Aniline does not undergo Friedel-Crafts reactions (alkylation and acetylation) due to salt formation with aluminum chloride, acting as a Lewis acid catalyst. The positive charge acquired by nitrogen in the aniline-AlCl₃ complex makes it a strong deactivating group for further reactions.
Why does aniline not undergo Friedel-Crafts reactions, and how does the interaction with aluminum chloride affect its reactivity?
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Aniline does not undergo Friedel-Crafts reactions because it is a poor electrophile. The amino group in aniline is strongly activating and ortho-para directing, preventing substitution at other positions. The lone pair on nitrogen, though enhancing electron density, also hinders the formation of a stable carbocation needed for Friedel-Crafts reactions. When aniline reacts with aluminum chloride (AlCl₃), the amino group coordinates with the Lewis acid, reducing its activating effect. This coordination disrupts the aromaticity of the ring, making it less nucleophilic. Consequently, aniline’s reactivity toward electrophiles is diminished in the presence of aluminum chloride.