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30/03/2026

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30/03/2026

Friedel craft alkylation mechanism

The Friedel–Crafts Alkylation mechanism is an important electrophilic substitution reaction where an alkyl group is introduced into an aromatic ring (like benzene).

🧪 Overall Reaction
Aromatic compound + Alkyl halide → Alkylbenzene
Catalyst: Lewis acid (usually AlCl3)

⚙️ Step-by-Step Mechanism
Step 1: Formation of Electrophile (Carbocation)
The alkyl halide reacts with the Lewis acid catalyst:

R–Cl + AlCl₃ → R⁺ (carbocation) + AlCl₄⁻

👉 The catalyst helps generate a strong electrophile (R⁺)

Step 2: Electrophilic Attack on Benzene
The benzene ring (rich in π-electrons) attacks the carbocation:

C₆H₆ + R⁺ → σ-complex (arenium ion)

👉 This intermediate is unstable and temporarily loses aromaticity.

Step 3: Deprotonation and Restoration of Aromaticity
The σ-complex loses a proton (H⁺):

σ-complex → Alkylbenzene + H⁺

The proton combines with AlCl₄⁻:

H⁺ + AlCl₄⁻ → HCl + AlCl₃

👉 The catalyst is regenerated.

🔁 Key Features
Reaction type: Electrophilic Aromatic Substitution (EAS)

Catalyst: Lewis acid (AlCl₃, FeCl₃)

Forms carbocation intermediate

Rearrangements may occur (more stable carbocation forms)

⚠️ Limitations
Carbocation rearrangement can give unexpected products

Polyalkylation (multiple substitutions) is common

Does not work well with strongly deactivated rings

✏️ Example
Benzene + CH₃Cl → Toluene (methylbenzene)

30/03/2026