Pyrolysis of Alkanes Free Radical Mechanism Detailed Explanation

🔥 Pyrolysis of Alkanes – Free Radical Mechanism (Class 11) By Neeraj Anand | Anand Classes

Pyrolysis, also known as cracking, is a fundamental chemical process in organic chemistry where alkanes are thermally decomposed into simpler hydrocarbons such as alkenes and smaller alkanes. This high-temperature reaction occurs via a free radical mechanism, which is crucial to understanding the behavior of hydrocarbons under thermal stress. In this detailed Class 11 chemistry lesson, Neeraj Anand of Anand Classes breaks down the step-by-step free radical mechanism of alkane pyrolysis, complete with diagrams, examples, and explanations aligned with CBSE, JEE, and NEET syllabi. This in-depth guide will help students strengthen their conceptual understanding and master key reaction pathways frequently asked in competitive exams.

What is Pyrolysis?

“Pyrolysis is defined as the conversion of a compound into smaller fragments in the absence of air through the application of heat”.

It is different from combustion. It happens in the absence of air and hence oxidation of compounds does not take place. Generally, pyrolysis of alkanes is also named as cracking.

Pyrolysis Process

In the absence of air when alkane vapours are passed through red-hot metal it breaks down into simpler hydrocarbons. This process takes place at high temperatures and high pressure without a catalyst. At low temperatures and pressure, the presence of a catalyst such as platinum or palladium is required for this reaction. The large hydrocarbons are generally obtained during the fractional distillation of crude oil (petroleum). In cracking, the hydrocarbon molecules randomly break into smaller hydrocarbon compounds. Some compounds obtained from cracking have carbon-carbon double bonds.

The factors responsible for the formation of products during cracking are:

• Nature of alkane
• Temperature and pressure
• Presence or absence of a catalyst

Pyrolysis Of Alkanes

In contrast to combustion, the rate of pyrolysis increases with the increase in the molecular weight and branching in an alkane. During the fission of C-C bonds, alkanes and alkenes are produced, whereas the fission of the C-H bond results in alkene and hydrogen. C-H bond fission takes place due to the catalytic action of Cr₂O₃, V₂O₂, MoO₃ and C-C bond fission which occurs under the presence of SiO₂, Al₂O₃, and ZnO.

The cracking of alkanes follows a free radical mechanism. It plays an important role in the petroleum industry. The higher molecules of alkanes are transformed into lower molecules (petrol C₆ to C₁₁) by cracking.

For example, Dodecane ( a component of kerosene oil) gives a mixture of heptane and pentane as pyrolysis products, by heating it to a temperature of 973 K under the catalytic action of platinum, palladium or nickel.

\(\begin{array}{l}C_{12}H_{26}\overset{Pd/Ni/Pt}{\rightarrow}C_{7}H_{16}+ C_{5}H_{10}+ other\ products\end{array} \)

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