Imagine an aromatic ring — benzene, say — as a very stable, electron-rich cloud. It's happy as it is. To make it react, you need to bring something that wants electrons badly enough to disturb that cloud. That something is an electrophile (electron-lover). The Friedel-Crafts reaction is simply a way to create that electrophile and then let it attack the ring.
The trick is that the electrophile you need — a carbocation (for alkylation) or an acylium ion (for acylation) — is too unstable to exist on its own. So you generate it in the reaction mixture using a Lewis acid catalyst like anhydrous AlClX3 or FeClX3.
The Precise Statement
Friedel-Crafts Alkylation: An alkyl group (R) is introduced onto an aromatic ring using an alkyl halide (R−X) and a Lewis acid catalyst.
Friedel-Crafts Acylation: An acyl group (RCO) is introduced onto an aromatic ring using an acyl halide (RCO−X) and a Lewis acid catalyst.
Both are electrophilic aromatic substitution reactions. The catalyst's job is to polarise the halogen-halide bond so strongly that the alkyl or acyl fragment breaks off as a powerful electrophile.
Alkylation:Ar−H+R−XAlClX3Ar−R+HX
Acylation:Ar−H+RCO−XAlClX3Ar−COR+HX
How It Works — Step by Step
Step 1: Generating the Electrophile
The Lewis acid (AlClX3) has an empty orbital. It grabs the lone pair on the halogen of the alkyl halide (R−X). This weakens the C−X bond enormously.
For alkylation, the complex R−X→AlClX3 can break to give a carbocation RX+ (if the alkyl group can stabilise a positive charge) or a highly polarised complex that behaves like one.
For acylation, the complex RCO−X→AlClX3 breaks to give an acylium ionR−CX+=O, which is resonance-stabilised and very stable.
Watch out
The acylium ion is much more stable than a simple alkyl carbocation. This is why acylation does not suffer from rearrangements (like 1,2-hydride shifts) that plague alkylation when using secondary or tertiary alkyl halides.
Step 2: Attack on the Ring
The electrophile (carbocation or acylium ion) attacks the electron-rich aromatic ring. This forms a sigma complex (arenium ion) — a positively charged, non-aromatic intermediate. The ring loses its aromaticity temporarily, which costs energy, but the positive charge is delocalised over three carbon atoms.
Step 3: Regaining Aromaticity
A base (often the AlClX4X− counterion) removes a proton from the sigma complex. The pair of electrons that was holding that proton goes back into the ring, restoring aromaticity. The product is an alkylbenzene or acylbenzene.
The Critical Difference Between Alkylation and Acylation
Feature
Alkylation
Acylation
Electrophile
Carbocation (RX+)
Acylium ion (R−CX+=O)
Catalyst
AlClX3 (catalytic)
AlClX3 (stoichiometric — it complexes with the product)
Rearrangements
Common (carbocations can rearrange)
None (acylium ion is stable)
Product
Alkylbenzene
Ketone (Ar−COR)
Further reaction
Can polyalkylate (product is more reactive than starting material)
Stops at monoacylation (product is less reactive)
Tip
If you need to introduce an alkyl chain without rearrangement, acylate first, then reduce the carbonyl (Clemmensen or Wolff-Kishner). This two-step route gives you the straight-chain alkylbenzene cleanly. …
Anhydrous AlCl3 is a Lewis-acid catalyst that generates an electrophilic alkyl cation from an alkyl halide, which then substitutes onto the benzene ring in a Friedel-Crafts alkylation. …
Anhydrous aluminium chloride (AlCl3) is a Lewis acid catalyst. It reacts with ethyl bromide (C2H5Br) to generate an electrophilic ethyl carbocation:
C2H5Br + AlCl3 --> C2H5+ + [AlBrCl3]-
This ethyl carbocation attacks the electron-rich benzene ring (electrophilic aromatic substitution). One hydrogen of benzene is replaced by the ethyl group, and HBr is eliminated. The overall reaction (Friedel-Crafts alkylation) is:
The statement is False: reacting benzene with CH3Cl over anhydrous AlCl3 to give toluene + HCl is Friedel-Crafts ALKYLATION, not acetylation.
Friedel-Crafts alkylation introduces an alkyl group (like -CH3) onto an aromatic ring using an alkyl halide (R-X) and a Lewis acid catalyst (anhydrous AlCl3): C6H6 + CH3Cl --(anhyd. AlCl3)--> C6H5-CH3 (toluene) + HCl. Friedel-Crafts ACETYLATION is a different reaction: it uses an acyl chloride (like CH3COCl, acetyl chloride) with anhydrous AlCl3 to introduce an acyl group (-COCH3), giving acetophenone: C6H6 + CH3COCl --(anhyd. AlC …
Q.The reaction of Alkyl Halides with Benzene, in presence of anhydrous AlCl3 is called -
(a) Wurtz Reaction
(b) Clemensen Reduction
(c) Friedel Crafts Reaction
(d) Kolbe's Reaction
›Reveal solutionSolution
Alkylation of benzene by an alkyl halide with anhydrous AlCl3 is the Friedel-Crafts reaction.
When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride (a Lewis acid catalyst), an alkyl group replaces a hydrogen atom on the benzene ring — this electrophilic aromatic substitution is called the …
Q.Benzene reacts with methyl chloride in presence of anhydrous AlCl3 to form:
(a) Chlorobenzene
(b) Benzyl chloride
(c) Xylene
(d) Toluene
›Reveal solutionSolution
Benzene + CH3Cl in the presence of anhydrous AlCl3 undergoes Friedel-Crafts alkylation to give toluene (methylbenzene).
This is the Friedel-Crafts alkylation reaction, one of the key electrophilic aromatic substitution reactions of benzene:
C6H6+CH3Clanhydrous AlCl3C6H5−CH3+HCl
Mechanism outline:
Anhydrous AlCl3 (a Lewis acid) coordinates with the chlorine of CH3Cl, polarising and eventually breaking the C-Cl bond to generate an electrophilic methyl carbocation-like species, CH3^+ (as part of a complex, CH3−AlCl4−).
This electrophile attacks the electron-rich benzene ring, forming a resonance-stabilised arenium (sigma-complex) intermediate.
Loss of a proton (H^+) from the sp3 carbon restores the aromatic ring, giving toluene, and regenerates the AlCl3 catalyst along with HCl.
…