: 200
The -NH2 group is the most powerful activating group for electrophilic aromatic substitution (+M effect), making aniline extremely reactive — often too reactive for controlled reactions.
Bromination: Aniline + Br2(aq) -> 2,4,6-tribromoaniline (no catalyst). For monosubstitution: protect -NH2 by acetylation, brominate (para), deprotect.
Nitration problem: Conc. HNO3 oxidises -NH2. In strong acid, -NH2 is protonated to -NH3+ (meta-directing, deactivating). Solution: (1) Acetylate. (2) Nitrate (gives mainly para-nitroacetanilide). (3) Hydrolyse.
The protection-reaction-deprotection strategy is essential for aniline chemistry. The acetamido group (-NHCOCH3) is moderately activating (resonance with C=O partially ties up the lone pair) and still o/p directing, allowing controlled monosubstitution.
Reactivity order: aniline > phenol > toluene > benzene > chlorobenzene > nitrobenzene. Aniline is so activated that Friedel-Crafts would give polyalkylation — use with caution.