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MCQEasyJEE 2025Diazonium Salts & Reactions

JEE Chemistry 2025 Question with Solution

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

  • A
    Reaction sequence starting from nitrobenzene with reagents Sn/HCl, then NaNO2/HCl, then beta-naphthol and NaOH, leading to azo dye formation pathway.
  • B
    Reaction sequence starting from benzenesulfonic acid with reagents SOCl2, then NH3, then chloromethylbenzene derivative, not corresponding to azo dye formation.
  • C
    Reaction sequence starting from benzonitrile with reagents 70 percent H2SO4, then PCl5, then aniline-like product, unrelated to diazotization and azo dye coupling.
  • D
    Reaction sequence starting from aniline with HCl and NaNO2 followed by toluene coupling partner shown, not the standard beta-naphthol azo dye sequence.

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: We need to identify which reaction sequence gives an azo dye.

Find: The correct option corresponding to azo dye formation.

An azo dye is formed through diazotization of a primary aromatic amine followed by coupling with an activated aromatic compound such as β\beta-naphthol.

The required sequence is:

  1. Reduction of NO2\text{NO}_2 to NH2\text{NH}_2 using Sn/HCl\text{Sn/HCl}.
  2. Diazotization using NaNO2/HCl\text{NaNO}_2/\text{HCl} to form the diazonium salt.
  3. Coupling with β\beta-naphthol in NaOH\text{NaOH} to form the azo dye.
Reaction scheme showing nitrobenzene reduced to aniline, converted to benzene diazonium chloride, then coupled with beta-naphthol in sodium hydroxide to form an azo dye.

Thus, the correct reaction sequence is option A.

Reaction Logic

Given: Azo dyes are obtained from aromatic diazonium salts by coupling reactions.

Find: Which sequence contains all necessary steps.

For azo dye formation, the substrate must first provide a primary aromatic amine. If the starting compound contains a nitro group, it must be reduced:

Ar-NO2→ Sn/HClAr-NH2\text{Ar-NO}_2 \xrightarrow[\ ]{\text{Sn/HCl}} \text{Ar-NH}_2

Then the amine is diazotized:

Ar-NH2→ NaNO2/HClAr-N2+Cl−\text{Ar-NH}_2 \xrightarrow[\ ]{\text{NaNO}_2/\text{HCl}} \text{Ar-N}_2^+\text{Cl}^-

Finally, coupling with β\beta-naphthol in alkaline medium gives the azo dye:

Ar-N2+Cl−+β-naphthol→ NaOHAzo dye\text{Ar-N}_2^+\text{Cl}^- + \beta\text{-naphthol} \xrightarrow[\ ]{\text{NaOH}} \text{Azo dye}

Only option A shows this complete sequence.

Common mistakes

  • Confusing diazotization with any reaction of an aromatic amine. Azo dye formation specifically requires formation of an aryl diazonium salt first, then coupling with an activated aromatic ring such as β\beta-naphthol.

  • Ignoring the need to reduce NO2\text{NO}_2 to NH2\text{NH}_2 before diazotization. A nitro group does not undergo diazotization directly; it must first be converted into a primary aromatic amine.

  • Choosing a sequence that lacks a proper coupling component. Even after diazonium salt formation, an azo dye is obtained only when it reacts with an activated aromatic compound like β\beta-naphthol in alkaline medium.

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