Cracking and alkenes

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

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

hexane → butane + ethene

C6H14 → C4H10 + C2H4

The total number of carbon and hydrogen atoms in the products is the same as in the starting alkane.

Question

C16H34 is an alkane which can be used as the starting chemical in cracking. One of the products of cracking this compound is an alkane which has 10 carbon atoms in it. Write a balanced symbol equation for this cracking reaction.

C16H34 → C10H22 + C6H12

Various methods can be used for cracking, eg catalytic cracking and steam cracking:

  • Catalytic cracking uses a temperature of approximately 550°C and a catalyst known as a zeolite which contains aluminium oxide and silicon oxide
  • Steam cracking uses a higher temperature of approximately 550°C and no catalyst

The slideshow describes the process of catalytic cracking.

Structure of hexane showing covalent bonds.

1. The structure of hexane

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. Very often, fractional distillation of crude oil 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.

Alkenes

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.

Alkenes will react with bromine water and turn it from orange/brown to colourless. This is the way to test for a double C=C bond in a molecule.

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