Cracking = breaking of C–C bonds of higher alkanes to give smaller alkanes + alkenes.

Types of Cracking

(A) Thermal Cracking

• High temperature (700–900°C)
• Free radical mechanism
• Produces more alkenes
• Less selective

(B) Catalytic Cracking

• 450–550°C
• Zeolite (acidic catalyst)
• Carbocation mechanism
• More branched alkanes
• Higher octane number

Thermal Cracking Example (Free Radical)

Example 1:

$$\text{n-Butane} \xrightarrow{800^\circ C} \text{Ethene} + \text{Ethane}$$

Example 2:

$$\text{n-Pentane} \rightarrow \text{Propene} + \text{Ethane}$$

Major product contains more substituted alkene due to radical stability

Example 3:

$$\text{n-Decane} \rightarrow \text{Pentane} + \text{Pentene}$$

Catalytic Cracking Example (Carbocation Mechanism)

Occurs in presence of zeolite catalyst (acidic sites).

$$\text{n-Hexane} \xrightarrow{\text{Zeolite}} \text{2-Methylpentane} + \text{Propene}$$

✔ Rearrangement possible
✔ Branched products favored

Example 5:

$$\text{n-Octane} \rightarrow \text{Isobutane} + \text{Butene}$$

More stable tertiary carbocation pathway preferred.