The reagent ‘R’ in the given sequence of chemical reaction is :

The correct answer is: CH₃CH₂OH (Ethanol)

Let us analyze the reaction sequence shown in the image step by step.

Step 1: Identify the Starting Material
The starting compound is 2,4,6-tribromoanline — a benzene ring bearing an −NH₂ (amine) group and three bromine (Br) substituents at positions 2, 4, and 6.

Step 2: First Reaction — Diazotization
The amine is treated with NaNO₂ / HCl at 0–5°C. This is a classic diazotization reaction. The primary aromatic amine (−NH₂) reacts with nitrous acid (HNO₂, generated in situ from NaNO₂ and HCl) at low temperature to form a diazonium salt (−N₂⁺ Cl⁻).

So the intermediate product is 2,4,6-tribromobenzenediazonium chloride, as clearly shown in the image with the N₂⁺ Cl⁻ group attached to the ring.

Step 3: Second Reaction — Treatment with Reagent 'R'
The diazonium salt is then treated with the unknown reagent R, and the final product shown in the image is 1,3,5-tribromobenzene ��� a benzene ring with only three bromine atoms and no nitrogen-containing group. The −N₂⁺ Cl⁻ group has been replaced by a hydrogen atom (−H).

Step 4: Identify the Reaction Type
The replacement of the diazonium group (−N₂⁺) by a hydrogen atom (−H) is known as deamination or reductive de-diazotization. In this reaction, nitrogen gas (N₂) is evolved, and a C−H bond is formed where the C−N₂⁺ bond existed.

Step 5: Which Reagent Achieves This?
The standard reagent used to replace a diazonium group with hydrogen is ethanol (CH₃CH₂OH). When a diazonium salt is warmed with ethanol, the following occurs:

Ar−N₂⁺ Cl⁻ + CH₃CH₂OH → Ar−H + N₂↑ + CH₃CHO + HCl

Ethanol acts as a reducing agent (specifically, a hydride source). It donates a hydrogen atom to the aryl ring, replacing the diazonium group. In the process, ethanol itself gets oxidized to acetaldehyde (CH₃CHO), and nitrogen gas is released.

Why not the other options?

• H₂O — Water with a diazonium salt would give a phenol (Ar−OH), not Ar−H.
• HI — While HI can also replace −N₂⁺ with −H via a radical pathway, the classical and most widely recognized reagent for this transformation in standard organic chemistry is ethanol.
• CuCN/KCN — This is a Sandmeyer reaction reagent that would replace −N₂⁺ with −CN (a nitrile group), not −H.

Therefore, the reagent R is CH₃CH₂OH (Ethanol).