18.10
Electrophilic Aromatic Substitution: Friedel–Crafts Acylation of Benzene
The Friedel–Crafts acylation reactions involve the addition of an acyl group to an aromatic ring. These reactions proceed via electrophilic aromatic s…
Unlike Friedel–Crafts alkylation, Friedel–Crafts acylation involves substituting hydrogen of an aromatic ring with an acyl group.
In this electrophilic aromatic substitution, benzene reacts with an acyl chloride in the presence of aluminum chloride, a Lewis acid catalyst, to form an aryl ketone.
The mechanism starts with the reaction of an acyl chloride with aluminum chloride to form a complex, which then dissociates to form an acylium ion, stabilized by resonance.
The acylium ion, acting as an electrophile, reacts with benzene to form an arenium ion.
Subsequently, the arenium ion is deprotonated, restoring aromaticity and giving an aryl ketone.
The resulting ketone, acting as a Lewis base, reacts with aluminum chloride to form a complex, which upon hydrolysis releases the free aryl ketone.
Interestingly, the carbonyl group of the aryl ketone can be reduced to a methylene group using the Clemmensen reduction, with HCl and amalgamated Zn.
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Q1: What is the role of aluminum chloride in Friedel-Crafts acylation?
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.
Q2: How does the acylium ion form in Friedel-Crafts acylation of benzene?
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.
Q3: What is the difference between Friedel-Crafts acylation and Friedel-Crafts alkylation?
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.
Q4: What happens to the arenium ion intermediate in this reaction?
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.
Q5: How can the carbonyl group of an aryl ketone be converted to a methylene group?
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.
Q6: Why is the acylium ion considered a strong electrophile in this reaction?
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.
Q7: What is the final product of Friedel-Crafts acylation of benzene?
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.