3.2.2 - Modification Of Alkanes By Cracking

Cracking

• long hydrocarbon chains = not useful ∴ are broken down to form smaller useful molecules

• The C-C are broken requiring harsh reaction conditions in order to break it down

• Endothermic reaction

• 2 main types of cracking resulting in different organic compounds :

  • thermal cracking

  • Catalytic cracking

Thermal Cracking

• alkanes are heated to ↑ temp (up to 1000°) and under ↑ pressure (up to 70 atm) the C-C bonds break up via homolytic fission forming free radicals

• These react to form shorter chain hydrocarbons producing smaller alkanes and many Alkenes + H₂

• This process is expensive due to high conditions

Catalytic cracking

• produces aromatic hydrocarbons (anything with benzene) + cycloalkane + H₂

• Lower temps used (around 500°) and used with slight pressure

• A zeolite catalyst consisting of silicon dioxide (SiO₂) and aluminium oxide (Al₂O₃) is needed to compensate these less harsh conditions

Economic reason for cracking alkanes

• 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)