Which among the following represent reagent 'A'?

Solution

Chemistry ma’am · 3 steps

NaOH plus an orange-red dye points straight to 2-naphthol, the classic coupling partner in alkali.

1. Read the medium. Benzenediazonium chloride is a weak electrophile, so it couples only with a strongly activated ring. NaOH turns a phenol or naphthol into its phenoxide ion. The O⁻ group activates the ring strongly enough for coupling. Amines are coupled in a mildly acidic medium instead.
2. Where it couples. In 2-naphthol, the diazonium ion attacks C-1, the position right next to the OH. This gives 1-phenylazo-2-naphthol, with an extended -N=N⁻ conjugated system that is orange-red.

   Option	Reagent	Coupling medium	Dye formed
   (a)	2-naphthol	alkaline (NaOH)	orange-red, 1-phenylazo-2-naphthol
   (b)	1-naphthol	alkaline	couples at C-4, not the standard orange-red dye
   (c)	aniline	mildly acidic	yellow p-aminoazobenzene
   (d)	1-naphthylamine	mildly acidic	amine coupling, not the NaOH reaction

3. So. Reagent A is 2-naphthol, option (a). In NaOH it couples with benzenediazonium chloride to give the orange-red azo dye.

The idea to keep. Match the medium to the partner: phenols and naphthols couple in alkali (phenoxide), amines in mild acid. Phenol gives orange, 2-naphthol orange-red, aniline yellow.

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