The Friedel-Crafts acylation reaction is an electrophilic aromatic substitution reaction in which an acyl group is introduced into an aromatic ring, typically using an acyl halide and a Lewis acid catalyst. The Friedel-Crafts alkylation reaction, on the other hand, involves the introduction of an alkyl group into the aromatic ring using an alkyl halide and a Lewis acid catalyst. Both reactions are used to functionalize aromatic compounds, but the acylation reaction is more useful for synthesizing monoacylated benzene derivatives due to the absence of carbocation rearrangement and over-alkylation issues.Mechanism of Friedel-Crafts Acylation:1. Formation of the acylium ion: The Lewis acid catalyst e.g., AlCl3 reacts with the acyl halide e.g., RCOCl to form a complex. This complex undergoes a heterolytic cleavage of the carbon-halogen bond, generating an electrophilic acylium ion RCO+ and a halide ion.RCOCl + AlCl3 RCO+ + AlCl4-2. Electrophilic attack on the aromatic ring: The acylium ion RCO+ acts as an electrophile and attacks the aromatic ring, forming a resonance-stabilized carbocation sigma complex .3. Deprotonation and regeneration of the aromatic system: A base e.g., AlCl4- abstracts a proton from the sigma complex, regenerating the aromatic system and releasing the Lewis acid catalyst.Example of Friedel-Crafts Acylation:Acetyl chloride CH3COCl reacts with benzene in the presence of aluminum chloride AlCl3 as a catalyst to form acetophenone C6H5COCH3 .CH3COCl + C6H6 + AlCl3 C6H5COCH3 + HCl + AlCl3In summary, the Friedel-Crafts acylation reaction is a useful method for synthesizing monoacylated benzene derivatives, as it avoids the issues of carbocation rearrangement and over-alkylation that can occur in the Friedel-Crafts alkylation reaction. The mechanism involves the formation of an electrophilic acylium ion, which attacks the aromatic ring, followed by deprotonation to regenerate the aromatic system.