Question
Explain the Friedel-Crafts alkylation and acylation of benzene with mechanisms. Why is acylation preferred over alkylation for preparing monosubstituted products?
(JEE Main 2023, similar pattern)
Solution — Step by Step
Benzene reacts with an alkyl halide in the presence of anhydrous AlCl (Lewis acid catalyst) to form an alkylbenzene:
Mechanism (electrophilic aromatic substitution):
- AlCl + RCl → R + AlCl (generation of electrophile — carbocation)
- R attacks the -electron cloud of benzene → arenium ion (sigma complex)
- Loss of H from the sigma complex restores aromaticity → product + HCl
- AlCl + H → AlCl + HCl (catalyst regenerated)
Benzene reacts with an acyl halide (RCOCl) in the presence of AlCl to form an aryl ketone:
Mechanism:
- AlCl + RCOCl → RCO + AlCl (acylium ion — the electrophile)
- RCO attacks benzene → arenium ion
- Loss of H → aryl ketone + HCl
The acylium ion (RCO) is stabilised by resonance (), so unlike alkylation, there is no carbocation rearrangement.
Problem with alkylation: The alkyl group is an electron-donating group (activating). So the product (alkylbenzene) is MORE reactive than benzene, leading to polyalkylation — getting a clean monosubstituted product is difficult.
Advantage of acylation: The acyl group (C=O) is an electron-withdrawing group (deactivating). So the product (aryl ketone) is LESS reactive than benzene. The reaction naturally stops at mono-substitution.
To get a monoalkylbenzene cleanly: first acylate, then reduce the C=O to CH using Clemmensen reduction (Zn-Hg/HCl) or Wolff-Kishner reduction (NHNH/KOH).
Why This Works
Both reactions are examples of electrophilic aromatic substitution — the fundamental reaction pattern of benzene. AlCl (a Lewis acid with an incomplete octet) acts as a catalyst by generating a strong electrophile from a mild one. The aromatic ring’s -electrons attack the electrophile, and the subsequent loss of H preserves aromaticity.
The acylation-then-reduction strategy (sometimes called the Friedel-Crafts acylation route) is one of the most important synthetic sequences in organic chemistry for JEE.
Alternative Method — Using Acid Anhydrides
Acid anhydrides can replace acyl halides in Friedel-Crafts acylation:
Friedel-Crafts reaction does NOT work with: (1) deactivated rings (nitrobenzene, benzoic acid), (2) vinyl or aryl halides, (3) NH-substituted rings (the amine coordinates with AlCl, deactivating the catalyst). These limitations are heavily tested in JEE.
Common Mistake
In alkylation, the intermediate carbocation can rearrange via hydride or methyl shifts. For example, using n-propyl chloride with AlCl gives mainly isopropylbenzene (cumene), not n-propylbenzene, because the primary carbocation rearranges to the more stable secondary carbocation. Students who ignore rearrangement predict the wrong product.