Bromination of phenol and aniline template

Phenol with bromine water gives a white precipitate of 2,4,6-tribromophenol with no Lewis acid, aniline gives 2,4,6-tribromoaniline, and acetylating the NH₂ (acetanilide) tames the ring enough to stop at 4-bromoaniline.

Mechanism step by step

OH and NH₂ push electrons into the ring so strongly that bromine water substitutes all three ortho/para positions

1. Attack at C4 – The O lone pair pushes through the ring; the para carbon attacks Br₂ (no FeBr₃ needed)
2. Loss of H⁺ – The C–H electrons restore the aromatic ring: 4-bromophenol
3. Both ortho positions – The same two steps at C2 and C6
4. Aniline – NH₂ is even more activating: 2,4,6-tribromoaniline at once (white precipitate)
5. Acetanilide: one Br – Acetylation moderates NH₂; bromination goes para; hydrolysis removes the acetyl group

Key points

• OH and NH₂ donate a lone pair into the ring (+M), so the σ-complex for ortho/para attack has a form with every atom an octet; they activate the ring more than a Lewis acid is needed for.
• In water, phenol is partly phenoxide (even more activating) and Br₂ is polarised by the solvent: all three o/p positions react. In a non-polar solvent (CS₂ or CHCl₃, about 0 °C) the main product is 4-bromophenol.
• In acetanilide the N lone pair is shared with C=O, so the ring is only moderately activated and the bulky NHCOCH₃ group blocks ortho: one Br, para.
• Aniline cannot be nitrated or acylated directly (oxidation, or protonation/Lewis-acid binding at N); protecting it as acetanilide solves this too.

Drawn with 133 atoms, 16 arrows, 7 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 21 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 17 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 11 Textbook