1. Chapter 15
Hydrocarbons
Learning Outcomes
• Recall that carbon forms branched and unbranched
chains and ring compounds
• Illustrate that carbon atoms can form single and double
bonds
• Write formulae to represent simple organic compounds
• List the general characteristics of a homologous series
• Deduce the homologous series given the fully displayed
formulae of compounds
• Define structural isomerism

2. Chapter 15
Hydrocarbons
Learning Outcomes
• Write general and molecular formulae for the
members of the alkane homologous series
• Write fully displayed structures and names of
branched and unbranched alkanes and their
isomers, given their molecular formulae
• Describe the reactions of alkanes
• Relate the characteristic reactions of alkanes to
their structures
• Relate the properties of alkanes to their uses

3. Chapter 15
Hydrocarbons
Learning Outcomes
• Write general and molecular formulae for the
members of the alkene homologous series
• Write fully displayed structures and names of
branched and unbranched alkenes and their
isomers, given their molecular formulae
• Describe the reactions of alkenes
• Relate the characteristic reactions of alkenes to
their structures
• Relate the properties of alkenes to their uses

4. Chapter 15
Hydrocarbons
Introduction to
Hydrocarbons
• Hydrocarbons belong to a group of compounds called the
organic compounds.
• This ability of carbon to form big molecules has enabled
nature to form living matter, from simple cells to complex
tissues.
• Gasoline or petrol is a mixture of hydrocarbons and is used
primarily as fuel in internal combustion engines of cars.

5. Chapter 15
Hydrocarbons
Bonding ability of carbon
• Carbon atoms are able to form a few types of covalent bonds
with other carbon atoms or atoms of other elements:
• Single bonds: C-C, C-H, C-O, C-X (X = halogens)
• Double bonds: C=C, C=O
• Triple bonds: C≡ C, C≡N
• Organic compounds can be classified as unbranched,
branched or ring compounds.
• There are five types of formulae which can be used to describe
and identify organic compounds:
1. Molecular formula
2. Empirical formula
3. Full structural or displayed formula
4. Condensed structural formula
5. General formula

6. Chapter 15
Hydrocarbons
Straight (unbranched)
Chain Compounds
• Straight-chain molecules that are unbranched may contain single,
double or triple bonds. E.g.:
Single bond Double bonds
Triple bonds

7. Chapter 15
Hydrocarbons
Branched- Chain
Compounds
• Branched chain compounds consist of one or more carbons
of a straight chain compound forming bonds with more than
two carbons.

8. Chapter 15
Hydrocarbons
Homologous Series
• A homologous series is a group or family of organic
compounds that has the following characteristics:
- They have the same general formula.
- They show similar chemical properties because they
have the same functional group.
- Each member differs from the previous one by -CH2.
- They show a gradual change in density, viscosity,
flammability, melting point and boiling point.
• A functional group is a group of atoms that is responsible for
the characteristic chemical properties of a homologous series.
• Alcohols have the –OH functional group, carboxylic acids have
the –COOH functional group and alkenes contain a C=C bond.

9. Chapter 15
Hydrocarbons
Structural Isomerism
• Isomers are organic compounds with the same
molecular formula but different structural formulae.
• Isomers that are in the same homologous series
have similar chemical properties but differ slightly in
melting points and boiling points.
Isomers of butane (C4H10)

10. Chapter 15
Hydrocarbons
Alkane Homologous
Series
• Alkanes have the general formula CnH2n+2,
• Their names end with “ane”.
• The carbon atoms are joined by single covalent bonds.
• Each member in the homologous series differs from the next one by -CH2.
No. of Chemical Molecular Relative Boiling
carbon name formula molecular point /
atoms mass °C
1 Methane CH4 16 –162
2 Ethane C2H6 30 –89
3 Propane C3H8 44 –42
4 Butane C4H10 58 –0.5
5 Pentane C5H12 72 36

11. Chapter 15
Hydrocarbons
Structure of Alkanes
• Alkanes are made up of covalent molecules. Each carbon atom
is joined to four other atoms by single covalent bonds.
• The structure of a methane molecule is drawn as
Name of alkane Molecular formula Structural formula
Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane C5H12

12. Chapter 15
Hydrocarbons
Chemical Properties of
Alkanes
• The alkanes are generally unreactive due to their
saturated carbon-carbon bonds. However, they can
undergo two types of chemical reactions.
Combustion of Alkanes
• Alkanes undergo combustion in air or oxygen to form
carbon dioxide and water vapour. The following two
equations illustrate this.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(g)

13. Chapter 15
Hydrocarbons
Chemical Properties of
Alkanes
Substitution Reactions with Halogens
• Alkanes react with halogens, such as fluorine, chlorine and bromine
under UV light or heat to form a mixture of halogenoalkanes.
E.g. methane reacts with chlorine as follows:
CH4 + Cl2  CH3Cl + HCl
CH3Cl + Cl2  CH2Cl2 + HCl
CH2Cl2 + Cl2  CHCl3 + HCl
CHCl3 + Cl2  CCl4 + HCl
• This type of reaction in which the hydrogen atom of the hydrocarbon is
replaced by the halogen atom is called a substitution reaction.

14. Chapter 15
Hydrocarbons
Uses of Alkanes and
Their Derivatives
Positive
• Combustion of alkanes are highly exothermic, so they are widely used as
fuels.
• Natural gas (mainly methane) – used in power stations, factories and more
recently as LNG (Liquefied Natural Gas) for running motor vehicles.
• Liquefied petroleum gas (LPG) – used for cooking and running motorcars.
• Substituted alkanes such as chlorofluorocarbons or CFCs are non-
flammable and not very toxic – used as refrigerants, propellants for
aerosols, making polystyrene or polyurethane foam, solvents for dry
cleaning and for general degreasing purposes.
Negative
• Unfortunately, CFCs are largely responsible for destroying the ozone layer.
• The carbon-chlorine bonds break to generate chlorine free radicals, which
destroy the ozone layer.
• CFCs are now being replaced by more environmentally friendly
compounds.

15. Chapter 15
Hydrocarbons
Quick Check 1
1. Consider the two isomers of pentane (an alkane after butane in the
homologous series) shown below. Are the following molecules also
isomers of pentane? Explain why.
2. What is the chemical formula of an alkane with
(a) eight carbon atoms;
(b) 20 carbon atoms?
Solution

16. Chapter 15
Hydrocarbons
Solutions to Quick Check 1
1. Yes, because they have the same molecular formula
(C5H12) but different structural formula.
2. (a) C8H18
(b) C20H42
Return

17. Chapter 15
Hydrocarbons
Alkene Homologous Series
• The alkenes are another family of hydrocarbons, ie.
they contain hydrogen and carbon only. All alkenes
share the following characteristics:
• They have the general formula CnH2n, where n is an
integer.
• Their names end with ‘ene’.
• There is a carbon-carbon double bond present in
the molecule.
• They are said to be unsaturated as other atoms can
be added to the molecule when the carbon-carbon
double bond opens up.

18. Chapter 15
Hydrocarbons
Alkene Homologous Series
Chemical Molecular Relative molecular Boiling point /
name formula mass °C
Ethene C2H4 28 –104
Propene C3H6 42 –48
Butene C4H8 56 –6
Pentene C5H10 70 30.1
• There is no organic compound with the formula CH2. Just
like the alkanes, as the number of carbon atoms increases,
the molecules become bigger and heavier.
• As the number of carbon atoms increases, the density,
melting point and boiling point increases.
• The first three alkenes are gases, the next few are liquids
and the rest are solids.

19. Chapter 15
Hydrocarbons
Structure of Alkenes
• Alkenes have a carbon-carbon double covalent bond.
Name of Molecular Structural Formula
alkene Formula
Ethene C2H4
Propene C3H6
But-1-ene C4H8
(1-butene)
Pent-1-ene C5H10
(1-pentene)

20. Chapter 15
Hydrocarbons
Isomers of Alkenes
• For propene, if we place the C=C double bond on the third
carbon atom, we will get a structure which is exactly the same
as the original when it is rotated through 180°. Hence, there is
no isomer for propene.
• It is possible for the C=C double bond to be in another position
in the carbon chain forming new isomers. For example, butene
has three isomers as shown below:
(i) (ii) (iii)

21. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
• The alkenes are much more reactive compared to
alkanes.
• Alkenes have an unsaturated carbon-carbon double
bond which can be easily broken and other atoms
can be added to form a molecule with single bonds.
• Alkenes take part in many important chemical
reactions and provide a basis for the manufacturing
of many important materials and chemicals in the
industry.

22. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
Combustion of Alkenes
• Alkenes undergo combustion in air or oxygen to form carbon
dioxide and water vapor.
• Alkenes do not burn as well as the alkanes and generally
produce a more sooty flame.
• Due to an insufficient supply of air, the combustion is incomplete
and carbon monoxide and soot are produced.
• The combustion reactions of alkenes:
C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(g)
2C3H6(g) + 9O2(g)  6CO2(g) + 6H2O(g)

23. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
Addition Reactions with Halogens
• Alkenes react readily with halogens to form haloalkanes.
• The reaction takes place quickly, without the need of UV light or catalyst.
E.g. Ethene reacts with chlorine to form dichloroethane.
C2H4 + Cl2  C2H4Cl2
ethene chlorine 1,2-dichloroethane
• This type of reaction in which the chlorine atoms are added across the
C=C double bond is called an addition reaction.
1,2-dichloroethane

24. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
Test for unsaturation
• Alkenes react with aqueous bromine (bromine water) and cause the
bromine solution to turn from brown to colourless.
• This reaction can be used as a test for unsaturation of any organic
compound.
• The equation for the reaction is:
C2H4 + Br2  C2H4Br2
ethene bromine 1,2-dibromoethane

25. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
Addition reaction with steam (hydration)
• Alkenes react with water (in the form of steam) and in the
presence of a catalyst to form alcohols. E.g.ethene reacts
with steam at 300 °C and with phosphoric(V) acid as catalyst,
to form ethanol.
C2H4(g) + H2O(g)  C2H5OH(l)
ethene water vapour ethanol
• It is a very important reaction used for the manufacture of
ethanol in the industry.
• The ethanol produced is usually mixed with a little methanol
to make it undrinkable and the mixture is used as ‘industrial
alcohol’.

26. Chapter 15
Hydrocarbons
Chemical Properties
of Alkenes
Addition reaction with hydrogen (hydrogenation)
• Alkenes react with hydrogen in the presence of the
catalyst nickel at a moderately high temperature to
form alkanes.
• E.g. C2H4(g) + H2(g)  C2H6(g)
ethene hydrogen ethane
• The addition of hydrogen to carbon-carbon double
bonds is used in the industry to change vegetable oils
into fats, which can be used for making margarine.
• When hydrogen is added to the unsaturated vegetable
oil, the melting point of the oil increases, and the oil
becomes solid (fats).

27. Chapter 15
Hydrocarbons
Uses of Alkenes
• Alkenes are very versatile in their uses and are the
starting materials for various chemicals.
E.g. ethene and propene are the starting materials
for polymers like poly(ethene), poly(propene),
polyvinylchloride (PVC) and Teflon etc.
• Alkenes are also used for the manufacture of
industrial chemicals such as alcohols and
aldehydes.
• The lower alkenes can be used as fuel and can be
obtained by the cracking of kerosene or petrol.

28. Chapter 15
Hydrocarbons
Quick Check 2
The diagram below shows the structural formula of an organic compound.
(a) Name the organic compound.
(b) Name the homologous series to which this compound belongs.
Solution

29. Chapter 15
Hydrocarbons
Solutions to Quick Check 2
(a) Propene
(b) Alkene
Return

30. Chapter 15
Hydrocarbons
Polyunsaturated Food
• Unsaturated hydrocarbons contain carbon-carbon double bonds.
• Polyunsaturated oils are oils that contain two or more C=C bonds in the
molecules.
• Due to the geometry of these C=C double bonds, the molecules are
harder to pack closely together and thus are liquids at room
temperature.
• Saturated oils which are mostly straight-chain molecules are more
easily packed and hence, they exist as solids at room temperature.

31. Chapter 15
Hydrocarbons
Polyunsaturated Food
• Saturated fats like animal fats and oils, butter and cream are
unhealthy in the diet as they raise the blood cholesterol of a
person.
• Polyunsaturated oils and fats are healthier as they do not clot
the blood vessels and raise the blood cholesterol level.
E.g. polyunsaturated oils are found in sunflower oil, palm oil,
fish oil, soya oil, linseed oil and nuts.