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Cracking – Definition, Mechanism, Types, Uses, Importance, Practice Problems and FAQ

Fuel is such an integral part of human civilization. Crude oil extracted from the earth is processed to obtain a multitude of beneficial petrochemical products. A combination of hydrocarbon molecules makes up crude oil.

Oil refineries produce the high-in-demand petrochemical products from the crude oil that makes the Earth run!

You name it— From aeroplanes to school buses, heating your cosy homes to running a large-scale industrial plant, fuel is the ultimate driver of all advancements that are happening across the globe.

Refineries are built to create the fuel and other related resources that the market demands. Fuel for motor vehicles is currently in the highest demand on global markets. However, the amount of petrol (gasoline) and petroleum products produced by standard fractional distillation of crude oil is insufficient to satisfy the increasing customer demands. The fractionation process recovers majorly longer hydrocarbons. Whereas Petroleum hydrocarbons (PHs) are made up of short-chain hydrocarbon substances such as smaller alkanes, paraffin, alicyclic, and aromatic chemicals.

Hence, longer hydrocarbons are subjected to a process called cracking that breaks them down to smaller and economically more beneficial small-chain hydrocarbons to meet the demands of the global sectors.


  • Cracking – Definition
  • Cracking in Organic Chemistry – Mechanism
  • Cracking – Types
  • Cracking – Importance
  • Cracking – Uses
  • Practice Problems
  • Frequently Asked Questions – FAQ

Cracking – Definition

Cracking is an integral part of oil refineries. Crude oil is refined using a method called fractional distillation. With this method, crude oil is divided into several fractions based on the boiling points of its constituent molecules. The fractions are heavy gas oil, lubricating oil, gas oil and diesel, kerosene, gasoline, naphtha, and gas, in that order. The length and arrangement of the molecules vary, and as a result, they display various characteristics, including boiling point.

Following separation, fractions can undergo additional processing to yield a variety of products needed every day, such as fuels, polymers, and medications.

  • Cracking can be defined as a process used in oil refineries, to break down large and complex hydrocarbon molecules into lighter, smaller hydrocarbons that are better suited for industrial or consumer usage. A crucial step in the refinement of crude oil is cracking.
  • Cracking is done by breaking the carbon-carbon single bonds present in long-chain hydrocarbons (alkanes) and usually produces smaller alkanes and alkenes. The products may vary depending on temperature and catalysts. Although a high temperature is very essential.
  • The first and most important step in the refining process is to fragment, or "crack," the unprocessed hydrocarbon molecules into smaller parts in order to assist transform the crude oil into a form that can be more extensively utilised. This process, known as "cracking," enables the conversion of crude oil into a range of commercially viable fuels, lubricants, and other goods.


  • Although the fundamental idea is always the same, there are numerous ways to carry out the cracking process. The process known as fluid catalytic cracking (FCC), which is used to create various distillate fuels as well as gasoline, is a typical application.
  • In the petroleum refining and petrochemical industry, this method is employed to create gasoline, diesel, and petrol. It is the process of dissolving complex alkanes into less complex alkenes and alkanes in chemistry. Similar to this, hydrocarbons are cracked when a complex long chain of hydrocarbons is broken down into smaller ones.

Cracking in Organic Chemistry – Mechanism

This method incorporates a variety of free radical-based chemical processes. Here are several crucial reactions that happen.

Initiation: Here, a single molecule disintegrates into two free radicals as the initial step. Only a small portion of the liberated radicals go through initiation, but it is essential to generate enough free radicals to continue the complete reaction.

CH3CH3 → 2CH3

Abstraction of Hydrogen: In this case, the free radical generated removes an atom of hydrogen from a third molecule, resulting in the formation of a free radical and a lower alkane.

CH3• + CH3CH3 → CH4 + CH3CH2

Radical Decomposition: In this step, the free radicals split into an alkene and another free radical. The byproducts of this reaction are alkenes.

CH3CH2• → CH2=CH2 + H•

Radical Addition: Here, a radical and an alkene interact to form a free radical.

CH3CH2• + CH2=CH2 → CH3CH2CH2CH2

Termination: Two free radicals join in this reaction to create products that are not free radicals. One is a lower alkane and the other is an alkene.

CH3• + CH3CH2• → CH3CH2CH3

CH3CH2• + CH3CH2• → CH2=CH2 + CH3CH3

Cracking – Types

There are three major types of cracking.

  1. Thermal Cracking

Conversion: Residual oil to diesel, fuel oil, petrol, and naphtha.

  • Large non-volatile hydrocarbons are broken down into gasoline during this process.
  • In this procedure, high pressures (7000 kPa) and temperatures are both used.
  • Homolytic fission takes place in the spaces between the bonds of the carbons that are found in hydrocarbons.
  • Many industrial processes utilise this reaction, most notably the manufacturing of burner fuels.
  • This method produces important alkenes necessary for the large-scale manufacturing of plastics and polymers.
  1. Steam Cracking
  • Saturated hydrocarbons are broken down into smaller, frequently unsaturated hydrocarbons by the petrochemical process known as steam cracking.
  • The process is the main commercial method for manufacturing the lighter olefins, such as ethene (or ethylene) and propene (or propylene).
  • Pyrolysis is another name for this process. This method is thought to be extremely helpful in creating alkenes due to the high levels of latent heat energy present in the steam.
  • Feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane, or butane is thermally split to yield lighter hydrocarbons in steam cracker units.
  • In the process of steam cracking, a gaseous or liquid hydrocarbon feed, such as naphtha, LPG, or ethane, is diluted with steam and heated for a brief period of time in an oxygen-free furnace. The reaction temperature is often very high, at about 850 °C
  1. Fluid Catalytic Cracking

Conversion: Gas oil or residual oil to diesel and petrol. Extremely cost-effective and energy efficient.

  • Petroleum refineries employ fluid catalytic cracking to the maximum level.
  • Solid acid catalysts (Lewis acids like silica-aluminates) must be present for the catalytic cracking process; these catalysts typically include zeolites and silica-alumina.
  • The catalyst leads to the breaking of the long chain C-C bonds by the creation of carbocations and promoting their rearrangements.
  • Cat-cracking occurs at lower temperatures than thermal cracking, which conserves energy.
  • Furthermore, the yield of alkenes is reduced while operating at lower temperatures.
  • These catalysts divide the lengthy molecular chains into their shorter counterparts.
  • Due to its olefinic composition, the gasoline generated in the fluid catalytic cracking unit has a higher octane rating but is less chemically stable than other gasoline constituents.
  • Due to the rising demand for gasoline (petrol), fluid catalytic cracking is more prevalent in the US.
  1. Hydrocracking

Conversion: Gas oil into petrol.

  • Catalytic cracking is aided by adding hydrogen gas during the hydrocracking process.
  • The C-C bonds are broken down using it. The major products from this process are diesels, jet fuel, and LPG.
  • It is based on the fusion of two distinct processes known as catalytic cracking and hydrogenation.
  • Feedstocks are used to crack in the presence of hydrogen in order to produce the desired products.
  • Vacuum gas oil, a heavy proportion of petroleum, serves as the major feedstock in this process.
  • Saturated hydrocarbons are the end products of this process; depending on the reaction circumstances (temperature, pressure, catalyst activity), these products can range from lighter hydrocarbons, such as ethane and LPG, to heavier hydrocarbons, primarily isoparaffins.
  • The elimination of contaminants including sulphur, nitrogen, and trace metals is also aided by hydrocracking. Gases include hydrogen sulphide created that is removable easily.

Cracking – Importance

  • It is beneficial to maintain the supply and demand fractions of fuel in the present-day world. Due to the high fuel efficiency of low carbon-chain hydrocarbons, it is important to crack down the long chain hydrocarbons obtained in refineries during fractional distillation, so as to produce effective fuel.
  • How much of a percentage an oil refinery produces is the supply. How much of a portion consumers want to purchase is the demand. In most cases, fractional distillation of crude oil results in a low quantity of the smaller hydrocarbons that consumers want and more of the larger hydrocarbons than can be sold.
  • Petrol (gasoline) and other smaller hydrocarbons work better as fuels than larger hydrocarbons do. The availability of fuels is increased because cracking breaks down large hydrocarbons into smaller ones. This aids in balancing supply and demand.
  • Another importance of cracking is the production of multidimensionally utilisable alkenes. It results in alkenes. Compared to alkanes, alkenes are more reactive. They serve as raw materials for the petrochemical sector, for making various polymers, plastics etc.

Cracking – Uses

  • Many significant compounds are made from the cracking byproducts, such as ethene, propene, buta-1,3-diene, and C4-alkenes.
  • To increase the octane rating, additional compounds such as branched and cyclic alkanes are added to the gasoline fraction left over after distilling crude oil.

Practice Problems

  1. After the conversion of long-chain hydrocarbons to smaller ones via cracking, we observe
  1. The product has a lower boiling point as compared to long-chain hydrocarbons
  2. The product has a higher boiling point as compared to long-chain hydrocarbons
  3. No change in boiling point is observed
  4. Unpredictable

Answer: A

Solution: Long chain hydrocarbons have high molar mass and hence there is stronger van der Waals force of attraction between them, unlike small chain hydrocarbons which have less intermolecular forces. Hence, the boiling point of small chain hydrocarbons obtained after cracking is expected to be lower than the long-chain hydrocarbons.

So, option A is the correct answer.

  1. Why are long chain hydrocarbons not efficient as fuels?

Solution: The intermolecular force between longer hydrocarbon molecules is stronger. They have greater boiling points, and so to separate them, more energy is required. So, being less volatile, they become less flammable and as a result, are less efficient as fuels.

  1. Name the catalyst used in fluid catalytic cracking.
  1. Zeolite
  2. Wilkinson’s Catalyst
  3. Ziegler-Natta Catalyst
  4. None of the above

Answer: A

Solution: Among the catalysts employed in fluid catalytic cracking procedure, the main ones are (FCC), aluminosilicate zeolite (ZSM-5), and silica-alumina (Al2O3/SiO2).

So, option A is the correct answer.

  1. What are kerogens?

Answer: Hydrocarbons that are insoluble in typical solvents like carbon tetrachloride but produce liquid or gaseous petroleum when heated are referred to as kerogen.

Frequently Asked Questions – FAQ

1. What is petrol’s composition?
 A combination of hydrocarbons with 5 to 10 carbon atoms can be found in petrol (gasoline). Straight-chain alkane content is high in the combination of C5-C10 hydrocarbons derived directly from from the distillation of crude oil. The engine of a car suffers severe harm if this mixture is utilised as gasoline. Hence straight-chain alkanes are transformed into branched-chain alkanes, cycloalkanes, and aromatic hydrocarbons in a number of processes in the refinery to finally obtain the commercially sourced petrol because they are much more knock-resistant.

2. What is Knocking?
In the cylinder of an automobile engine, gasoline with a high concentration of straight chain alkanes has a tendency to ignite as the piston builds pressure and before the cylinder reaches the ideal position. A spark is generated and this ignites the mixture of air and gasoline at a specific location on the piston in the cylinder causing a knocking sound. This is called as knocking

Engine knock and pre-ignition are two terms used to describe this issue with early ignition. When pre-ignition is audible, the phrase knock is employed. Serious engine damage might result from a severe knocking.

3. What is octane number/octane rating?
 An octane rating is a measurement of how heat resistant a fuel is to avoid knocking. Octane does not improve combustion; rather, it stops the air-fuel mixture inside an engine from igniting earlier than it should. The resistance of a fuel to knocking increases with increasing octane.

The resistance of the gasoline mixture to knocking increases with increasing octane number. Higher compression ratios, turbocharging, and downsizing/downspeeding are also made possible by the use of higher octane fuels, all of which increase engine performance and efficiency.

4. Why is catalytic cracking beneficial as compared to thermal cracking?
The industry of refining crude oil makes extensive use of catalytic cracking to transform viscous feedstocks into lighter and more valuable gasoline and petrol-based fractions. Catalytic cracking has replaced thermal cracking as a result of the rise in demand for gasoline with a higher octane rating, which is highly efficient for the automobile industry. Also, catalytic cracking requires less temperature and pressure and is hence energy efficient.

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