Explain how the reactivity of aniline in electrophilic substitution is controlled. Show how p-nitroaniline is prepared from aniline, and explain why AlCl₃ cannot be used as a catalyst with aniline.

1. 1 · State the directing effect. -NH₂ is a strong activating, ortho-para directing group: its lone pair is delocalised into the ring.
2. 2 · Explain the problems of direct nitration. HNO₃ oxidises aniline (tarry products), and in acid aniline becomes the anilinium ion, which is meta-directing: about 47% m-nitroaniline.
3. 3 · Protect by acetylation. C₆H₅NH₂ + (CH₃CO)₂O → (pyridine) C₆H₅NHCOCH₃ + CH₃COOH. The lone pair is shared with C=O, so the group is milder.
4. 4 · Nitrate and hydrolyse. Acetanilide + HNO₃ → (conc. H₂SO₄, 288 K) 4-nitroacetanilide; hydrolysis (H⁺ or OH⁻) → 4-nitroaniline.
5. 5 · Draw the sequence. Draw aniline → acetanilide → 4-nitroacetanilide → 4-nitroaniline as ring structures, with the reagent on each arrow and the NO₂ at the para position.
6. 6 · Explain no Friedel-Crafts. Aniline forms a salt with AlCl₃ (a Lewis acid); the positive N strongly deactivates the ring.
7. 7 · Close in one line. So protecting -NH₂ turns an uncontrollable reaction into a clean para substitution.

Scoring tip: Mention both reasons for protection, oxidation and anilinium ion formation, and show the hydrolysis step at the end.