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If –NH₂ is an ortho and para director, why does nitrating aniline give almost half meta product? And why do chemists bother turning aniline into acetanilide first, only to turn it back later? Both questions have the same answer. This post explains the protecting-group idea, follows the route step by step and works a lab yield calculation.
Where does this appear in the syllabus?
The rationalised NCERT Class 12 chapter on Amines (Reprint 2026-27) covers acylation and electrophilic substitution of aniline in Section 9.7. The JEE (Main) 2026 syllabus lists the preparation of acetanilide and p-nitroacetanilide under practical chemistry, and the CBSE 2026-27 curriculum includes both as practical preparations. The JEE (Advanced) 2026 brochure lists alkylation and acylation reactions of amines. Check the syllabus for your exam year.
What goes wrong in direct nitration?
NCERT gives two problems. First, direct nitration of aniline "yields tarry oxidation products in addition to the nitro derivatives". Second, the nitrating mixture is strongly acidic, so much of the aniline is protonated to the anilinium ion, C₆H₅NH₃⁺. That group has no lone pair to donate and is meta directing.
The result, shown in NCERT for nitration with HNO₃ and H₂SO₄ at 288 K, is 51% p-nitroaniline, 47% m-nitroaniline and 2% o-nitroaniline. Nearly half the product comes from the anilinium ion, not from aniline.
Bromination shows the opposite problem. Aniline is so activated that with bromine water at room temperature it gives a white precipitate of 2,4,6-tribromoaniline at once. You cannot stop at one bromine.
How does acetylation help?
Treat aniline with acetic anhydride and it becomes acetanilide, C₆H₅NHCOCH₃. NCERT explains that the lone pair on nitrogen now interacts with the carbonyl oxygen by resonance, so it is less available to push into the ring. The –NHCOCH₃ group is still ortho and para directing, but a weaker activator than –NH₂.
NCERT's conclusion is that, with the –NH₂ group protected this way, "the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product".
For bromination, NCERT shows acetanilide with Br₂ in acetic acid giving mainly p-bromoacetanilide, which hydrolyses to 4-bromoaniline.
The three-step route
Protect. Aniline + (CH₃CO)₂O, with pyridine in NCERT's scheme, gives acetanilide.
React. Acetanilide + HNO₃/H₂SO₄ at 288 K gives p-nitroacetanilide as the major product.
Deprotect. Hydrolysis with OH⁻ or H⁺ gives p-nitroaniline.
In the NCERT Class 12 lab manual (Unit 10), step 2 is done by dissolving 2 g of acetanilide in glacial acetic acid, adding concentrated H₂SO₄, cooling to 0–5 °C and adding the nitrating mixture drop by drop so the temperature does not rise above 10 °C. The manual notes that the small amount of yellow o-nitroacetanilide stays in the mother liquor when the para product is recrystallised from alcohol.
Worked example: a yield calculation
These are sample numbers for practice. You nitrate 2.00 g of acetanilide and collect 1.85 g of dry p-nitroacetanilide. Find the percentage yield, and the maximum mass of p-nitroaniline you could get from it.
Molar masses: acetanilide, C₈H₉NO = 135.17 g mol⁻¹; p-nitroacetanilide, C₈H₈N₂O₃ = 180.16 g mol⁻¹; p-nitroaniline, C₆H₆N₂O₂ = 138.12 g mol⁻¹.
The reaction is 1 : 1, so the theoretical yield = 2.00 × 180.16 / 135.17 = 2.67 g.
Percentage yield = 1.85 / 2.666 × 100 = 69.4%.
Hydrolysis is also 1 : 1, so the maximum p-nitroaniline = 1.85 × 138.12 / 180.16 = 1.42 g.
Notice that the mass falls in step 4 even at 100% conversion, because the acetyl group is removed. Students who expect the mass to stay the same have confused moles with grams.
Why not use Friedel-Crafts instead?
NCERT notes that aniline does not undergo Friedel-Crafts alkylation or acylation. The –NH₂ group forms a salt with the Lewis acid AlCl₃, giving nitrogen a positive charge that strongly deactivates the ring. It is the same lesson as nitration: a basic –NH₂ in an acidic medium stops behaving like an activator.
Which mistakes cost marks?
Writing p-nitroaniline as the sole product of direct nitration. NCERT's figures include 47% meta.
Forgetting the deprotection step in a conversion question.
Saying acetylation makes the group meta directing. –NHCOCH₃ is still ortho and para directing, just weaker.
Using aniline's molar mass in a yield calculation that starts from acetanilide.
A practice routine
Day 1: Draw the resonance of acetanilide that pulls the lone pair towards oxygen.
Day 2: Write the three-step routes to p-nitroaniline and to 4-bromoaniline from memory.
Day 3: Two yield calculations with your own sample masses.
Day 4: Mixed conversions using our amines and diazonium salts guide.
When a multi-step conversion falls apart halfway, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer, so you can see why each step comes where it does.
Tonight, explain in three sentences why direct nitration of aniline gives meta product, using the word anilinium.
