1 Chapter 29Alkanols 29.1Introducing alkanols 29.2Manufacture of ethanol 29.3Uses of alkanols 29.4Properties of alkanols 29.5Alcohol power 29.6Problems.

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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

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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.

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4

Figure 29.1

The two simplest alkanols.

ETHANOL — THE MOST IMPORTANT ALKANOL

Ethanol is the most important alkanol.


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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.

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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).


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Figure 29.4 Some raw materials for fermentation.


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.


(b) Yeast under microscope. Note the

oval-shaped cells.

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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

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(a) (b)

Figure 29.6

(a) Brewing beer by the traditional met

hod of fermentation.


(b) Brewing beer in a local brewery.

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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.


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solution of glucose with yeast

limewater

short rubber tubing

Figure 29.7 Fermentation of glucose in the laboratory.


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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.


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To prepare ethanol by fermentation.

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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.


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.


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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.


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Figure 29.11 These products contain ethanol as the solvent.


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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.


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Figure 29.13

Beer and wine are

produced by

fermentation alone.

Brandy has to be

obtained by

distillation of wine.


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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


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Figure 29.14

Ordinary vinegar is

colourless. It may be brown

due to a colouring matter

added.


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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.


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Figure 29.15 Fruits contain natural esters.


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Figure 29.16 Cosmetics contain synthetic esters.


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Figure 29.17 Synthetic esters are often added to food and drinks as flavourings.

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Figure 29.18

This nail varnish remover contains

an ester.


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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.

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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.


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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.


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Figure 29.20

This after-shave lotion

contains ethanol.


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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)


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Figure 29.21 Ethanol burns in air.


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Oxidation

Alkanols in the form of RCH2OH can be oxidized in two stages:

[O] [O]

e.g. CH3CH2OH CH3CHO CH3COOH

ethanol ethanal ethanoic acid


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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.


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In the laboratory, the above reaction is better carried out using

Quickfit apparatus.

Step 1: Heating the reaction mixture under reflux


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Figure 29.24 Oxidizing ethanol to ethanoic acid by heating under reflux.


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Step 2: Distilling the product mixture

Figure 29.25 Distilling ethanoic acid from the product mixture.


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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.


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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.


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A29.4


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Naming of esters

For example,

methyl propanoate


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A29.5


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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%).

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Figure 29.30

Sugar cane farm.

29.5 ALCOHOL POWER

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Figure 29.31 Making ethanol fuel from sugar cane.

29.5 ALCOHOL POWER

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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

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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

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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

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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

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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!


WHAT HAPPENS AFTER DRINKING ALCOHOL?

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Figure 29.34

How alcohol

goes around the

body.


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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.


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Table 29.3

Typical physiological effects

at various blood alcohol

concentrations.


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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


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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.


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SOCIAL PROBLEMS ASSOCIATED WITH ALCOHOL

Social problems with alcoholics

Besides having bad health, alcoholics often have family and social

problems.


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Figure 29.35

An alcoholic sleeping in

the street.


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Drinking and driving

Alcohol is one of the main causes of traffic accidents.

Figure 29.36

Drinking and driving

don’t mix.


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Figure 29.37

A scene of a car

accident.


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Figure 29.38

One type of breathalyser tube contains potassium dichromate. Its colour changes

from orange to green with ethanol.


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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.


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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?


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(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?


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(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?


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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.


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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

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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

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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.

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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.

1 Chapter 29Alkanols 29.1Introducing alkanols 29.2Manufacture of ethanol 29.3Uses of alkanols 29.4Properties of alkanols 29.5Alcohol power 29.6Problems.

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