Nitrating aniline.

Aniline in nitrating acid: a slider for how much is protonated to meta-directing anilinium shifts the product split towards 51% p / 47% m / 2% o; protecting as acetanilide restores para; AlCl₃ shows the Friedel-Crafts dead end.

Get p-nitroaniline above 90% of the product.

Direct nitration of aniline gives only ortho/para products.

• The resonance structures of aniline pile electron density onto the ortho and para positions, so –NH₂ is a powerful activating, ortho- and para-directing group.
• Bromination: aniline and bromine water at room temperature give a white precipitate of 2,4,6-tribromoaniline at once.
• To stop at one substituent, –NH₂ is protected by acetylation with acetic anhydride (pyridine), the substitution is done on acetanilide, and the amide is then hydrolysed back to the amine. Acetanilide with Br₂ in CH₃COOH gives mainly the para product, and hydrolysis gives 4-bromoaniline.
• In acetanilide the nitrogen lone pair is also shared with the carbonyl oxygen by resonance, so it is less available to the ring and –NHCOCH₃ activates less than –NH₂.
• Nitration: direct nitration (HNO₃, H₂SO₄, 288 K) gives tarry oxidation products as well as nitroanilines. In strong acid much of the aniline becomes the anilinium ion, which is meta-directing, so the product mix is 51% para, 47% meta and 2% ortho.
• Nitrating acetanilide instead, then hydrolysing, gives p-nitroaniline as the main product.
• Sulphonation: aniline with concentrated H₂SO₄ forms anilinium hydrogensulphate, which on heating at 453–473 K gives p-aminobenzenesulphonic acid (sulphanilic acid) as the main product; it exists as a zwitter ion.
• Aniline gives no Friedel-Crafts reaction: it forms a salt with the AlCl₃ catalyst, and the positively charged nitrogen then strongly deactivates the ring.

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