Why does the direct nitration of aniline with a concentrated nitrating mixture (HNO3 + H2SO4 at 288 K) unexpectedly yield a substantial amount (47%) of meta-nitroaniline alongside 51% para-nitroaniline?

Answer and explanation

Correct answer: A. In acidic medium, aniline protonates to the strongly meta-directing anilinium ion

  1. Step 1: Under neutral conditions, the amino group () of aniline is a powerful ortho/para-directing activating group due to resonance.
  2. Step 2: However, nitration requires a strongly acidic medium ().
  3. Step 3: In this acidic environment, the basic group accepts a proton to form the anilinium ion: .
  4. Step 4: The group possesses a full positive charge, which exerts a strong inductive electron withdrawal and lacks a lone pair, making it strongly deactivating and meta-directing.
  5. Step 5: Consequently, electrophilic attack by the nitronium ion () occurs significantly at the meta position, yielding 51% para, 47% meta, and 2% ortho-nitroaniline.
Answer
In strongly acidic solution, aniline is extensively protonated to form the anilinium ion (-NH3+), which is a powerful meta-directing deactivating group
Why not the others: 'The amino group undergoes thermal isomerization...' is chemically false; 'Sulfuric acid acts as a Lewis base...' is incorrect as acts as a strong Bronsted acid; 'The reaction proceeds through free radicals...' contradicts the standard electrophilic aromatic substitution mechanism ().

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