Chemistry Reference
In-Depth Information
Where do we get alkenes from?
Very few alkenes are found in nature. Most of the
alkenes used by the petrochemical industry are
obtained by breaking up larger, less useful alkane
molecules obtained from the fractional distillation
of crude oil. This is usually done by a process
called
catalytic cracking
. In this process the alkane
molecules to be 'cracked' (split up) are passed over a
mixture of aluminium and chromium oxides heated
to about 500 °C.
dodecane
C
12
H
26
(
g
)
(found in kerosene)
H
H
H
H
C
C
C
C
H
H
H
H
Figure 14.9
The bonding in ethene, the simplest alkene.
unsaturated
, because it is possible to break one of
the two bonds to add extra atoms to the molecule.
The chemical test to show the difference between
saturated and unsaturated hydrocarbons is discussed
on p. 225.
decane
C
10
H
22
(
g
)
shorter alkane
+
+
ethene
C
2
H
4
(
g
)
alkene
Naming the alkenes
All alkenes have names ending in -
ene
. Alkenes,
especially ethene, are very important industrial
chemicals. They are used extensively in the plastics
industry and in the production of alcohols such as
ethanol and propanol. See Table 14.2 and Figure 14.10.
Another possibility is:
C
12
H
26
(
g
) → C
8
H
18
(
g
) + C
4
H
8
(
g
)
There is a further cracking process which is more
versatile, called
thermal cracking
. Thermal cracking
is carried out at a higher temperature than catalytic
cracking, 800-850 °C. This process is more expensive
owing to the higher temperature used. However,
larger alkane molecules can be more successfully
cracked using this process than by the catalytic
method.
Note that in these reactions hydrogen may also be
formed during cracking. The amount of hydrogen
produced depends on the conditions used. Since
smaller hydrocarbons are generally in greater demand
than the larger ones, cracking is used to match
demand (Table 14.3).
Table 14.2
The first three alkenes and their physical properties.
Alkene
Formula
Melting
point/°C
Boiling
point/°C
Physical state at
room temperature
Ethene
C
2
H
4
−169
−104
Gas
Propene
C
3
H
6
−185
−47
Gas
Butene
C
4
H
8
−184
−6
Gas
ethene
H
H
CC
H
H
Table 14.3
Percentages of the fractions in crude oil and the demand for
them.
propene
H
H
Fraction
Approx % in crude oil
Approx % demand
H
Refinery gas
2
5
H
C
C
C
Gasoline
21
28
H
H
Kerosene
13
8
Diesel oil
17
25
butene
Fuel oil and bitumen
47
34
H
H
H
H
H
C
C
C
C
This means that oil companies are not left with large
surpluses of fractions containing the larger molecules.
Figure 14.11 shows the simple apparatus that
can be used to carry out cracking reactions in the
laboratory. You will notice that in the laboratory we
may use a catalyst of broken, unglazed pottery.
H
H
H
Figure 14.10
Structure and shape of the first three alkenes.
