Electrophilic Aromatic Substitution: Friedel–Crafts Acylation of Benzene

Aluminum chloride acts as a Lewis acid catalyst in Friedel-Crafts acylation. It reacts with the acyl chloride to form a complex, which then dissociates to generate the acylium ion. After the reaction, the aryl ketone product complexes with aluminum chloride again, and hydrolysis releases the free ketone.

The acylium ion forms when an acyl chloride reacts with aluminum chloride to create a complex. This complex then dissociates, cleaving the carbon-chlorine bond and generating the acylium ion. The acylium ion is stabilized by resonance, with a positive charge on the carbon atom.

Friedel-Crafts acylation substitutes a hydrogen atom on an aromatic ring with an acyl group to form an aryl ketone, while electrophilic aromatic substitution friedel ndash crafts alkylation replaces hydrogen with an alkyl group. Acylation uses an acyl chloride as the electrophile, whereas alkylation uses an alkyl halide.

The arenium ion intermediate is formed when the acylium ion electrophile attacks the benzene ring. Subsequently, the arenium ion is deprotonated, which restores aromaticity to the ring and produces the final aryl ketone product. This deprotonation step completes the electrophilic aromatic substitution mechanism.

The carbonyl group of an aryl ketone can be reduced to a methylene group using the Clemmensen reduction. This process employs hydrochloric acid and amalgamated zinc, which effectively convert the carbonyl functional group into an alkyl group. This reduction is useful for further synthetic transformations.

The acylium ion is a strong electrophile because it carries a positive charge on the carbon atom and is stabilized by resonance. This resonance stabilization makes the acylium ion highly reactive toward the electron-rich benzene ring, enabling efficient electrophilic aromatic substitution chlorination and bromination of similar aromatic compounds.

The final product of Friedel-Crafts acylation is an aryl ketone, formed when the arenium ion intermediate is deprotonated and aromaticity is restored. The aryl ketone initially complexes with aluminum chloride, which is then hydrolyzed to release the free ketone product.