octane number improvement

Mechanism of isomerisation of Alkane

Isomerisation of alkane is the process in which a straight-chain alkane (n-alkane) is converted into its branched-chain isomer without changing the molecular formula.

Conditions Required

  • Catalyst:
    • Anhydrous AlCl₃
    • HF
    • Pt/Al₂O₃
  • Temperature: 250–400°C (industrial process)
  • Occurs via carbocation mechanism

Mechanism (Simplified for JEE/NEET)

  1. Formation of carbocation
  2. Hydride shift or methyl shift
  3. Formation of more stable branched carbocation
  4. Deprotonation → Branched alkane

Industrial Importance

  • Used in petroleum refining
  • Improves octane number of petrol
  • Branched alkanes burn more smoothly (less knocking)

Example:
n-Pentane → Isopentane
n-Hexane → Isohexane

The mechanism for butane:

it probably begins by the protonation of butene impurities.

Step 1: Initiation

Propagation:

Termination:

Isomerisation of Pentane (C₅H₁₂)

During catalytic isomerisation of n-pentane (using AlCl₃/HCl or Pt/Al₂O₃), rearrangement proceeds via a carbocation intermediate.

🔹 Possible products:

2,2-Dimethylpropane (Neopentane)

2-Methylbutane (Isopentane)

Major Product → 2-Methylbutane (Isopentane)

✔ Why is it major?

  1. Formation occurs via secondary carbocation, which is reasonably stable.
  2. It requires only one methyl shift from n-pentane.
  3. Formation of neopentane requires further rearrangement and is less favored.
  4. Under equilibrium, moderately branched alkane forms in higher amount.

Isomerisation of n-Hexane — Major Product (Isohexane)

When n-hexane (C₆H₁₄) undergoes catalytic isomerisation (AlCl₃/HCl, Pt/Al₂O₃, zeolites), branched isomers are formed via carbocation rearrangement.

🔹 Possible Isohexanes (Structural Isomers)

1) 2-Methylpentane ✅ Major Product

2) 3-Methylpentane

3) 2,2-Dimethylbutane

4) 2,3-Dimethylbutane

✅ Major Isohexane → 2-Methylpentane

✔ Reason (JEE/NEET Concept):

  • Forms via secondary carbocation with minimal rearrangement.
  • Requires only one hydride/methyl shift.
  • Moderately branched alkane is favored kinetically.
  • Highly branched (like 2,2-dimethylbutane) needs further rearrangement → less formed.

Practically (Industrial Catalytic Isomerisation)

  • Commercially important for C₄ to C₇/C₈ alkanes
  • Used in petroleum refining to increase octane number.
  • Higher alkanes (> C₈) tend to undergo cracking along with isomerisation.