Britannica Sources used in this answer

How does cracking convert heavy hydrocarbons into lighter molecules?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

structures of common hydrocarbon compounds — hydrocarbon Structures assumed by hydrogen (H) and carbon (C) molecules in four common hydrocarbon compounds.

Cracking is a fundamental process in petroleum refining used to break down heavy hydrocarbon molecules into lighter, more valuable products like gasoline and diesel fuel. The conversion occurs through two primary mechanisms:1

Thermal Cracking: Under conditions of extreme heat, the electron bonds between carbon atoms in a hydrocarbon molecule break, creating free radicals—molecules with an unpaired electron. These radicals trigger a chain reaction that reduces the molecular size of the original feedstock.2
Catalytic Cracking: This process uses a catalyst to facilitate reactions under less severe conditions than thermal cracking. According to the carbonium ion theory, the catalyst promotes the removal of a hydride ion or the addition of a proton to form a positively charged carbonium ion. This intermediate compound transfers the charge through the hydrocarbon, weakening carbon-carbon bonds and causing the molecule to crack into smaller components.2

Additionally, hydrocracking employs hydrogen gas to improve the hydrogen-carbon ratio while simultaneously breaking down the molecules.1