Friedel–Crafts reaction

A type of electrophilic aromatic substitution reaction

The Friedel–Crafts reaction is a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring. These reactions are of two main types: the Friedel–Crafts alkylation and the Friedel–Crafts acylation. Both reactions proceed via an electrophilic aromatic substitution mechanism.

Friedel–Crafts Alkylation

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General mechanism of Friedel–Crafts alkylation

The Friedel–Crafts alkylation involves the introduction of an alkyl group into an aromatic ring. This is typically achieved by reacting an aromatic compound with an alkyl halide in the presence of a Lewis acid catalyst, such as aluminum chloride (AlCl_). The reaction proceeds through the formation of a carbocation intermediate, which then acts as the electrophile in the substitution reaction.

Mechanism

1. Formation of the Electrophile: The alkyl halide reacts with the Lewis acid to form a complex, generating a carbocation. 2. Electrophilic Attack: The aromatic ring donates a pair of electrons to the carbocation, forming a sigma complex. 3. Deprotonation: The sigma complex loses a proton to regenerate the aromaticity of the ring, yielding the alkylated aromatic compound.

Limitations

- Carbocation Rearrangement: The carbocation intermediate can undergo rearrangement, leading to unexpected products. - Polyalkylation: The product can be more reactive than the starting material, leading to multiple alkylations.

Friedel–Crafts Acylation

File:Friedel-Crafts acylation.png

General mechanism of Friedel–Crafts acylation

The Friedel–Crafts acylation involves the introduction of an acyl group into an aromatic ring. This is typically achieved by reacting an aromatic compound with an acyl chloride or acid anhydride in the presence of a Lewis acid catalyst.

Mechanism

1. Formation of the Electrophile: The acyl chloride reacts with the Lewis acid to form an acylium ion, which is the active electrophile. 2. Electrophilic Attack: The aromatic ring attacks the acylium ion, forming a sigma complex. 3. Deprotonation: The sigma complex loses a proton, restoring the aromaticity and yielding the acylated aromatic compound.

Advantages

- No Rearrangement: The acylium ion does not rearrange, leading to more predictable products. - Single Substitution: The carbonyl group deactivates the ring, preventing further substitution.

Applications

Friedel–Crafts reactions are widely used in the synthesis of pharmaceuticals, dyes, and fragrances. They are also employed in the production of polystyrene and other polymers.

Related pages

• Electrophilic aromatic substitution
• Lewis acid
• Carbocation
• Aromaticity