Download Products From Oil

CRACKING
Some of the heavier fractions obtained from crude oil are not very useful. In the process
of cracking, these larger hydrocarbon molecules are broken down ("cracked") to
produce smaller, more useful molecules. This process involves heating the
hydrocarbons to vaporise them and passing the vapours over a hot catalyst (about
450oC) for a few seconds. A THERMAL DECOMPOSITION reaction then occurs
→
C10H22
C5H12 + C3H6 + C2H4
Some of the products of cracking are useful as fuels. Other products of cracking are
used as feedstock for the petrochemicals industry; these compounds have carboncarbon double bonds (they are unsaturated) and can be used to make plastics
(polymers) such as poly(ethene) and poly(propene).
ALKENES
The alkenes are another group of hydrocarbons which all have the same general
formula (CnH2n).
Like the alkanes, they contain ONLY carbon and hydrogen atoms. Carbon atoms form
the spine of the molecules. Each carbon atom forms four covalent bonds; each
hydrogen forms one covalent bond.
However, in the alkenes, two of the carbon atoms are joined by a double covalent bond.
These two carbon atoms are therefore sharing four electrons. The molecule no longer
contains the maximum possible number of hydrogen atoms for its particular number of
carbon atoms. The molecule is said to be unsaturated.
Alkenes are UNSATURATED hydrocarbons.
The first three alkenes are:
H
C
ethene (C2H4)
C
H
C
H
H
H
H
C
C
C
H
H
H
TOPIC 10.1.5: PRODUCTS FROM OIL 1
C
propene (C3H6)
H
H
butene (C4H8)
H
H
H
H
H
C
C
H
H
Note that each carbon
atom has formed four
bonds, but they are not all
joined to four other atoms,
because of the double
bond. Each hydrogen atom
has formed one bond.
Carbon=carbon double bonds are weaker and are more easily broken than carboncarbon single bonds. When the double bond breaks, the carbon atoms are able to join
with more atoms. A double bond in a molecule is therefore a sign of chemical reactivity.
Alkenes are REACTIVE.
Test for Alkenes
The difference in reactivity between alkanes and alkenes provides a useful way of
distinguishing between them.
If bromine water is added to an alkene, the initial brown colour of the bromine water
is lost and the mixture becomes colourless.
There is no reaction with an alkane, and the mixture stays brown.
This test with bromine water is a test for unsaturation.
Polymerisation
Alkenes are one of the products obtained from the cracking of crude oil fractions.
Alkenes react with lots of other substances to form useful products, but a particularly
important reaction occurs when they react with themselves in a process called
POLYMERISATION.
Polymerisation is the joining together of a large number of small molecules, called
monomers, to form one very large molecule, called a polymer.
All the common plastics are polymers.
When unsaturated monomers join together to form a polymer with no other substance
being produced in the reaction, the process is called ADDITION POLYMERISATION.
The following are examples of addition polymerisation:
1. Poly(ethene), also called polythene
H
n
H
C
H
(
C
H
ethene
(monomer, unsaturated)
H
C
C
H
H
)n
poly(ethene)
(polymer, saturated)
n is a very large number, often greater than 10,000
Poly(ethene) is used for making plastic bags and bottles.
TOPIC 10.1.5: PRODUCTS FROM OIL 2
H
2. Poly(propene)
H
n
CH3
C
(
C
H
H
propene
(monomer, unsaturated)
H
CH3
C
C
H
H
)n
poly(propene)
(polymer, saturated)
Poly(propene) is used to make crates and rope.
3. Poly(chloroethene), also called p.v.c.
H
n
Cl
C
H
(
C
H
chloroethene
(monomer, unsaturated)
H
Cl
C
C
H
H
)n
poly(chloroethene)
(polymer, saturated)
New polymers
Shape memory polymers are polymeric smart materials that have the ability to return
from a deformed state (temporary shape) to their original (permanent) shape induced by
an external stimulus (trigger), such as temperature change. These materials can be
used to make stitches to hold a cut together. The polymer tightens as it is warmed by
the body and applies just the right amount of pressure.
We can make light sensitive polymers that are used on plasters. When this polymer is
exposed to light, it becomes less sticky and so is easy to remove.
Hydrogels are used as wound dressings since they hold an incredible amount of water
so wounds can heal in moist sterile conditions.
TOPIC 10.1.5: PRODUCTS FROM OIL 3
Waste plastic
We are making more and more waste, and there are fewer holes in the ground to put it.


Landfill can be hazardous, if toxic chemicals get into water;
Burning waste can give off toxic materials in the waste gases.
Recycling can turn waste material into useful raw materials to be made into new useful
things, for example rulers made from old coffee cups.
The cost of recycling is a lot less than making completely new materials. Many polymer
(plastic items) are non-biodegradable and will remain unaffected in landfill for many
hundreds of years. They can also be dropped as litter which is unsightly, especially
when caught up in fences and trees. This is a kind of pollution. They can be eaten by
animals, which can be harmed. They drift about the oceans, causing deaths of many
marine creatures.
It is far better to recycle. Plastic items have codes on to state what they are, for
example:



HDPE - high density polyethene;
PS - polystyrene;
PP - polypropene.
Biodegradable materials can be broken down by micro-organisms in the environment.
Many polymers are not biodegradable, and can last for many years in land-fill. Others
are, and will break down. Scientists have found ways to speed up this decomposition of
polymers by adding starch which encourages micro-organisms in the soil to feed on the
starch and break down the polymer.
Other polymers made using cornstarch biodegrade fully very quickly.
Using plant materials to make polymers does mean that these crops are not being used
for the food chain as in the case of biofuels mentioned earlier.
TOPIC 10.1.5: PRODUCTS FROM OIL 4
ALCOHOLS
The alcohols form a homologous series with the general formula CnH2n+1OH. They
contain the functional group –OH, which is called a hydroxyl group. It is the presence
of this functional group which gives alcohols their characteristic properties. The first
members of the series are:
H
H
C
OH
H
methanol
H
H
H
C
C
H
H
ethanol
OH
H
H
H
H
C
C
C
H
H
H
OH
propan-1-ol
ETHANOL C2H5OH
Ethanol can be produced in two ways:
 by the fermentation of sugars
 by the hydration of ethene.
1. Fermentation of Sugars
Fermentation is used to produce ethanol as an industrial chemical. The raw material,
usually glucose, is dissolved in water to make a solution containing about 15% by mass
of glucose. Yeast is then added, and the temperature is maintained at about 25 oC for 3
to 5 days. Yeast is a living organism which contains the enzyme, zymase. Enzymes are
biological catalysts. Under these conditions, the sugar reacts to form ethanol and
carbon dioxide.
C6H12O6
2C2H5OH + 2CO2
The carbon dioxide is allowed to escape and air is prevented
from entering the fermentation vessel, often by using a water
trap.
When the fermentation is complete, the yeast is filtered off, and
the ethanol is separated from the reaction mixture by fractional
distillation.
Fermentation is also used:
 to produce ethanol in beer and wine
 to produce the bubbles of CO2 which make bread dough
rise
fermentation
mixture
N.B. The simple laboratory test for carbon dioxide is that it turns limewater milky.
TOPIC 10.1.5: PRODUCTS FROM OIL 5
2. Hydration of Ethene
Ethene undergoes an addition reaction with steam at high temperature and pressure in
the presence of a strong acid catalyst (phosphoric acid), to form ethanol.
C2H4 + H2O
ethene
C2H5OH
ethanol
COMPARISON OF THE TWO PROCESSES
Fermentation
Disadvantages:
 a slow reaction (3 to 5 days)
 the product is a mixture containing only about 10% ethanol, from which
the ethanol is separated by distillation
 the process is a batch process
Advantages
 the raw material (glucose) is renewable
 the process is low-tech: it needs only inexpensive equipment and unskilled
labour
Hydration
Advantages:
 a fast reaction (seconds)
 the product is pure
 the process is a continuous process
Disadvantages
 the raw material (ethene) is obtained from crude oil, which is nonrenewable
 the process is high-tech: it requires expensive equipment and semi-skilled
labour
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