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Friedel-Crafts acylation adds an acyl group (RCO-) to benzene via an acylium ion (RCO⁺) intermediate, generated from an acyl chloride or anhydride with a Lewis acid catalyst. Unlike alkylation, acylation does not undergo rearrangement and readily forms aromatic ketones. The reaction is more reliable for synthesizing aromatic ketones from acyl chlorides.
From your study of electrophilic aromatic substitution (EAS), you know the general pattern: an electrophile attacks the pi electron cloud of benzene, forming a resonance-stabilized carbocation intermediate (the arenium ion or sigma complex), and then a proton is lost to restore aromaticity. Friedel-Crafts acylation follows this same template, but with an acylium ion (RCO⁺) serving as the electrophile — and this particular electrophile solves several problems that plague Friedel-Crafts alkylation.
The mechanism begins with generation of the electrophile. An acyl chloride (RCOCl) reacts with a Lewis acid catalyst, typically AlCl₃, which coordinates to the chlorine and pulls it away, generating the acylium ion (R–C≡O⁺). This ion is resonance-stabilized: one resonance structure shows the positive charge on carbon, while the other places it on oxygen with a triple bond between C and O. This stabilization is the key advantage over alkylation — because the acylium ion is resonance-stabilized, it does *not* rearrange. In Friedel-Crafts alkylation, the carbocation intermediate can undergo hydride or methyl shifts to form a more stable cation, giving unexpected products. The acylium ion's built-in stability eliminates this problem entirely.
Once formed, the acylium ion attacks the benzene ring in the standard EAS fashion. The pi electrons of benzene attack the electrophilic carbon of R–C≡O⁺, forming the arenium ion intermediate. Deprotonation by AlCl₄⁻ (the conjugate base generated in the first step) restores aromaticity and yields the aromatic ketone product. However, there is an important stoichiometric detail: the carbonyl oxygen of the product coordinates strongly to AlCl₃, forming a stable complex. This means a full equivalent of Lewis acid is consumed, not just a catalytic amount. The product-Lewis acid complex is broken apart during aqueous workup.
Friedel-Crafts acylation has a built-in self-limiting feature that makes it particularly useful. The acyl group placed on the ring is an electron-withdrawing group (through both inductive and resonance effects of the carbonyl). This deactivates the ring toward further electrophilic attack, so the reaction stops cleanly after one substitution — you get the monosubstituted ketone without worrying about polysubstitution. This stands in contrast to Friedel-Crafts alkylation, where the alkyl group activates the ring and can lead to multiple substitutions. For this reason, acylation followed by reduction (Clemmensen or Wolff-Kishner) is often the preferred route to alkylbenzenes, avoiding the rearrangement and overreaction problems of direct alkylation.