Ocsillation

1. Chapter
15
HYDROCARBONS
1. Hydrocarbons
2. Uses of hydrocarbons
3. Homologous series
4. Reactions of alkanes
5. Pollution from burning of hydrocarbons
6. Substitution reaction
7. alkenes
Table of Contents

2. Hydrocarbons
 Introduction
Hydrocarbons
Simple organic compounds containing carbon and
hydrogen elements only are called hydrocarbons
For example;
HYDROCARBON IN USE
 Crude oil is our main source of hydrocarbons.
 They provide us with fuels such as petrol, diesel and
kerosene.
 Hydrocarbons are also the starting compounds we use to
make many new compounds,
S.NO. Hydrocarbons Chemical formula
1 Methane CH4
2 Ethene C2H4
3 Benzene C6H6

3. Hydrocarbons
Saturated hydrocarbons
Hydrocarbons in which C-C bonds are single covalent bonds,
resulting in the maximum number of H-atoms in the molecule
Alkanes
Saturated hydrocarbons with general formula CnH2n+2.
For example,
The molecular formula of pentane, in which n=5, is C5H12.
some different ways of representing pentane molecules.

4. Hydrocarbons
Homologous series of alkanes
A series of hydrocarbons in which the adjacent members is
differ by methylene group(-CH2) is called homologous series.
The first ten members of this homologous series given below

5. Hydrocarbons
Sources of alkanes
Crude oil
It is a complex mixture of hydrocarbon- alkanes, cycloalkane
and aromatic hydrocarbons.
Natural gas
Natural gas is 90% methane with lesser amounts of C2, C3
and C4

6. Hydrocarbons
Cyclic Hydrocarbons
Hydrocarbons having closed ring of carbon atoms.
General formula of cycloalkanes is CnH2n
HOME WORK
BOOK PAGE 306
QUESTION: 1.a.b.c.d

7. Hydrocarbons
Reactions of alkanes
Alkanes are generally unreactive compounds.
However, the alkanes do react with oxygen in combustion
reactions and also undergo substitution by halogen in sunlight.
1) Combustion of alkanes
Alkanes are often used as a fuels. We burn them for many
reasons.
• To generate electricity in power stations.
• To heat our homes and cook our food
• To provide energy needed in industrial processes
• To provide fuel for ships, aero planes, trains, lorries, buses,
cars and motorbikes.

8. Hydrocarbons
Reactions of alkanes
1) Combustion of alkanes
If we burn an alkane in plenty of oxygen, the alkane will
undergo complete combustion. All the carbon will be oxidised
fully to form CO2 and all the H will be oxidised to form water.
Alkane + oxygen Carbon dioxide + water
For example,
octane can be found in petrol. Some of it will undergo
complete combustion in a car engine
Complete
combustion

9. Hydrocarbons
Reactions of alkanes
Pollution from burning hydrocarbons fuels
When petrol or diesel is mixed with air inside a car engine,
there is a limited supply of oxygen.
Under these conditions, not all the carbon in the hydrocarbon
fuel is fully oxidised to carbon dioxide.
Some of the carbon is only partially oxidised to form carbon
monoxide(CO) gas.
This is called incomplete combustion.
For example;

10. Hydrocarbons
Reactions of alkanes
Harmful effect of Carbon monoxide
Carbon monoxide is a toxic gas that combine with the
haemoglobin in your blood. Haemoglobin then failed to
transport oxygen around your body.
If the victim not removed from the toxic gas, the victim will die.
Harmful effect of oxides of nitrogen
Along with carbon monoxide, road traffic also releases acidic
nitrogen oxides mainly NO and NO2.
These gases causes the problem of acid rain, which can kill
trees and aquatic animals in lakes.
Acid raid also corrodes metals, such as iron etc.

11. Hydrocarbons
Reducing traffic emissions
Cars can now be fitted with a catalytic converter in their exhaust
system (Figure 15.9). Once warmed up, a catalytic converter
can cause the following reactions to take place:
 the oxidation of carbon monoxide to form carbon dioxide
 the reduction of nitrogen oxides to form harmless
nitrogen gas
 the oxidation of unburnt hydrocarbons to form carbon dioxide
and water
Unfortunately, catalytic converters can do nothing to reduce the
amount of carbon dioxide given off in the exhaust gases of cars.
Reaction take place in converter
2CO + 2NO 2CO2 + N2
2CO + O2 2CO2

12. Hydrocarbons
2. Substitution reactions of alkanes
The alkanes will goes substitution reactions with halogens in the
presence of sunlight.
For example;
The reaction between methane and chlorine in sunlight
In this reaction a hydrogen atom in the methane molecule gets
replaced by a chlorine atom. However,
the reaction does not take place
in darkness as shown in figure,
So what role does the sunlight play
in the mechanism of the
substitution reaction?

13. Hydrocarbons
Steps involved in substitution reaction alkane
1) Initiation step
The first step in the mechanism is the breaking of the Cl-Cl
bond by ultraviolet light from the Sun. This is an example of
homolytic fission of a covalent bond.
2) Propagation step
Free radicals are very reactive. They will attack the normally
unreactive alkanes. A chlorine free radical will attack the
methane molecule:
In propagation step a C-H bond broke homolytically. A methyl
free radical, CH3, is produced. This can then attack a chlorine
molecule, forming chloromethane and regenerating a chlorine
free radical

14. Hydrocarbons
Steps involved in substitution reaction alkane
Propagation step
The more chlorine gas in the reaction mixture to start
with, the greater the proportions of CH2Cl2, CHCl3 and CCl4
formed as products.

15. Hydrocarbons
Steps involved in substitution reaction alkane
3) Termination steps
Whenever two free radicals meet they will react with each other.
A single molecule is the only product.
As no free radicals are made that can carry the chain reaction st
Examples;

16. Hydrocarbons
Unsaturated hydrocarbon
Hydrocarbons having carbon to carbon multiple bonds are called
unsaturated hydrocarbons.
Types of unsaturated hydrocarbons
Unsaturated hydrocarbons are two types
 Alkenes
 Alkynes
Alkenes
unsaturated hydrocarbons which contain at least one carbon to
carbon double bond per molecule are called alkenes.
Alkenes with one double bond per molecule have
the general formula CnH2n.
Example;
Ethene, C2H4

17. Hydrocarbons
Industrial preparation of alkenes
1) Oil refineries provide useful alkenes for the chemical industry.
On page 203 we saw how crude oil is separated into fractions at
a refinery.
2) In the oil companies some of the excess
heavier fractions are converted to lighter
hydrocarbons by the cracking process.
Example of a cracking reaction is:

18. Hydrocarbons
Unsaturated hydrocarbon
Addition reactions of alkenes
In these reactions one of the two bonds in the carbon–carbon
double bond is broken and a new single bond is formed from
each of the two carbon atoms.
The general addition reactions are shown below

19. Hydrocarbons
Unsaturated hydrocarbon
Addition of hydrogen
When hydrogen and an alkene are passed over a finely
divided nickel catalyst at 140 °C, the addition reaction produces
an alkane:
The addition reaction with hydrogen is used in the manufacture
of margarine
Hydrogenation
The addition reaction
of alkenes with
hydrogen is called
hydrogenation

20. Hydrocarbons
Unsaturated hydrocarbon
Addition of hydrogen halides, HX(aq)
When an alkene is bubbled through a concentrated solution of a
hydrogen halide (HF, HCl, HBr, HI) at room temperature, the
product is a halogen alkane.
For example:
Addition of steam
hydration is used in industry to make alcohols. Steam and the
gaseous alkene, in the presence of concentrated phosphoric
acid are reacted at a temperature of 330 °C and a pressure of 6
MPa. When the alkene is ethene, the product is ethanol
However, the ethanol found in alcoholic drinks is always
produced by the fermentation of glucose

21. Hydrocarbons
Unsaturated hydrocarbon
Addition of halogens, X2(aq)
If we bubble an alkene through a solution of chlorine or bromine
at room temperature, we again get an addition reaction. The
colour of the halogen molecules in solution is removed in the
reaction mixture.
In fact, bromine water is used
to test for the presence of the
C-C double bond in compounds.
The compound to be tested is
shaken with bromine water.
If it is unsaturated,
the bromine water will be decolorised

22. Hydrocarbons
Unsaturated hydrocarbon
The mechanism of electrophilic addition to alkenes
There are total of 4 electrons between C-C double bond,
So ethene is a non-polar molecule, there is a high electron
density around the C-C double bond, which makes the alkenes
to attack by electrophiles (electron accepter).
HBr is a polar molecule because of the difference in E.N.
between the H atom and the Br atom.
In HBr, the H atom carries a partial positive charge and the Br
atom carries a partial negative charge.
In the mechanism of addition, the H atom acts as the
electrophile, accepting a pair of electrons from the C-C bond in
the alkene.

23. Hydrocarbons
Unsaturated hydrocarbon
The mechanism of electrophilic addition to alkenes
But how can a non-polar molecule such as Br2 act as an
electrophile?
As the bromine molecule and ethene molecules approach each
other, the area of high electron density around the C-C bond
repels the pair of electrons in the Br-Br bond away from the
nearer Br atom. This makes the nearer Br atom slightly positive
and the further Br atom slightly negative.
Figure 15.16 shows the mechanism of electrophilic addition.
HOME WORK PAGE 314
Question 5
a, b, c, d, e, f, g

24. Hydrocarbons
Oxidation of the alkenes
Alkenes can be oxidised by acidified by KMnO4(aq), which is a
powerful oxidising agent.
The products formed will depend on the conditions chosen for
the reaction. In Figure 15.19, the R, R1 and R2 are alkyl groups.
Practical activity

25. Hydrocarbons
Hot concentrated acidified KMnO4 solution
Under these harsher conditions, the C=C bond in the alkene is
broken completely. The O-H groups in the diol formed initially
are further oxidised to ketones, aldehydes, carboxylic acids or
carbon dioxide gas. The actual products depend on what is
bonded to the carbon atoms involved in the C=C bond. Figure
15.17 shows the oxidation products from each type of group
bonded to a carbon atom in the C=C bond.

26. Hydrocarbons
Summaries oxidation of alkenes
We can summaries the oxidations of alkenes under harsh
conditions in three reactions.
HOME WORK BOOK PAGE 315

27. Hydrocarbons
Addition polymerisation
Probably the most important addition reaction of the alkenes
forms the basis of much of the plastics industry. Molecules of
ethene, as well as other unsaturated compounds, can react with
each other under the right conditions to form polymer molecules.
Polymer
A polymer is a long-chain molecule made up of many repeating
units.
Monomers
The small, reactive molecules that react together to make the
polymer are called monomers.
Up to 10 000 ethene monomers can bond together to form the
polymer chains of Polyethene.
Polyethene is commonly used to make carrier bags.
Other alkenes also polymerise to make polymers with different
properties.
Examples; Polypropene and polyphenylethene.

28. Hydrocarbons
Addition polymerisation
The reaction of many monomers containing at least one double
C=C bond to form the long-chain ploymers as the only product is
called addition polymerization
As in other addition reactions, addition polymerisation is also
involves the breaking of the π bond in each C=C bond, then the
monomers link together

29. Hydrocarbons
More about addition polymerisation
We can also use substituted alkenes, such as
Chloro ethene, as monomers;
The [–H2C—CHCl –] section of the polymer chain is the repeat
unit of poly(chloroethene)

30. Hydrocarbons
Disposal of Polyethene plastics
Plastics are widely used in many aspects of everyday life.
However, the large-scale use of poly(alkene)s has created a
problem when we come to dispose of them.
During their useful life, one of the poly(alkene)s’ useful
properties is their lack of reactivity.
As they are effectively huge alkane molecules, they are
resistant to chemical attack. So they can take hundreds of years
to decompose when dumped in landfill sites, taking up valuable
space. They are non-biodegradable. Therefore throwing away
poly(alkenes) creates rubbish that will pollute the environment
for centuries (Figure 15.23).

31. Hydrocarbons
Burning plastic waste
One way to solve this problem would be to burn the
poly(alkene)s and use the energy released to generate
electricity. As we have seen on page 204, if hydrocarbons burn
in excess oxygen the products are carbon dioxideand water. So
this solution would not help combat global warming, but would
help to conserve our supplies of fossil fuels that currently
generate most of our electricity. However, we have also seen
that toxic carbon monoxide is produced from incomplete
combustion of hydrocarbons

32. Hydrocarbons
Burning plastic waste
Another problem is the difficulty recycling plants have in
separating other plastic waste from the poly(alkene)s when
objects have just been thrown away without being sorted
according to their recycling code. Then if poly(chloroethene) is
burnt, acidic hydrogen chloride gas will be given off, as well as
toxic compounds called dioxins. Acidic gases would have to be
neutralised before releasing the waste gas into the atmosphere
and very high temperatures used in incinerators to break down
any toxins