Following is a confirmatory test for aromatic primary amines. Identify reagent (A) and (B).

Solution

Chemistry ma’am · 3 steps

Diazotise in the cold, then couple with β-naphthol in alkali to get the scarlet dye.

1. Reagent A. Turning -NH₂ into -N₂⁺ is diazotisation. It needs nitrous acid, made in situ from NaNO₂ and HCl at 0 to 5 °C. The cold matters because the diazonium salt decomposes on warming.

   C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

2. Reagent B. The confirmatory dye test couples the diazonium salt with 2-naphthol (β-naphthol) dissolved in NaOH. In alkali it forms the strongly activated naphthoxide ion, which couples at C-1, next to the -OH. The product, 1-phenylazo-2-naphthol, is a scarlet-red azo dye.
3. So. A is NaNO₂ + HCl at 0 to 5 °C and B is 2-naphthol, which is option (a).

The idea to keep. Aromatic primary amine test: diazotise with NaNO₂ + HCl in the cold, then couple with β-naphthol in NaOH. A scarlet-red dye confirms ArNH₂.

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