Cracking is a reaction in which larger hydrocarbon molecules are broken down into smaller, more useful hydrocarbon molecules:

  • the original starting hydrocarbons are alkanes¸containing carbon-to-carbon single bonds only
  • the products of cracking include alkanes and alkenes, members of a different homologous series which contain one carbon-to-carbon double bond

For example, hexane can be cracked to form butane and ethene:

hexane → butane + ethene

C6H14 → C4H10 + C2H4

The larger alkanes are heated to around 650°C and their vapours are passed over a hot catalyst containing aluminium oxide. This causes covalent bonds to break and reform. The slideshow describes this process.

Structural formula for hexane, an alkane.

Reasons for cracking

Cracking is important for two main reasons:

  1. it helps to match the supply of fractions with the demand for them
  2. it produces alkenes, which are useful as feedstock for the petrochemical industry

Supply and demand

The supply is how much of a fraction an oil refinery produces. The demand is how much of a fraction customers want to buy. Fractional distillation of crude oil often produces more of the larger hydrocarbons than can be sold, and less of the smaller hydrocarbons than customers want.

Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.


Alkanes and alkenes both form homologous series of hydrocarbons, but:

  • alkanes are saturated (their carbon atoms are only joined by C-C single bonds)
  • alkenes are unsaturated (they contain at least one C=C double bond)

As a result, alkenes are more reactive than alkanes. Alkenes can take part in reactions that alkanes cannot. For example, ethene molecules can react together to form poly(ethene), a polymer.

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