Cracking process in chemistry- WorldOfChemicals

Cracking: Breaking up larger molecules

cracking process

Cracking in petro chemistry, organic chemistry and petroleum geology is the process in which complex large hydrocarbons are broken down into lighter hydrocarbons molecules, by breaking carbon-carbon bonds. The end products from this process are strongly dependent on the temperature and catalysts present. For the commercial production of gasoline and diesel, Cracking is the most important process.

Chemistry in Cracking:

During the cracking process, huge number of chemical reactions takes place. Most of the reactions which take place are free radicals based. Following are the main reactions that take place during cracking:

  • Initiation:

A single molecule of an element breaks down into two free radicals in these initiation reactions. Only few portions of the feed molecules undergo these reactions but initiation is important for producing the free radicals which drives the rest of the reactions.

  • Hydrogen abstraction:

In hydrogen abstraction a hydrogen atom is removed from another molecule by a free radical, which turns second molecule as a free radical.

CH3+ CH3CH3 → CH4 + CH3CH2

  • Radical decomposition:

During these reactions a free radical breaks into two molecules; alkene and a free radical. This is the process whose outcome is alkene products.

CH3CH2 → CH2=CH2 + H

  • Radical addition:

It is the reveres of radical decomposition reaction; a radical reacts with an alkene forming a large single radical. These reaction forms aromatic products.


  • Termination:

In radical decomposition, two free radicals come into reaction with each other producing free radicals products. There are two forms of termination, one is recombination where two radicals comes together forming one larger molecule, another form is disproportionation, where hydrogen atom gets transferred to other by one radical, giving an alkene and an alkane.


CH3CH2+ CH3CH2 → CH2=CH2 + CH3CH3

Cracking methods/types:

 I.Thermal methods

Thermal method of cracking was the first method of hydrocarbon cracking to be developed.

  • Thermal cracking:

Thermal cracking is one such reaction where entropy dominates energetic of this reaction rather than enthalpy. Thermal cracking operates at about 7000 kPa absolute pressures. An overall disproportionation process can be observed, forming “light”, hydrogen-rich products at the cost of heavy molecules which condense and hydrogen depleted. This reaction is actually known as homolytic fission which produces alkenes, which acts as the basis for production of polymers.

  • Steam cracking:

Steam cracking is a petrochemical process, where saturated hydrocarbons are broken down into smaller hydrocarbons which are often unsaturated. It is the key industrial method for the production of lighter alkenes, including ethene or propene. In steam cracking process, a gaseous or liquid hydrocarbons feed is diluted with steam and then heated in a furnace in the absence of oxygen.

II.Catalytic methods:

This is the commonly used process in oil refinery. This process involves converting larger hydrocarbons with high boiling point into olefinic, gasoline and other products.

  • Hydrocracking:

Hydrocracking is a catalytic process of cracking aided by the presence of added hydrogen gas. This process uses hydrogen to break carbon-carbon bonds. The resultant products of this reaction are saturated hydrocarbons; the product ranges from ethane, LPG to heavy hydrocarbons which consists mostly of isopraffins.

The major resultant products of hydrocracking are jet fuels and diesel, along with low sulphur naphtha and LPG. uses cookies to ensure that we give you the best experience on our website. By using this site, you agree to our Privacy Policy and our Terms of Use. X