Which one of the following reaction sequences will give an azo dye?

Solution

Chemistry ma’am · 3 steps

An azo dye needs two things: a diazonium salt and an electron-rich ring for it to couple with.

1. What makes azo. An azo dye has an Ar-N=N-Ar' link. It forms when a diazonium ion, a weak electrophile, attacks a strongly activated ring such as a phenol or an aromatic amine.

   Option	What the sequence does	Azo link formed?
   (a)	PhSO₃H → PhSO₂Cl → PhSO₂NH₂ → PhSO₂NHCH₂Ph, a sulphonamide	No, nitrogen never forms a diazonium salt
   (b)	PhCN is hydrolysed to the acid, PCl₅ gives PhCOCl, and aniline gives benzanilide, an amide	No, it is acylation, not coupling
   (c)	PhNO₂ → PhNH₂ → PhN₂⁺Cl⁻, then coupling with β-naphthol in NaOH	Yes, gives 1-phenylazo-2-naphthol
   (d)	Diazonium salt forms, but the partner is toluene	No, a -CH₃ group does not activate the ring enough

2. Why (c) works. Sn/HCl reduces nitrobenzene to aniline. Cold NaNO₂/HCl at 0 to 5 °C turns it into benzenediazonium chloride. In NaOH, β-naphthol becomes the naphthoxide ion, a very electron-rich ring, and coupling at C-1 gives an orange-red azo dye.
3. So. Only sequence (c) makes both partners of a coupling reaction, so the answer is (c).

The idea to keep. For an azo dye, check that both partners are present: a diazonium salt made from a primary aromatic amine, and a phenol or amine ring for it to couple with.

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