Q.30 Which one is the major product of the following reaction?

The major product is the mono‑azo dye in which diazotized sulphanilic acid couples at the para position of resorcinol, giving a single $-N=N-$ linkage to the sulfonated ring (Option D).

Reaction background

In this question, 4‑aminobenzenesulfonic acid (sulphanilic acid) is first diazotized with NaNO₂/HCl at about 5 °C to form a benzenediazonium sulfonate, an electrophile that readily undergoes azo coupling with activated aromatic systems such as phenols. Resorcinol (1,3‑dihydroxybenzene) is a strongly activated ring where hydroxyl groups direct electrophilic substitution predominantly to the para positions, and if para is occupied, to the ortho positions.​

Mechanism and position of coupling

• Step 1: Diazonium formation

  • Sulphanilic acid $\text{H₂N–C₆H₄–SO₃H}$ reacts with NaNO₂ and HCl at low temperature to give the diazonium ion $\text{N₂⁺–C₆H₄–SO₃H}$.​

• Step 2: Azo coupling with resorcinol

  • Under mildly alkaline to neutral conditions, resorcinol forms a phenoxide, which is highly nucleophilic and attacks the diazonium ion in an electrophilic aromatic substitution step.​

  • For phenols, coupling normally occurs at the para position to the hydroxyl group; if para is blocked, the ortho position is used.​

  • In resorcinol (OH at positions 1 and 3), the most activated available site relative to one OH is the para position (position 4), giving a mono‑azo product rather than di‑ or tri‑azo under the single‑equivalent diazonium conditions described.​

Thus, a single azo bond connects the resorcinol ring at its para position to the sulphanilic acid ring bearing SO₃H, matching the structure in Option D.

Detailed analysis of each option

Option A

• Structure: Disazo compound where resorcinol appears to have two azo linkages (one from each activated position) attached to sulfonated rings.

• Problem: The question shows only one equivalent of sulphanilic acid undergoing diazotization, so only one diazonium electrophile is available, making a disazo product stoichiometrically impossible under the given conditions.​

• Additionally, forming two azo bonds on resorcinol requires either excess diazonium reagent or stepwise coupling under controlled pH, which is not indicated here.​

• Therefore Option A is not the major product.

Option B

• Structure: Appears to depict an azo dye where the resorcinol ring is tri‑substituted by OH groups (an extra OH is drawn) along with the azo linkage.

• Problem: No step in the reaction introduces an additional hydroxyl group; only diazotization and coupling occur, which install an $-N=N-$ linkage but not extra OH groups.​

• Hence the structure in Option B does not correspond to the actual transformation and is incorrect.

Option C

• Structure: Azo dye where the diazonium-derived ring seems unsubstituted (no SO₃H) while resorcinol bears the correct number of OH groups and an azo bond.

• Problem: The starting amine is sulphanilic acid, which necessarily carries an SO₃H group that remains on the aromatic ring after diazotization and coupling, imparting water solubility to the dye.​

• Option C omits this sulfonic acid substituent, so it cannot be the correct product.

Option D (Correct)

• Structure: Mono‑azo dye where

  • One ring is resorcinol with its two OH groups and a single azo linkage at the para position relative to one OH.

  • The other ring is the sulphanilic acid fragment retaining the SO₃H group, connected through the $-N=N-$ bond.

• Consistency:

  • Matches the expected electrophilic aromatic substitution pattern for coupling of diazotized sulphanilic acid with resorcinol under standard azo dye synthesis conditions, giving the well‑known mono‑azo “resorcinol yellow” type structure.​

  • Honors both stoichiometry (one diazonium equivalent) and directing effects (para‑coupling on the phenolic ring, sulfonic group preserved on the diazonium ring).​

• Therefore Option D correctly represents the major product.

SEO‑optimized introduction

Understanding the major product of diazotization of sulphanilic acid followed by coupling with resorcinol is essential for mastering aromatic electrophilic substitution and azo dye formation, topics frequently tested in competitive exams. This reaction illustrates how diazonium salts generated from sulphanilic acid selectively couple with activated phenols like resorcinol to yield brightly colored azo dyes used as acid dyes and pH indicators. By analyzing the mechanism and directing effects, the correct MCQ option can be identified confidently as the mono‑azo dye bearing a sulfonic acid group on the diazonium‑derived ring (Option D).​