Amines

Chemistry · Class 12

Simulation · Chemistry · Class 12

Nitrating aniline

From the lesson Electrophilic substitution in aniline in Amines. Change the values and watch what happens.

Nitrating anilineChemistry · Class 12

The idea behind it

NCERT §9.6

  • 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.