L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 30: Alkane
Chpt.30: P.1
HYDROCARBONS
I.
Introduction
Hydrocarbons are covalent compounds containing hydroqen and carbon only. They can be aliphatic 脂
肪族, alicyclic 脂環族 or aromatic 芳香族.
(A)
Aliphatic Hydrocarbons
Aliphatic hydrocarbons are compounds with open chains of carbon atoms. The open chains may he
unbranched of branched; and may contain single, double or triple bonds or combination of these.
The three most important groups of aliphatic hydrocarbons are alkanes 烷烴 , alkenes 烯烴 and and
alkynes 炔烴
(B)
Alicyclic Hydrocarbons
Alicylic hydrocarbon are compounds with closed rings of carbon atoms. The ring may contain single or
multiple bonds.
Examples
(C)
cyclobutane
cyclohexene
Aromatic Hydrocarbons
Aromatic hydrocarbons are compounds containing at least one benzene ring. They are also known as
arenes.
Examples
methylbenzene
Naphthalene
Note : Saturated hydrocarbons : ______________________________
Unsaturated hydrocarbons: _____________________________
30.1
ALKANES
The alkanes are a homologous series of saturated hydrocarbons containing only the C—C single
and C—H single bonds.
All carbon atoms are sp3 hybrize , and are consequently surrounded tetrahedrally by hydrogen
or other carbon atom.
The carbon atoms join into straight or branched chain : CnH2n+2
The carbon atoms join into a ring (cycloalkane ) : CnH2n
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 30: Alkane
I.
Chpt.30: P.2
Natural Source of Alkanes
The natural source of alkanes is crude oil which is a mixture of various types of hydrocarbons.
Different types of hydrocarbons can he obtained by two process and cracking.
(A)
Fractional distillation of crude oil
Crude oil can be fractionally distilled into fraction of a mixture of hydrocarbons which have
different boiling points. The fractions of Mid-east crude oil is listed below
Refinery gas
Gasoline
Naphtha
Kerosene.
Gas oil
Heavy oil
(B)
液化石油氣
汽油
石腦油
煤油
氣油
重油
(C1 - C4 )
(C5 - C10 )
(C5 - C7 )
(C11 - C12)
(C13 - C25)
(C25 or above)
5%
10%
5%
20%
15%
45%
Cracking
Cracking is the process in which C—C bonds in long chain alkane molecules are broken, producing
smaller molecules of both alkanes and alkenes.
The composition of the products depends on the conditions under which the cracking takes place.
<1> Thermal Cracking 熱裂解
This type of cracking is also known as pyrolysis. The alkane is heated to a temperature between
450 and 700°C. At these temperature, carbon -carbon bonds in the alkane molecules undergo
homolytic fission. The reactions proceed by chain mechanisms involving a varity of radicals.
Example
Cracking of butane
(1)
Initiation: In this stage methyl, ethyl and propyl radicals are produced
(2)
Propagation
radicals.
: In this stage, the alkyl radicals react with butane molecules to form butyl
The C—C bonds in the butyl radicals then undergo homolytic fission produce alkene molecules
and more radicals.
A variety of other propagation steps are also possible.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 30: Alkane
(3)
Chpt.30: P.3
Termination: This occurs when two radicals combine to form a molecule.
e.g.
CH3 + CH3  CH3—CH3
CH3 + CH3CH2

CH3—CH2CH3
In practice the thermal cracking of butane produces a mixture of methane, ethene, propene
but-1-ene, but –2-ene and hydrogen
<2> Catalytic Cracking
In this method of cracking the alkane is passed over a catalyst at a temperature between 400°C
and 500°C. The catalyst used is normally aluminium oxide mixed with either silica or
chromium (VI) oxide. Like thermal cracking, catalytic cracking produces a mixture of shorter
chain alkanes and shorter chain alkenes. However, unlike thermal cracking. catalytic cracking
has an ionic mechanism. The acidic oxides used as catalyst promote the formation of carbon
ions.
Note：There is a high demand for the use of small hydrocarbons as fuel. Thus, cracking is
important in economics.
II.
Reactions of alkanes
Since an alkane molecule contains a saturated carbon skeleton with non-polar C—C and C—H
bond, it is unreactive towards polar or ionic reagents like acids, alkalis, dehydrating agents or
aqueous oxidizing agents.
The non-polar alkane molecules are reactive towards other non-polar reagents like oxygen and
halogens as chlorine and bromine. Energy (heat or light) should be supplied before the reactions
can occur. Also, since the bonding atoms of alkanes are of similar electro negativities, bond
breaking is normally homolytic, producing reactive free radicals,
(A)
Combustion of alkanes
The most important reaction of the alkanes is combustion. They burn to form the harmless
products, carbon dioxide and water in an exothermic reaction.
CH4
C8H18
+ O2

CO2 + 2H2O
1
+ 12 O2 
2
Note ：<1>
8CO2 + 9H2O
CH4 is the main component of natural gas and domestic gas. C8H18 is the
component of petrol used for internal combustion engines. The large amount of
energy released during combust Ion of these alkanet products makes them
excellent fuels.
<2> In a limited supply of air, carbon monoxide can be produced.
CH4
+ O2 
CO + H2O
Carbon monoxide is toxic as it displaces oxygen from blood haemoglobin, thus
making it ineffective in carrying oxygen to various tissues of the body.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 30: Alkane 烷烴
(B)
Chapt. 30: p.4
Photochemical Reactions with halogens
Alkanes do not react with chlorine in the dark. However, in the presence of sunlight they
undergo a series of substitution reactions.
Example:
Methane and chlorine react explosively in sunlight. producing a mixture of
chloromethane, dichloromethane, trichloromethane and tetrachloromethane.
The reaction is a photochemical chain reaction. It takes place in 3 stages.
Stage 1 : Initiation
This is the homolytic fission of the Cl—Cl bond producing chlorine radicals.
(Bond energy term of Cl—Cl : 242 kJ mol-1 , C—H ：435 kJ mol-1 )
Stage 2 : Propagation
This involves a number of reactions resulting in the formation of the products
CH3Cl, CH2Cl2, CHC13 and CCl4
Stage 3：Termination
This occurs when two radicals combine.
Note : <1> In stage 2, further chlorination can produce CH2Cl2 ,CHCl3 and CCl4.
<2> A mixture of products is often found from such free radical reactions.