Friedel-Crafts reaction

The Friedel-Crafts reaction is a set of organic chemistry reactions developed by Charles Friedel and James Crafts in 1877. It involves the alkylation or acylation of aromatic compounds using alkyl halides or acyl halides, respectively, in the presence of a Lewis acid catalyst such as aluminium chloride (AlCl₃) or iron(III) chloride (FeCl₃). The Friedel-Crafts reaction is a key transformation in the synthesis of complex organic molecules, including pharmaceuticals, dyes, and polymers.

Types of Friedel-Crafts Reactions

There are two main types of Friedel-Crafts reactions: the Friedel-Crafts alkylation and the Friedel-Crafts acylation.

Friedel-Crafts Alkylation

The Friedel-Crafts alkylation involves the introduction of an alkyl group into an aromatic ring. The general mechanism includes the generation of a carbocation from the alkyl halide by the action of the Lewis acid, followed by the electrophilic attack of this carbocation on the aromatic ring to form a new carbon-carbon bond.

Friedel-Crafts Acylation

The Friedel-Crafts acylation involves the introduction of an acyl group into an aromatic ring. This reaction uses an acyl halide and a Lewis acid to form an acylium ion, which then acts as the electrophile in the reaction with the aromatic compound, leading to the formation of a ketone.

Mechanism

The mechanism of the Friedel-Crafts reaction involves the formation of a complex between the Lewis acid and the halide of the alkyl or acyl halide, followed by the generation of a carbocation or acylium ion. This electrophilic species then attacks the aromatic ring, undergoing a series of steps to form the final product and regenerate the Lewis acid catalyst.

Limitations

The Friedel-Crafts reactions have several limitations. They often do not proceed well with sterically hindered halides or aromatic compounds. Furthermore, the Friedel-Crafts alkylation can result in polyalkylation, where multiple alkyl groups are added to the aromatic ring. The reaction is also sensitive to the presence of functional groups that can interact with the Lewis acid, deactivating the catalyst.

Applications

The Friedel-Crafts reactions are widely used in the synthesis of a variety of organic compounds. They are particularly valuable in the construction of complex molecules in the pharmaceutical and agrochemical industries, as well as in the synthesis of polymers and materials.

See Also

• Electrophilic aromatic substitution
• Lewis acids and bases
• Organic synthesis