JEE Main 2023 · 11 April · morning · Single correct · +4 / −1

o-Phenylenediamine →(HNO₂) `X'
'X' is

  1. (a)
    Option (a)
  2. (b)
    Option (b)
  3. (c)
    Option (c)
  4. (d)
    Option (d)
Answer and solution

Answer (b)

Solution

Chemistry ma’am · 4 steps

Diazotise one NH2 and the NH2 right next door grabs the diazonium nitrogen to close a five-membered ring.

  1. First diazotisation. HNO₂ (from NaNO₂ + HCl) turns one -NH₂ group into a diazonium group, -N₂⁺. This gives o-aminobenzenediazonium ion, the structure in option (a).
  2. Neighbour attacks. The second -NH₂ sits on the adjacent carbon, close to the terminal nitrogen of -N₂⁺. Its lone pair attacks that electrophilic nitrogen. This intramolecular coupling closes a ring before the second amine can be diazotised.
  3. Ring size. The new ring holds two ring carbons and three nitrogens, so it is five-membered and strain-free. Loss of H⁺ gives a neutral, aromatic fused ring: 1H-benzotriazole, with the N-H on one nitrogen and N=N in the ring.
  4. So. The major product X is benzotriazole, option (b).

The idea to keep. When two groups sit ortho to each other and one becomes an electrophile, look for intramolecular ring closure. A five- or six-membered ring forms faster than any second intermolecular step.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Amines & Diazonium Salts

Asked about 170 times in JEE Main since 2020. This question is from 2023.