Diazonium Salts

The diazonium salt question on NEET is a reagent-recall trap. You see a benzenediazonium chloride (C₆H₅N₂⁺Cl⁻) reacting with a copper salt or water, and the answer hinges on whether you remember which copper reagent gives which product. Confuse Sandmeyer with Gattermann and you lose four marks — the wrong option is designed to look right.

What is a diazonium salt? When a primary aromatic amine (like aniline) reacts with nitrous acid (NaNO₂ + HCl) at 273–278 K, the amino group is replaced by the diazonium group (–N₂⁺). The product, benzenediazonium chloride, is the gateway to a wide range of aromatic substitutions that are otherwise difficult or impossible by direct methods (NCERT Class 12 Chemistry Chapter 9, page 16).

Why is this synthetically important? The –N₂⁺ group can be replaced by –Cl, –Br, –CN, –I, –OH, –H, –F, and –NO₂ through specific named reactions. This makes diazonium chemistry the single most versatile route to substituted benzene rings in NCERT organic chemistry (NCERT Class 12 Chemistry Chapter 9, page 18).

The high-frequency trap — Sandmeyer vs. Gattermann reagent confusion. Both reactions replace –N₂⁺ with a halide, but the reagents differ:

• Sandmeyer reaction: ArN₂⁺ + CuCl → ArCl (or CuBr → ArBr, CuCN → ArCN). Reagent is cuprous salt in solution.
• Gattermann reaction: ArN₂⁺ + Cu powder + HCl → ArCl (or HBr → ArBr). Reagent is copper metal powder with the hydrogen halide.

Other key conversions: hydrolysis with warm water gives ArOH (phenol). Reaction with HBF₄ followed by heating gives ArF (Balz–Schiemann). Reaction with H₃PO₂ gives ArH (deamination). Azo coupling with phenol or aniline in alkaline medium gives azo dyes.

Watch out: NEET distractors routinely swap the Sandmeyer reagent (CuX) with the Gattermann reagent (Cu + HX). Anchor the distinction: Sandmeyer = cuprous salt; Gattermann = copper powder + acid.

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