PETROLEUM & ALKANES
PETROLEUM
Petroleum (crude oil) was formed over millions of years from the accumulated remains
of sea creatures which became buried on the ocean bed. The conditions required for
the formation of petroleum (and natural gas) are:
 high temperature
 high pressure (compression by overlying sediments)
 absence of oxygen
Petroleum is a complex mixture of hydrocarbons, mostly alkanes.
ALKANES
The alkanes are a homologous series of saturated hydrocarbons which all have the
general formula CnH2n+2.
A HOMOLOGOUS SERIES is a group of compounds which have:
 the same general formula
 similar chemical properties
HYDROCARBONS are compounds which are made from ONLY carbon
and hydrogen atoms.
In a SATURATED compound, there are only single bonds between
carbon atoms.
Carbon atoms form the spine of hydrocarbon molecules. Each carbon atom forms
four covalent bonds; each hydrogen forms one covalent bond.
When the carbon atoms are joined only by single covalent bonds, the molecule contains
the maximum possible number of hydrogen atoms for its particular number of carbon
atoms. This is why the molecule is said to be saturated.
Physical Properties
The alkanes have simple molecular structures. The carbon and hydrogen atoms within
each molecule are joined by strong covalent bonds; there are weak van der Waals’
forces between molecules. The strength of the van der Waals’ forces increases as the
surface area of the molecule increases.
Down a homologous series, the molecular mass and therefore the boiling point
increases. Thus the lower alkanes are gases at room temperature; the higher members
are liquids and solids.
TOPIC 12.12: ALKANES
1
Straight chain alkanes
Name
methane
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane
undecane
dodecane
tridecane
tetradecane
pentadecane
hexadecane
heptadecane
octadecane
nonadecane
eicosane
Formula
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
C11H24
C12H26
C13H28
C14H30
C15H32
C16H34
C17H36
C18H38
C19H40
C20H42
m.p. /oC b.p. /oC
-182
-162
-183
-89
-188
-42
-138
-0.5
-130
36
-95
69
-91
98
-57
126
-54
151
-30
174
-26
196
-10
216
-5.5
235
6
254
10
271
18
287
22
302
28
316
32
330
37
343
Density /g.cm-3
0.626
0.659
0.684
0.703
0.718
0.730
0.740
0.749
0.756
0.763
0.769
0.773
0.778
0.782
0.786
0.789
Isomeric alkanes
Isomers have different boiling points because these depend on the strength of the
intermolecular forces. The strength of the intermolecular forces, and therefore the
boiling point, decreases as the amount of chain branching increases. Straight chain
alkanes are approximately sausage shaped, but as the amount of branching increases,
the shape becomes more spherical. This can be seen in the diagrams below:
pentane
b.p. 36oC
2-methylbutane
b.p. 28oC
2,2-dimethylpropane
b.p. 10oC
The more spherical the structure, the smaller the surface area is and so the weaker the
van der Waals’ forces are. Therefore the boiling point decreases.
Chemical Properties
Alkanes contain only C-C and C-H bonds, which are strong and non-polar. Alkanes are,
therefore, unreactive towards acids, alkalis, electrophiles and nucleophiles. They do,
however, readily undergo combustion and are important as fuels.
TOPIC 12.12: ALKANES
2
FRACTIONAL DISTILLATION
The properties of each substance in a mixture are unchanged. This makes it possible to
separate substances in a mixture by physical methods including distillation. The
complex mixture of hydrocarbons in crude oil can be separated into simpler mixtures or
fractions by fractional distillation. Fractions contain molecules with a similar number of
carbon atoms and have a narrow boiling point range.
The crude oil is heated to about 400oC and the liquid/vapour mixture is then pumped
into a tall tower called a fractionating column. Most of the hydrocarbons have been
converted to vapour by the heating and start to rise up the column. The lower the boiling
point of a hydrocarbon, the further it will rise up the column before it cools enough to
condense. In this way, the different fractions are collected at different points up the
column. The number of different fractions which are collected and the amount of each
which is produced depends on the source of the crude oil.
Most of the fractions from crude oil are burned as fuels.
The residue from this primary distillation contains useful materials such as lubricating oil
and waxes. If these were distilled at atmospheric pressure, the temperature needed to
vaporise them would be so high that thermal decomposition would occur. Therefore, the
residue is distilled in a separate column under reduced pressure; reducing the pressure
lowers the boiling point and prevents decomposition.
The quantities of the different fractions produced by fractional distillation do not usually
match up with the market requirements for each fraction. There is a shortage of light
fractions, especially gasoline and an excess of the heavier fractions. To resolve this
problem, some of the heavier fractions (larger molecules) are converted into lighter,
higher value fractions (smaller molecules) by cracking.
CRACKING
In the process of cracking, large hydrocarbon molecules are broken down ("cracked") to
produce smaller, more useful molecules. Molecules may break down in more than one
way and will give a mixture of products which can be separated by a further distillation
process. During cracking carbon-carbon bonds are broken; in addition to smaller alkane
molecules, alkenes and hydrogen are produced. For example:
C14H30
alkane
C14H30
alkane
There are two main types of cracking:
TOPIC 12.12: ALKANES
C7H16 + C3H6 + 2C2H4
alkane
alkenes
C12H24 + C2H4 + H2
alkenes
thermal cracking
catalytic cracking
3
FRACTIONAL
DISTILLATION
PETROLEUM (bottled gases)
GASES
100oC

b.p.decreases

Mr decreases

size of molecule
decreases

viscosity
decreases

volatility
increases

easier to ignite
GASOLINE
(fuel for cars)
NAPHTHA
(feedstock for
petrochemicals
)KEROSINE
200oC
(fuel for jet aircraft)
GAS OIL
(diesel: fuel for cars
& large vehicles)
vapour
300oC
LUBRICATING
OIL & WAXES
CRUDE
OIL
VAPOUR
liquids
FUEL OIL
(fuel for ships &
industrial heating)
360oC
BITUMEN
(road surfacing)
TOPIC 12.12: ALKANES
4
Thermal Cracking
In this process, the bonds are broken by heating the hydrocarbon vapour to a high
temperature under a high pressure for a few seconds.
Temperature:
Pressure:
400 – 900oC
7MPa
The higher the temperature at which the cracking is carried out, the closer to the end of
the chain the C-C bond breaks.
Homolytic fission of the carbon-carbon bond takes place, forming two alkyl radicals.
HOMOLYTIC FISSION
When a bond breaks homolytically, each of the bonded atoms takes one electron
from the shared pair, forming two particles with unpaired electrons called (free)
radicals.
e.g.
CH3Cl
CH3. + Cl.
Thermal cracking produces a high percentage of alkenes.
Catalytic Cracking
In this process, the bonds are broken by heating the hydrocarbon vapour to a high
temperature under a high pressure for a few seconds.
Temperature:
Pressure:
Catalyst:
450oC
slight
zeolite (crystalline aluminosilicates)
Catalytic cracking proceeds by a carbocation (C+) mechanism; heterolytic fission of the
carbon-carbon bond takes place, forming two ions.
HETEROLYTIC FISSION
When a bond breaks heterolytically, one of the bonded atoms takes both electrons
from the shared pair, forming a positive ion and a negative ion.
e.g.
(CH3)3CCl
(CH3)3C+ + ClCatalytic cracking is used mainly to produce motor fuels (branched-chain alkanes) and
aromatic hydrocarbons.
Economics of Cracking
The lower Mr branched-chain alkanes produced by the cracking of heavy fractions are
more useful as fuels and are therefore of higher value.
The alkenes produced by cracking can be used to make plastics (polymers) such as
poly(ethene) and poly(propene).
TOPIC 12.12: ALKANES
5
COMBUSTION OF ALKANES
Most of the hydrocarbon fractions obtained from petroleum are used as fuels, because
their combustion reactions are very exothermic. The products of combustion depend on
whether the combustion is complete or incomplete.
Complete Combustion
When alkanes burn in a plentiful supply of air or oxygen, complete combustion takes
place, forming carbon dioxide and water.
CH4 + 2O2
C8H18 + 121/2O2
CO2 + 2H2O
8CO2 + 9H2O
H = -890 kJ.mol-1
H = -5512 kJ.mol-1
A graph of enthalpy of combustion against no. of carbon atoms for straight chain
alkanes is a straight line.
Incomplete Combustion
When the supply of air or oxygen is restricted, incomplete combustion of alkanes
takes place, forming water together with carbon monoxide or carbon. The design
of the burner affects the product of incomplete combustion.
Bunsen burners, which are intended for use in open laboratories, produce carbon when
combustion is incomplete (the luminous flame obtained when the air hole is closed is
sooty).
CH4 + O2
C + 2H2O
The design of gas fires is such that if the flue becomes blocked, restricting the air
supply, incomplete combustion takes place to form carbon monoxide. Carbon monoxide
is toxic. Every year there are a number of accidental deaths caused by carbon
monoxide from poorly maintained gas fires and central heating boilers.
CH4 + 11/2O2
CO + 2H2O
Pollutants from Combustion
The principal products of the internal combustion engine are carbon dioxide and water.
Carbon dioxide is a greenhouse gas and contributes to global warming.
Sulphur-containing compounds are often present as impurities in alkanes obtained by
the fractional distillation of petroleum. When these hydrocarbons are burned in air or
oxygen, the sulphur is oxidised to sulphur(IV) oxide, SO2, and possibly to sulphur(VI)
oxide, SO3. Both these oxides are toxic and also dissolve in atmospheric moisture,
causing acid rain. This happens on a massive scale when power stations burn fossil
fuels to produce electricity.
Flue Gas Desulphurisation is a process used to prevent SO2 escaping into the
atmosphere. Waste gases containing SO2 are passed through a flue (chimney)
TOPIC 12.12: ALKANES
6
containing calcium oxide (CaO) which absorbs the SO2 producing calcium sulphite
(CaSO3).
CaO
+
SO2
CaSO3
This can easily be oxidised to to make hydrated calcium sulphate (CaSO 4), also known
as gypsum, which is used to make plasterboard for the building industry.
Carbon monoxide (petrol engine) and carbon (diesel engine) are formed as a result of
incomplete combustion. Carbon monoxide is toxic; carbon particles are irritant.
Unburned hydrocarbons pass through the engine and enter the exhaust gases.
At the high temperatures produced in the engine (up to 2500 oC), the nitrogen and
oxygen molecules in air have enough energy to combine to form nitrogen oxide.
N2 + O2
2NO
On cooling and in the presence of more oxygen, nitrogen oxide reacts to form other
oxides of nitrogen (NOx), especially nitrogen dioxide, NO2. With water and more
oxygen, nitrogen dioxide reacts to form nitric acid, which contributes to acid rain.
2NO + O2
2NO2
4NO2 + 2H2O + O2
4HNO3
Oxides of nitrogen are irritant, toxic gases. They combine with unburned hydrocarbons
in the presence of sunlight to form photochemical smog. This is a particular problem in
Los Angeles.
Catalytic Converters
Catalytic converters are fitted to the exhaust systems of cars to remove pollutant gases.
They consist of a honeycomb of ceramic material which is coated with a thin layer of a
catalyst containing platinum (Pt) and rhodium (Rh). Up to 90% of pollutant gases are
removed.
The catalyst system catalyses two important reactions:
 the reaction between carbon monoxide and nitrogen oxide, forming carbon
dioxide and nitrogen
2NO + 2CO
TOPIC 12.12: ALKANES
N2 + 2CO2
7

the reaction between nitrogen oxide and unburned hydrocarbon fuel, forming
carbon dioxide and nitrogen
C8H18 + 25NO
8CO2 + 121/2N2 + 9H2O
The principal exhaust gases are therefore carbon dioxide, nitrogen and water vapour.
These gases are harmless, but carbon dioxide causes environmental problems.
TOPIC 12.12: ALKANES
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