Pyrolysis of alkanes

Introduction:
Pyrolysis is a process in which a compound is subjected to high temperatures in the absence of oxygen. Alkanes are hydrocarbons that consist of only carbon and hydrogen atoms and are known for their relatively low reactivity. Pyrolysis of alkanes involves breaking the carbon-carbon and carbon-hydrogen bonds to produce smaller molecules.

Answer:
The correct answer is option 'A': Free radical decomposition reaction.

Explanation:
Pyrolysis of alkanes involves the breaking of carbon-carbon and carbon-hydrogen bonds, leading to the formation of smaller molecules. This process occurs through a series of free radical reactions, making it a free radical decomposition reaction. Here is a detailed explanation:

1. Initiation:
The pyrolysis reaction starts with the initiation step, where a small amount of energy is required to break the weak carbon-hydrogen bond in the alkane molecule. This process forms highly reactive alkyl free radicals (R·). The initiation step can be represented as:

RH → R· + H·

2. Propagation:
Once the alkyl free radical is formed, it can react with another alkane molecule, leading to the propagation step. In this step, the alkyl free radical abstracts a hydrogen atom from the second alkane molecule, forming a new alkyl free radical and a hydrogen molecule. This process can be represented as:

R· + RH → RHR' + ·H

The newly formed alkyl free radical (R') can further react with another alkane molecule, continuing the propagation process.

3. Termination:
The free radical reactions can also undergo termination steps, where two free radicals combine to form a stable molecule. These termination steps help in consuming the highly reactive free radicals and eventually stop the reaction. Some examples of termination steps include:

·R + ·R → R-R
·R + H· → RH
·R + ·X → RX

In summary, the pyrolysis of alkanes involves the initiation, propagation, and termination steps, which are characteristic of free radical decomposition reactions. The high temperatures in the absence of oxygen provide the energy required to break the carbon-carbon and carbon-hydrogen bonds.

Pyrolysis of alkanes is free radical decomposition reaction.