Pyrolysis of Hydrocarbons Alkanes

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What is Pyrolysis?

Pyrolysis is described as converting the compound into smaller fragments in the absence of air with the application of heat, and it varies from combustion. It happens in the absence of air, and thus oxidation of compounds does not occur. In general, the pyrolysis of alkanes is also named cracking.


Pyrolysis Process

When alkane vapours are passed in the absence of air through red-hot metal, it breaks down into smaller or simpler hydrocarbons. This specific process occurs at high pressure and high temperatures without a catalyst. At low pressure and temperatures, the presence of a catalyst such as palladium or platinum is needed for this reaction. Generally, large hydrocarbons are obtained during the fractional distillation of crude oil (such as petroleum). In cracking, the hydrocarbon molecules break into smaller hydrocarbon compounds randomly—a few compounds obtained from the cracking hold carbon-carbon double bonds.

The factors that are responsible for the formation of products during cracking are listed as follows:

  • Temperature and pressure

  • Nature of alkane

  • Presence or absence of a catalyst

Pyrolysis of Alkanes

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

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The cracking of alkanes follows a mechanism, which is a free radical mechanism. It plays an essential role in the petroleum industry. The alkane’s higher molecules are transformed into the lower molecules (it means from petrol C6 to C11) by cracking.


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

C₁₂H₂₆ \[\overset{pt/pd/ni}{\rightarrow}\] C₇H₁₆ + C₅H₁₀ + other products


Pyrolysis

  • The decomposition of a compound using an application of heat is known as pyrolysis.

  • Pyrolysis of higher alkanes to produce a mixture of lower alkanes, alkenes are referred to as cracking. Usually, it is carried out by heating the higher alkanes to high temperatures under pressure ranging from 6-7 atm, either in the presence or absence of a catalyst. The chemical reaction for the same can be given as follows:

C6H14 → C6H12 + H2 + C4H8 + C2H6 + C3H6 + C2H4 + CH4

  • Alkane’s pyrolysis involves breaking C-H and C-C bonds and takes place by a free radical mechanism.

  • The preparation of petrol gas from petrol and oil gas from kerosene oil is according to the process of pyrolysis. For suppose, Dodecane, which is a constituent of kerosene oil, on heating up to 973 K in the presence of Pd, Ni, or Pt, forms a mixture of pentene and heptane along with other products. The chemical reaction for the same can be given as follows:

C12H26 → C7F116 + C5H10 + other products.

Uses of Alkanes

  • Methane in the form of natural gas can be used for running cars, scooters, buses, and more vehicles. LPG (which is a mixture of isobutane and butane) can be used as a fuel in homes and in the industry as well.

  • Methane can also be used to form carbon black, which is used in the manufacturing of paint, automobile tyres, and printing inks.

  • Catalytic oxidation of alkanes forms aldehydes, carboxylic acids, and alcohols.

  • Higher alkanes in the form of kerosene oil, gasoline, lubricating oils, paraffin wax, and diesel are widely used.

  • Methane can also be used in the manufacturing of halogen-containing compounds such as CHCI3, CH2Cl2, CCI4, and more, which are used as solvents both in industry and in the laboratory.

Process of Cracking or Pyrolysis

Cracking: Alkanes, when subjected to high temperatures (ranging from 670-970 K) in the presence of a catalyst, and are decomposed into very smaller molecules. The method of dissolving the less volatile higher hydrocarbons into different forms of more volatile lower hydrocarbons by heat application is referred to as pyrolysis or cracking. The alkane’s cracking involves the cleavage of C-H and C-C bonds. For example,

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Pyrolysis of alkanes is supposed to take place by the free radical mechanism. The preparation of petrol gas or oil gas petrol or from kerosene oil involves the pyrolysis principle. For example, heating to 973K in the presence of Pd, Ni, Pt provides a mixture of pentene and heptane.

C12H26 \[\xrightarrow[973 k]{Pt/Pd/Ni}\]  C7H16    +    C5 H10 + other products

Dodecane     Heptane      Pentene    


Cracking is of Two Types

  • Thermal Cracking: 

It can be carried out either in the liquid phase or in the vapour phase. It is quite difficult to control and gives rise to complex product mixtures.

  • Catalytic Cracking: 

It can be carried out at a temperature ranging from 670-820 K using alumina and silica as a catalyst. Catalytic cracking can be useful as it forms gasoline having a higher octane number.

The products that are formed during cracking depends upon:

  • the pressure applied,

  • the structure of starting alkane,

  • use of a catalyst.

FAQs (Frequently Asked Questions)

Q1. Explain Stereoisomerism?

Answer: Isomers that contain similar structural formulas but with the different relative arrangements of groups or atoms in space are known as stereoisomers, and this particular phenomenon is referred to as stereoisomerism.


Since every different spatial arrangement of atoms characterizes a specific stereoisomer known its configuration, the stereoisomers hold the same molecular structure but with different configurations.

Q2. What are Alicyclic Hydrocarbons?

Answer: Hydrocarbons that contain a ring of either three or more carbon atoms and with properties similar to those of aliphatic hydrocarbons are referred to as alicyclic hydrocarbons.

Alicyclic hydrocarbons are further divided into three categories, which are listed as follows:

  • Cycloalkanes

  • Cycloalkanes

  • Cycloalkynes

Q3. What are Benzenoid Aromatic Compounds and Non-Benzenoid Aromatic Compounds?

Answer: 


Benzenoid Aromatic Compounds: Hydrocarbons, including their alkenyl, alkynyl, and alkyl derivatives, which contain either one or more benzene rings, which are isolated or fused in their molecules, are known as benzenoid aromatic hydrocarbons. They are also known as arenes.


Non-benzenoid Aromatic Compounds: Aromatic Hydrocarbons that do not contain a benzene ring but instead contain other highly unsaturated rings are known as non -benzenoid aromatic compounds.

Q4. Give the Applications of Alkanes?

Answer: Alkanes’ applications depend on the carbon atom count. The first four alkanes are primarily used for cooking and heating purposes and for electricity generation in some countries. Ethane and Methane are the primary components of natural gas; normally, they are stored as gases under pressure. However, it is easier to transport them as liquids: This needs both cooling and compression of the gas.