Chemistry · Amines · NEET
The -NH2 group is a very strong activating group. Its nitrogen lone pair enters the ring by resonance and pushes electron density onto both ortho and both para positions. This makes the ring extremely reactive, so bromine substitutes at every available o/p position at once. With three such positions (2, 4 and 6), all three get brominated in one step, giving 2,4,6-tribromoaniline. You cannot stop at mono unless you first reduce the reactivity.
Benzene itself needs a Lewis acid catalyst (FeBr3) to polarise Br2 because benzene is not very reactive. Aniline's ring is already electron-rich due to the -NH2 group, so it reacts with bromine directly - even with bromine water at room temperature. Adding a catalyst is unnecessary; the ring is activated enough on its own.
The orange-brown colour of bromine water fades because the Br2 is consumed in substituting the activated aniline ring (forming tribromoaniline and HBr). This is why aniline, like phenol, decolourises bromine water. Remember: here it is ring substitution, not addition across a double bond as in alkenes.
Protect the -NH2 group first. Acetylate aniline with acetic anhydride to form acetanilide. In acetanilide the nitrogen lone pair is pulled toward the carbonyl oxygen by resonance, so the -NHCOCH3 group is a milder activator. Bromination now gives mainly p-bromoacetanilide (para preferred, ortho blocked by size). Finally hydrolyse to remove the acetyl group and get p-bromoaniline.
-NH2 is ortho and para directing and strongly activating, because its lone pair raises electron density at the o and p carbons. It becomes meta directing only in strong acid, where it is protonated to -NH3+ (a deactivating, meta director). Free aniline in neutral bromine water directs to 2, 4 and 6 - all o/p positions.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A white precipitate of 2,4,6-tribromoaniline, formed at room temperature without any catalyst.
Aniline's ring is already highly activated by -NH2, so it reacts with mild bromine water directly. FeBr3 is only needed for unactivated rings like benzene.
Acetylate aniline to acetanilide first. This lowers the activating power of nitrogen, so bromination gives mainly p-bromoacetanilide, which is then hydrolysed to p-bromoaniline.
It decolourises bromine water too, but by ring substitution (giving tribromoaniline + HBr), not by addition across a double bond.
Positions 2, 4 and 6 - the two ortho and the one para positions - because -NH2 is ortho/para directing.
