Cracking of Alkanes (CIE A Level Chemistry)

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
• This is a separating technique in which a wide range of different hydrocarbons are separated into fractions based on their boiling points

A fractional distillation column

Crude oil is initially separated into fractions with similar boiling points in a process called fractional distillation

• However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones
• Therefore, some of the excess heavier fractions are broken down into smaller, more useful compounds
• These more useful compounds include alkanes and alkenes of lower relative formula mass (M_r)
• This process is called cracking

The process of cracking

 

The heavier fractions that are obtained in fractional distillation are further cracked into useful alkanes and alkenes with lower M_r values

Cracking

• The large hydrocarbon molecules are fed into a steel chamber, heated to a high temperature and then passed over an aluminium oxide (Al₂O₃) catalyst
  • The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide
• When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
  • E.g. octane and ethene from decane

 Cracking of long-chain hydrocarbons

The long hydrocarbon fraction is cracked into two smaller ones

 

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

• The low-molecular mass alkenes are more reactive than alkanes due to their double bond
• This makes them useful for the chemical industry as the starting compounds (feedstock) for making new products
  • E.g. they are used as monomers in polymerisation reactions

Using alkenes to form other useful compounds

  

Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds