Modification of Alkanes by Cracking (AQA A Level Chemistry): Revision Note

Cracking

• Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by the cracking of longer alkane chains

Crude oil

• Crude oil is a mixture of hydrocarbons containing alkanes, cycloalkanes, and arenes (compounds with a benzene ring)

• The crude oil is extracted from the earth in a drilling process and transported to an oil refinery

• At the oil refinery, the crude oil is separated into useful fuels by fractional distillation

• Supply of long-chain fractions exceeds demand, while shorter alkanes/alkenes are more valuable (fuels and feedstock for polymers), so cracking converts low-value surplus into high-demand products

• These more useful compounds include alkanes and alkenes of lower relative formula mass (M_r)

• This process is called cracking

Production of alkanes from cracking

• In cracking, these large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules

• Cracking involves breaking C–C bonds in alkanes

• The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature, and then passed over a zeolite (or aluminosilicate) catalyst

  • The chamber does not contain any oxygen to prevent the combustion of the hydrocarbon to water and carbon dioxide

• When a large hydrocarbon is cracked, a smaller alkane and one (or more) alkene molecules are formed

  • e.g., octane and ethene from decane

decane $\stackrel{\mathrm{zeolite}}{\to }$octane + ethene

C₁₀H₂₂ $\stackrel{\mathrm{zeolite}}{\to }$C₈H₁₈ + C₂H₄

• The low-molecular-mass alkanes formed make good fuels and are in high demand

• There are two types of cracking:

  • Thermal cracking requires high temperatures (up to 1000 $°$C) and high pressure (up to 70 atmospheres) and produces alkanes and a lot of alkenes

  • Catalytic cracking uses a lower temperature (around 450 $°$C) and slight pressure in the presence of a catalyst such as a zeolite or aluminosilicate to produce motor fuels and mainly aromatic hydrocarbons

Related topics

• Fractional Distillation of Crude Oil