Question
Explain the Friedel-Crafts alkylation of benzene. Give the reaction conditions, write the mechanism, and name one limitation of this reaction.
Solution — Step by Step
Friedel-Crafts alkylation introduces an alkyl group onto a benzene ring using an alkyl halide and a Lewis acid catalyst (most commonly anhydrous AlCl₃).
General reaction:
Conditions:
- Anhydrous aluminium chloride (AlCl₃) as Lewis acid catalyst
- Anhydrous conditions (water decomposes AlCl₃)
- Mild temperature (room temperature to ~50°C)
- The reaction is an example of electrophilic aromatic substitution (EAS)
AlCl₃ is a Lewis acid — it has an empty orbital and can accept electron pairs. It attacks the alkyl halide (say, CH₃Cl) and pulls away the Cl:
A carbocation (electrophile) is generated. For primary alkyl halides, a tight ion pair forms rather than a fully free carbocation, but the net effect is the same — a highly electrophilic carbon species attacks benzene.
The carbocation (CH₃⁺) attacks the π electron cloud of benzene. One carbon of the benzene ring forms a new C–C bond with the methyl group. The ring loses its aromaticity temporarily — this intermediate is called the arenium ion or sigma complex (also known as the Wheland intermediate).
The two electrons of one π bond are now involved in the new C–C sigma bond. The positive charge is delocalised over the remaining ring carbons (at positions ortho and para to the attack).
AlCl₄⁻ (the anion formed in Step 2) abstracts the H⁺ from the carbon that formed the new C–C bond. This restores aromaticity:
AlCl₃ is regenerated — it is technically a catalyst, but in practice it is used in stoichiometric amounts because it forms a complex with the product.
Once one methyl group is added to benzene, the ring becomes more electron-rich (alkyl groups are +I electron donors → activating groups). This makes the product (toluene) more reactive toward electrophiles than benzene itself. So further alkylation occurs, leading to di-, tri-, and poly-substituted products.
This is the major limitation: Friedel-Crafts alkylation cannot be easily controlled to give mono-substituted product only. The product mixture requires separation, reducing yield efficiency.
Why This Works
The mechanism is electrophilic aromatic substitution, not addition. The key is that benzene’s aromatic stability (extra ~150 kJ/mol stabilisation from delocalization) is preserved by eliminating H⁺ rather than adding across the double bond. The driving force is recovering aromaticity — this is why EAS gives substitution not addition products.
The Lewis acid catalyst’s role is purely to generate the electrophile. Without AlCl₃, alkyl halides are not electrophilic enough to attack the π cloud of benzene — the activation energy would be too high.
Alternative Method — With Alkenes (No AlCl₃)
Alkylation can also be done with alkenes in the presence of a protic acid catalyst (like H₂SO₄ or HF):
The acid protonates the alkene to form a carbocation. Same mechanism — just a different way of generating the electrophile.
Common Mistake
Students often write the mechanism as an addition to benzene followed by elimination — that is the mechanism for alkenes, not arenes. In Friedel-Crafts alkylation, the ring is temporarily disrupted (arenium ion forms) but aromaticity is restored by loss of H⁺ in the same step, not as a separate elimination step.
Also, do not confuse Friedel-Crafts alkylation (uses alkyl halide, product is alkylbenzene) with Friedel-Crafts acylation (uses acyl halide + AlCl₃, product is a ketone like acetophenone). Acylation does not undergo polysubstitution — that’s its advantage over alkylation.
JEE asks: (1) identify mechanism type — EAS; (2) why alkylation undergoes polysubstitution but acylation does not — acyl group is deactivating (−M effect); (3) which intermediate — sigma complex / arenium ion. These three points cover all Friedel-Crafts JEE MCQs.