Aniline does not undergo

Friedel – Crafts alkylation

The correct answer is option 2. Friedel – Crafts alkylation.

1. Sulphonation: Aniline can undergo sulphonation with reagents like \(H_2SO_4\) and \(SO_3\). The \(SO_3\) group replaces a hydrogen atom on the aromatic ring.
2. Friedel – Crafts alkylation: Aniline does not undergo Friedel – Crafts alkylation because the Lewis acid catalyst \((AlCl_3)\) used in this reaction reacts with the lone pair of electrons on the amino group of aniline, forming a stable ammonium salt. This salt prevents the electrophilic species needed for alkylation from attacking the ring.
3. Bromination: Aniline can undergo bromination with \(Br_2\) in the presence of \(FeBr_3\). The bromine atom substitutes a hydrogen atom on the aromatic ring.
4. Nitration: Aniline can undergo nitration with \(HNO_3\) and \(H_2SO_4\), although the reaction conditions have to be controlled to avoid over-nitration. The nitro group substitutes a hydrogen atom on the aromatic ring.

Therefore, the unique reactivity of aniline due to its amino group prevents it from undergoing Friedel – Crafts alkylation while allowing it to participate in other reactions like sulphonation, bromination, and nitration under appropriate conditions