Question
Explain the coupling reaction of benzene diazonium chloride with phenol. What is the product formed and why is this reaction important?
(NCERT Class 12, Chapter 13)
Solution — Step by Step
Benzene diazonium chloride is prepared by treating aniline with NaNO + HCl at 0–5°C (diazotisation):
The low temperature is critical — diazonium salts decompose above 5°C.
Benzene diazonium chloride reacts with phenol in weakly alkaline medium (NaOH) to form a brightly coloured azo compound:
The product is p-hydroxyazobenzene — an orange-yellow azo dye. The diazonium ion (a weak electrophile) attacks the para position of the activated phenol ring.
Phenol is a weak activating group. In acidic medium, it doesn’t activate the ring enough for the weak diazonium electrophile to attack. In alkaline medium, phenol converts to phenoxide ion (CHO), which is a much stronger activating group. This makes the coupling reaction feasible.
However, strongly alkaline conditions destroy the diazonium salt (converting it to diazotate). So we use weakly alkaline conditions — a careful balance.
Why This Works
The coupling reaction is an electrophilic aromatic substitution where the diazonium ion () acts as the electrophile. It is a weak electrophile, so it can only attack highly activated aromatic rings — like phenols (in alkaline medium) and aromatic amines.
The -N=N- group (azo group) is a chromophore that absorbs visible light, giving the product its characteristic colour. This is the basis of the azo dye industry — one of the most important classes of synthetic dyes.
Alternative Method — Coupling with Aniline
Diazonium salts also couple with aniline in weakly acidic medium:
Product: p-aminoazobenzene (yellow dye). Note: strongly acidic conditions would protonate the amine, deactivating it.
The coupling always happens at the para position. If the para position is blocked, coupling occurs at the ortho position. If both are blocked, coupling fails. This selectivity is a frequent JEE question.
Common Mistake
Students forget the temperature condition. Diazonium salts are prepared and used at 0–5°C. At higher temperatures, they decompose to phenols (with water) or aryl fluorides/chlorides (Balz-Schiemann or Sandmeyer reactions). If a question mentions “room temperature” with a diazonium salt, decomposition has likely occurred.