan-1-ol layer
water
uniform mixture (water + ethanol)
Figure 29.19
Different solubilities of ethanol and hexan-1-ol in water.
29.4 PROPERTIES OF ALKANOLS
33
CHEMICAL PROPERTIES OF ALKANOLS
All alkanols are non-electrolytes. They are neutral (pH value = 7).
Most of their properties are due to the —OH functional group.
A29.2
No; no.
Combustion
Alkanols burn readily in air to form carbon dioxide and water,
giving a lot of heat. For example,
CH3CH2OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)
29.4 PROPERTIES OF ALKANOLS
35
Oxidation
Alkanols in the form of RCH2OH can be oxidized in two stages:
[O] [O]
e.g. CH3CH2OH CH3CHO CH3COOH
ethanol ethanal ethanoic acid
29.4 PROPERTIES OF ALKANOLS
36
Figure 29.23 shows how the oxidation can be performed on a
test-tube scale.
ethanol
acidified potassium dichromate solution
solution containing Cr3+(aq) ions
Figure 29.23 Oxidizing ethanol with acidified potassium dichromate solution.
29.4 PROPERTIES OF ALKANOLS
37
In the laboratory, the above reaction is better carried out using
Quickfit apparatus.
Step 1: Heating the reaction mixture under reflux
29.4 PROPERTIES OF ALKANOLS
38
Figure 29.24 Oxidizing ethanol to ethanoic acid by heating under reflux.
29.4 PROPERTIES OF ALKANOLS
39
Step 2: Distilling the product mixture
Figure 29.25 Distilling ethanoic acid from the product mixture.
29.4 PROPERTIES OF ALKANOLS
40
29.4 PROPERTIES OF ALKANOLS
Oxidation of ethanol to ethanoic acid by acidified potassium
dichromate solution.
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A29.3
Ethanol in wine is slowly oxidized in air by the action of micro-
organisms. It is changed to ethanoic acid, which is sour. On the
other hand, brandy has an ethanol content much higher than
wine. Micro-organisms cannot function in such a high ethanol
concentration.
Esterification
ESTERIFICATION is the reversible reaction of an alkanoic acid
with an alkanol to form an ester and water.
29.4 PROPERTIES OF ALKANOLS
43
Take an example:
Figure 29.28
Reaction between ethanoic acid and ethanol (in the presence of a little concentrated
sulphuric acid) to form ethyl ethanoate.
29.4 PROPERTIES OF ALKANOLS
47
29.5 ALCOHOL POWER
29.5 ALCOHOL POWER
ALCOHOL POWER FROM CROPS
Crops can be fermented and distilled to produce ethanol.
ETHANOL AS A FUEL FOR MOTOR VEHICLES
Sugar cane is a fast-growing tropical plant. The sugar it provides c
an be fermented to make ethanol.
Brazil is a country that extensively uses ethanol as a fuel for
motor vehicles. Most vehicles use gasohol. Gasohol is a mixture o
f petrol (78%) and ethanol (22%).
50
For the affirmative side:
Ethanol burns to form carbon dioxide and water only, so it is a
cleaner fuel than petrol.
Ethanol has a higher octane number (a measure of anti-knock
characteristics) than ordinary petrol.
Ethanol can be produced from the fermentation of various
crops (e.g. cane sugar), which are renewable energy sources.
(But petrol is a non-renewable fuel.)
29.5 ALCOHOL POWER
Activity 6
51
In some places (e.g. Brazil), ethanol is much cheaper than
petrol.
Ethanol is easy to transport within and between nations.
A petrol engine would take 10% ethanol in the petrol without
any carburetor adjustments. Even engines specially designed
to run on pure ethanol are only slightly more expensive than
ordinary engines.
29.5 ALCOHOL POWER
52
For the negative side:
The calorific value of ethanol is only 60% that of petrol.
Ethanol has different burning characteristics compared with
petrol. As a result, car engines need to be modified or
specially designed before pure ethanol can be used.
The above two points may have serious economic
implications.
In many places, ethanol is even more expensive than petrol.
Ethanol can be reserved for other purposes, e.g. alcoholic
drinks, solvents and making chemicals.
Poisonous effluents from distilleries may cause pollution to
nearby rivers.
29.5 ALCOHOL POWER
53
A little ethanal is produced on burning ethanol. Ethanal is
toxic, causing damage to the skin, eyes and lungs and
harming vegetation.
Huge areas of arable land would have to be cultivated to
produce ethanol, at the expense of other agricultural uses.
Besides, land disturbance is high and soil erosion is
increased.
29.5 ALCOHOL POWER
54
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
29.6 PROBLEMS ASSOCIATED WITH
ALCOHOLIC DRINKS
Most alkanols are poisonous. The first member of the series, meth
anol, is very toxic.
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Figure 29.32
Methanol is toxic!
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
WHAT HAPPENS AFTER DRINKING ALCOHOL?
57
Alcohol is removed from the blood mainly by action of the
liver.
PHYSIOLOGICAL EFFECTS OF ALCOHOL
Alcohol can suppress feelings of fear and tension, making people
feel relaxed. Excessive drinking would be harmful.
Blood alcohol concentration (BAC)
Drinking affects the blood alcohol concentration (BAC). BAC is the
mass of alcohol (in mg) present in 100 cm3 of blood.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
58
Table 29.3
Typical physiological effects
at various blood alcohol
concentrations.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
59
HEALTH PROBLEMS ASSOCIATED WITH DRINKING
Excessive drinking over a number of years can cause many
health problems:
hepatitis (inflammation of the liver)
cirrhosis (scarring of the liver)
brain damage
depression, forgetfulness and other
psychiatric and emotional disorders
problems with the nervous system
high blood pressure
alcohol increases the workload of the liver and kills liver cells
alcohol can destroy brain cells, and dead brain cells cannot be renewed
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
60
stomach ulcer (alcohol irritates stomach linings)
vitamin deficiency
muscle disease
cancer of the mouth, throat and gullet
body weight problem
A29.6
Alcohol in a pregnant woman‘s blood can pass into the blood of
the unborn baby, causing much harm.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
61
SOCIAL PROBLEMS ASSOCIATED WITH ALCOHOL
Social problems with alcoholics
Besides having bad health, alcoholics often have family and social
problems.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
63
Drinking and driving
Alcohol is one of the main causes of traffic accidents.
Figure 29.36
Drinking and driving
don’t mix.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
65
Figure 29.38
One type of breathalyser tube contains potassium dichromate. Its colour changes
from orange to green with ethanol.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
66
Activity 7
Students may be divided into 3 groups:
One works on the physiological effects of alcoholic drinks;
One works on the health problems associated with excessive
drinking;
One works on the social problems associated with excessive
drinking.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
67
Some useful guiding questions for the library search and subsequ
ent discussion:
(a) On physiological effect:
(1) What is the constituent in wine that causes the
physiological effects?
(2) How does the body cope with the intake of wine?
(3) Which parts of the body or organs are affected by
drinking wine?
(4) What are the symptoms of excessive drinking?
(5) How are the symptoms/effects changed with the amount
of wine drunk?
(6) Does excessive drinking cause addiction?
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
68
(b) On health problem:
(1) Which parts of the body or organs would be damage
d by excessive drinking?
(2) What are the diseases likely to be induced by excess
ive drinking?
(3) What are the bad effects of excessive drinking on a
pregnant woman?
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
69
(c) On social problem:
(1) How does excessive drinking affect our society on the
aspects of
(i) family relationship
(ii) law and order
(iii) economy?
(2) Do you think that the problem is serious in Hong Kong?
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
70
Some additional information:
The life expectancy of heavy drinkers is estimated to be 10 –
12 years shorter than the general public.
Children of alcoholics are 3 – 4 times more likely to become
alcoholics.
In the U.S.A., about 15% of the people suffer from desertion,
divorce, impoverishment or child displacement due to
alcoholism.
29.6 PROBLEMS ASSOCIATED WITH ALCOHOLIC DRINKS
71
SUMMARY
1. Alkanols (or alcohols) are a homologous series of organic co
mpounds with the general formula CnH2n+1OH.
2. Ethanol (CH3CH2OH) is the most important alkanol.
3. Ethanol can be manufactured by the catalytic hydration of eth
ene:
SUMMARY
72
SUMMARY
4. Ethanol in alcoholic drinks is made from starch/sugars by fer
mentation. The reaction is catalysed by enzymes in yeast.
5. Fermentation is the slow breakdown of large organic molecul
es (e.g. starch) to smaller molecules (e.g. ethanol) by micro-o
rganisms (e.g. yeast).
6. General uses of lower alkanols:
As fuels
As solvents
Production of esters
In addition, ethanol
is present in all alcoholic drinks
can be oxidized to produce vinegar
73
SUMMARY
7. Chemical properties of an alkanol are mainly due to the
— OH functional group.
8. Ethanol can be oxidized first to ethanal and then to ethanoic
acid in the laboratory. The oxidizing agent commonly used is
acidified potassium dichromate solution.
9. Esterification is the reversible reaction of an alkanoic acid wit
h an alkanol to form an ester and water.
74
SUMMARY
10. Esters have the general formula RCOOR’. Lower esters are li
quids with pleasant sweet fruity smell. They are often used in
flavourings and solvents.
11. Ethanol is used as a fuel for vehicles in some countries (e.g.
Brazil, USA). Ethanol is a cleaner fuel than petrol. It is also a
renewable energy source.
12. Excessive drinking causes many health and social problems.
13. Drink driving is very dangerous.