Question

Write a note on pyrolysis of alkane.

Answer 1

We first need to understand what is pyrolysis. When a compound, in the absence of air, is converted into smaller fragments with the help of heat, the process is known as pyrolysis. Oxidation does not take place during pyrolysis due to the absence of air. It is also different from combustion.

Complete answer:
Now, the pyrolysis of alkanes is generally referred to as cracking.
Alkane vapors break into simpler and smaller hydrocarbons when they are passed through red hot metal in the absence of air.
The breaking of alkanes into simpler hydrocarbons takes place randomly and might even contain compounds containing a double bond between two carbon atoms.
A catalyst is not required during this process and it takes place at high temperatures and pressures. For enabling the reaction at low temperatures and pressures, catalysts like platinum or palladium are used.
The larger hydrocarbons are obtained during fractional distillation of crude oils like petroleum.
Hence temperature, pressure, catalyst, and nature of alkane are the factors that determine the formation of hydrocarbons after pyrolysis of alkanes.
The free radical mechanism is followed as C-H bonds and C-C bonds are broken during the pyrolysis of alkanes.
For example, oil gas can be prepared from kerosene oil through pyrolysis.
Heating of dodecane, a constituent of kerosene oil, in the presence of catalysts like either platinum, palladium, or nickel, to 973 K gives a mixture of products of pentene, heptane, etc.
${{C}_{12}}{{H}_{26}}\xrightarrow{\Delta }{{C}_{7}}{{H}_{16}}+{{C}_{5}}{{H}_{10}}+...$

Note:
It should be noted that there are two types of methods of pyrolysis of alkanes or cracking.
- Catalytic cracking involves the use of catalysts like alumina or silica at a temperature in the range of 670 K - 820 K. Gasoline having a high-octane number can be obtained by catalytic cracking.
- Thermal cracking can be carried out in either the vapor phase or the liquid phase but gives rise to a mixture of complex products and is very difficult to control.