Choose the correct colour of the product for the following reaction.

Solution

Chemistry ma’am · 4 steps

This is the nitrite test: the diazonium salt couples with 1-naphthylamine to give a red azo dye.

1. Spot the reagent. The left compound is CH₃COO-N=N-C₆H₄-SO₃H, the diazonium acetate of sulphanilic acid. It forms when a nitrite is treated with sulphanilic acid in acetic acid.
2. Coupling reaction. 1-Naphthylamine has a strongly activating -NH₂ group. The diazonium ion attacks the ring at position 4, para to -NH₂, and the -N=N⁻ link joins the two rings.

   HO₃S-C₆H₄-N₂⁺ + C₁₀H₇NH₂ → HO₃S-C₆H₄-N=N-C₁₀H₆-NH₂ + H⁺

3. Why red. The product is an azo dye. Its long conjugated system runs through a benzene ring, the -N=N⁻ group and the whole naphthalene ring, so it absorbs visible light strongly and looks red.
4. So. The product is a red azo dye, option (c). This is the Griess–Ilosvay test used to detect NO₂⁻ in salt analysis.

The idea to keep. Sulphanilic acid plus 1-naphthylamine with a nitrite always gives a red azo dye. Remember it as the confirmatory test for NO₂⁻.

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