Cracking is a chemical process used in petroleum refining in which large hydrocarbon molecules are broken into smaller ones by the cleavage of carbon–carbon bonds. It is mainly applied to heavy fractions obtained during the fractional distillation of crude oil. Crude oil is primarily composed of a complex mixture of hydrocarbons, which are organic molecules consisting solely of carbon and hydrogen atoms. Their largest proportion is alkane. ^[1-4]

During cracking, long-chain alkanes break into a mixture of smaller hydrocarbons, including shorter alkanes and alkenes. For example, the alkane pentadecane (C₁₅H₃₂) can split into smaller molecules such as ethene (C₂H₄), propene (C₃H₆), and octane (C₈H₁₈):

C₁₅H₃₂ → 2 C₂H₄ + C₃H₆ + C₈H₁₈

Classification

Cracking can be classified into several types based on the reaction conditions and the use of catalysts. ^[1-8]

Among these processes, thermal cracking and catalytic cracking are widely used in petroleum refining.

Mechanism ^[5-7]

1. Free-Radical Mechanism for Thermal Cracking

i. Initiation: Bond Breaking and Radical Formation

At high temperatures, a C–C bond breaks evenly so that each atom takes one electron (homolytic cleavage), forming two free radicals. Because long-chain hydrocarbons contain many C–C bonds, cleavage can occur at different positions along the chain, producing different combinations of radicals.

Example of one possible cleavage:

C₁₀H₂₂ → C₃H₇· + C₇H₁₅·

(Decane → Propyl radical + Heptyl radical)

ii. Propagation: Radical Breakdown and Alkene Formation

The radicals formed in the initiation step undergo β-scission (splitting at the β-position), forming a smaller radical and an alkene.

C₇H₁₅· → C₃H₇· + C₄H₈ 

(Heptyl radical → Propyl radical + Butene)

iii. Termination: Radical Removal

Two radicals combine to form a stable molecule, removing reactive radicals from the system and slowing the chain reaction:

C₃H₇· + C₃H₇· → C₆H₁₄

(Propyl radical + Propyl radical → Hexane)

Steam cracking follows a free-radical mechanism similar to thermal cracking. Steam is added to dilute the hydrocarbons, lower their partial pressure, and reduce the formation of coke, a carbon-rich solid that can deposit on reactor surfaces.

2. Carbocation Mechanism for Catalytic Cracking

i. Protonation: Formation of Carbocation

The hydrocarbon interacts with a Brønsted acid site on the zeolite catalyst, leading to protonation and the formation of a carbocation intermediate.

R–CH₂–CH₂–CH₂–R’ + H⁺ → R–CH₂–CH⁺–CH₂–R’

ii. β-Scission: Breakdown of Carbocation

The carbocation undergoes cleavage at the β-position, resulting in the formation of an alkene and a smaller carbocation.

R–CH₂–CH⁺–CH₂–R’ → R–CH=CH₂ + R’–CH₂⁺

iii. Rearrangement: Formation of Final Products

Carbocations may undergo rearrangements, such as 1,2–hydride or 1,2–alkyl shifts, leading to the formation of branched hydrocarbons. The reaction sequence eventually produces stable hydrocarbons and regenerates the catalyst.

In catalytic cracking processes, some reactions produce coke on the catalyst surface and gradually reduce its activity. The deposited coke is periodically burned off to regenerate the catalyst.

Importance ^[8]

Crude oil contains a large proportion of heavy hydrocarbons, while transportation fuels and petrochemical feedstocks require smaller and more reactive molecules. Cracking increases the economic value of petroleum by converting heavy fractions into lighter, more useful hydrocarbons:

• Gasoline and LPG are widely used as fuels.
• Ethylene and propylene are key feedstocks for producing plastics, synthetic fibers, and many industrial chemicals.
• Branched hydrocarbons and high-octane gasoline burn more smoothly in internal combustion engines and reduce engine knocking.