Modification of Alkanes by Cracking

Specification Reference Organic chemistry, Alkanes 3.3.2.2

Quick Notes

Thermal cracking – produces a high percentage of alkenes, occurs at high pressure and temperature.
Catalytic cracking – used to produce motor fuels and aromatic hydrocarbons, occurs at high temperature and low pressure using a zeolite catalyst.

Converts large hydrocarbons into more valuable smaller ones. (Greater demand for shorter chain hydrocarbons)
Increases yield of useful products (e.g., petrol and alkenes).

Crude oil contains long-chain hydrocarbons that have limited uses.

Shorter hydrocarbons are in higher demand as fuels.

Cracking helps to balance supply and demand by breaking down large hydrocarbons into more useful products.

Alkenes produced from cracking are important for making polymers (plastics) and as starting reagents for organic processes.

1. Thermal Cracking

Conditions:
High temperature
High pressure

Products:
Produces a high percentage of alkenes (e.g., ethene, propene).

Reaction Example: C₁₀H₂₂ → C₅H₁₂ + C₃H₆ + C₂H₄

2. Catalytic Cracking

Conditions:
High temperature
Slight pressure
Zeolite catalyst (aluminosilicate).

Products:
Produces higher proportion of branched alkanes, cycloalkanes, and aromatic hydrocarbons (used in motor fuels).

Usage:
Improves petrol quality (produces more useful fuels).

Reaction Example: C₁₂H₂₆ → (CH₃)₃CCH₂CH(CH₃)₂ + C₄H₈

Cracking increases the yield of petrol and diesel from crude oil, which are in high demand.
Produces valuable alkenes for making plastics and starting reactants in industrial processes.
Reduces waste by converting large hydrocarbons into useful fuels.

Type	Conditions	Products	Uses
Thermal Cracking	High temp, high pressure	Shorter Chain Alkanes and Alkenes	Plastics, polymers
Catalytic Cracking	High temp, slight pressure, zeolite catalyst	Branched alkanes, aromatics	Petrol, motor fuels