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Most students can write "benzenediazonium chloride gives chlorobenzene". Fewer can say which copper reagent the Sandmeyer reaction uses, how Gattermann's version differs, or why a diazonium salt lets you put a group where direct substitution never would. This post sorts out the named reactions, flags the most common mix-up, and works two syntheses and a yield calculation.
What exactly does NCERT say?
NCERT's Class 12 unit Amines (2026-27 reprint) states that Cl⁻, Br⁻ and CN⁻ "can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction." Chlorine or bromine can also be introduced "by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder", which NCERT calls the Gattermann reaction, adding that "the yield in Sandmeyer reaction is found to be better". The Haloalkanes and Haloarenes unit repeats the Sandmeyer route with cuprous chloride or bromide and notes that iodine needs no cuprous halide: shaking the diazonium salt with potassium iodide is enough.
The names are historical. Traugott Sandmeyer's 1884 paper reported replacing the amino group by chlorine, bromine and cyanide in aromatic compounds, and Ludwig Gattermann published his studies on diazo compounds in 1890.
Is it in your exam?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus both list "Diazonium Salts: Importance in synthetic organic chemistry". The JEE (Advanced) 2026 brochure is more specific: "Sandmeyer and related reactions of diazonium salts".
How do you tell the reactions apart?
All of them start from a diazonium salt made at 273 to 278 K from the amine, sodium nitrite and HCl:
Sandmeyer: CuCl/HCl gives ArCl, CuBr/HBr gives ArBr, CuCN/KCN gives ArCN. The copper is copper(I).
Gattermann: copper powder with HCl or HBr gives ArCl or ArBr, in lower yield.
No copper needed: KI gives ArI. Fluoroboric acid precipitates the diazonium fluoroborate, which on heating gives ArF.
Other replacements in NCERT: warming to 283 K gives phenol; H₃PO₂ or ethanol replaces the group with H.
Worked example 1: p-toluidine to 4-methylbenzoic acid
Diazotise p-toluidine (4-methylaniline) with NaNO₂ and HCl at 273 to 278 K.
Add CuCN/KCN (Sandmeyer): 4-methylbenzonitrile.
Hydrolyse the nitrile with acid or alkali. NCERT's aldehydes, ketones and carboxylic acids unit notes that nitriles are hydrolysed to amides and then to acids. Product: 4-methylbenzoic acid.
Why not swap a chlorine for CN instead? NCERT is direct: "the cyano group cannot be introduced by nucleophilic substitution of chlorine in chlorobenzene", but cyanobenzene "can be easily obtained from diazonium salt".
Worked example 2: p-toluidine to 3-bromotoluene
Brominating toluene directly gives ortho and para products. To get bromine meta to the methyl group, borrow the amino group as a temporary director.
Acetylate with acetic anhydride. NCERT describes this as the way to control the strong activating effect of –NH₂.
Brominate. The –NHCOCH₃ group directs ortho to itself (its para position is taken by CH₃), which puts the bromine meta to CH₃.
Hydrolyse the amide back to the amine.
Diazotise at 273 to 278 K.
Treat with H₃PO₂, which replaces the diazo group with H. The product is 3-bromotoluene.
Here the diazo group is removed, not replaced by a halogen, but the logic is the same: an –NH₂ group places substituents and then leaves.
Worked example 3: how much product?
Sample numbers. You start with 9.30 g of aniline (C₆H₇N, 93.13 g/mol) and run a Sandmeyer bromination with a 70% yield.
Aniline = 9.30 ÷ 93.13 = 0.0999 mol.
Every step is 1 : 1, so the theoretical bromobenzene (157.01 g/mol) is 0.0999 × 157.01 = 15.7 g.
At 70%, you collect 0.70 × 15.7 = 11.0 g.
The same 0.0999 mol of diazonium salt releases 0.0999 mol of N₂, which is why these reactions fizz.
Which mistakes cost marks?
Writing CuCl₂ for Sandmeyer. The reagent is a copper(I) salt.
Adding a copper salt for iodide. KI alone does it.
Letting the diazonium solution warm up, which gives phenol instead of the halide.
Storing the diazonium salt: NCERT says it "is used immediately after its preparation".
Confusing Gattermann with Gattermann-Koch.
For conversion questions, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can check each step of your route.
A practice routine
Make a five-row card: reagent, product, named reaction, copper form, temperature.
Do NCERT Exercise 9.8(i), nitrobenzene to benzoic acid, using worked example 1 as the template.
Plan 3-chlorotoluene from p-toluidine using worked example 2.
Repeat worked example 3 for chlorobenzene and benzonitrile.
For basicity and the rest of the amines chapter, see our guide to amines and diazonium salts. Tonight, write the five-row card from memory and check it against NCERT.
