FriedelCraftsSubstitutionen

Friedel–Crafts reactions refer to two related electrophilic aromatic substitution processes developed by Charles Friedel and James Crafts in the late 19th century. The reactions introduce an alkyl or an acyl group onto an aromatic ring, typically benzene, using a Lewis acid catalyst such as aluminum chloride (AlCl3).

Friedel–Crafts alkylation uses an alkyl halide and AlCl3 to generate a carbocation, which then attacks the π system of the aromatic ring to form an arenium ion. Deprotonation restores aromaticity and yields the alkylated product. Key limitations include carbocation rearrangements that can lead to unexpected isomers, the tendency for polyalkylation on activated rings, and deactivation of the ring after substitution, which can complicate control of the reaction.

Friedel–Crafts acylation uses an acyl chloride and AlCl3 to generate an acylium ion, which likewise attacks the ring to form an aryl ketone after deprotonation. This route generally avoids carbocation rearrangements and the acyl group is strongly deactivating, making further substitution on the same ring less likely. Multiple acylations are less common unless forcing conditions are used.

Directivity and substrate scope differ: alkylation is activating and ortho/para-directing, but prone to overreaction; acylation is deactivating and meta-directing, often used to install a directing block before further chemistry. Historically, Friedel–Crafts reactions remain fundamental in organic synthesis for building substituted arenes and aryl ketones, though modern methods offer alternatives to address rearrangements and poly-substitution.