JEE Mains Chemistry · Amines

Diazonium Salts: Stability and Replacement Reactions

A primary aryl amine with nitrous acid in the cold gives an arenediazonium salt, whose N₂ group can be replaced by halogen, cyanide, OH or H — so NH₂ can direct a substitution and then be removed.

All Amines notes JEE Mains Chemistry strategy

Why this matters

Twenty-nine PYQs, three numerical, six from 2026, the largest page in the chapter. Four rank diazonium salts by stability; twelve ask what a reagent puts in place of the diazo group; thirteen ask for the reagents, in order, of a synthesis that uses the amino group as a temporary director.

Concept 1 of 3: Diazotisation and the stability of diazonium salts

Nitrous acid, made in the cold from sodium nitrite and hydrochloric acid, converts

\mathrm{ArNH_2}

into

\mathrm{ArN_2^+}

. On an aryl group the positive charge is spread into the ring, so the salt survives in cold solution. On an alkyl group there is no such spreading, and the ion loses

\mathrm{N_2}

at once. Groups that push electrons into the ring spread the charge further and make the salt more stable.

Definition

Diazotisation is done at 273–278 K and the salt is used at once; on warming the solution gives phenol and $\mathrm{N_2}$ .
Arenediazonium ions are stabilised by resonance with the ring; alkanediazonium ions are not.
Stability of para-substituted salts: electron donors raise it, electron acceptors lower it, for example $\mathrm{OCH_3 > CH_3 > H > NO_2}$ .
The nitrogen must be on the ring: benzylamine behaves as an aliphatic amine and gives no stable diazonium salt.
Benzenediazonium fluoroborate, $\mathrm{C_6H_5N_2^+BF_4^-}$ , is stable enough to isolate as a solid.
o-Phenylenediamine with nitrous acid diazotises one $\mathrm{NH_2}$ , which is captured by the other to give benzotriazole.

Diazotisation of aniline

\mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273{-}278\ K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O}

Worked example

Arrange in decreasing stability: 4-methylbenzenediazonium chloride, benzenediazonium chloride and ethanediazonium chloride.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 24 Jan 2026 Shift 1 · Q32Moderate

Example 1 · Amines · Diazonium Salts: Stability and Replacement Reactions

The correct stability order of the following diazonium salts is (A) (B) (C) (D)

Electron-withdrawing groups destabilise the diazonium salt

A para nitro or cyano group pulls electrons away from a ring that is already carrying a positive charge, so the salt becomes less stable. A para methoxy group does the opposite.

Only a nitrogen on the ring gives a stable salt

Diazotisation at 273–278 K gives a usable salt only from a primary aromatic amine. Aliphatic amines, benzylamine included, lose nitrogen at once.

Drill 3 more on diazotisation and the stability of diazonium salts

Concept 2 of 3: Replacement reactions of arenediazonium salts

\mathrm{N_2}

is one of the best leaving groups in chemistry, so the diazo group can be swapped for almost anything. Each replacement has its own reagent, and questions test the pairing: which reagent gives which product, and which halogens a copper(I) salt can deliver.

Definition

Sandmeyer: CuCl/HCl, CuBr/HBr or CuCN/KCN give ArCl, ArBr or ArCN. Gattermann: copper powder with HCl or HBr does the same for Cl and Br.
Iodine needs no catalyst: KI gives ArI.
Fluorine: $\mathrm{HBF_4}$ gives the fluoroborate, which on heating gives ArF (Balz–Schiemann).
Warm water gives phenol.
Reduction to ArH: hypophosphorous acid ( $\mathrm{H_3PO_2}$ ) or ethanol; ethanol is oxidised to ethanal.
ArCN can be hydrolysed to ArCOOH, which is a way to put COOH on a ring.

Reagent	Product from ArN₂⁺	Name or note
$\mathrm{CuCl/HCl}$	ArCl	Sandmeyer
$\mathrm{CuBr/HBr}$	ArBr	Sandmeyer
$\mathrm{CuCN/KCN}$	ArCN	Sandmeyer
Cu powder with HCl or HBr	ArCl or ArBr	Gattermann
KI	ArI	No copper needed
$\mathrm{HBF_4}$ , then heat	ArF, with $\mathrm{BF_3}$ and $\mathrm{N_2}$	Balz–Schiemann
$\mathrm{H_2O}$ , warm	ArOH	Phenol and $\mathrm{N_2}$
$\mathrm{H_3PO_2}$ and $\mathrm{H_2O}$	ArH	Reductive removal; $\mathrm{H_3PO_3}$ forms
$\mathrm{CH_3CH_2OH}$	ArH	Ethanol is oxidised to ethanal

Sandmeyer delivers Cl, Br and CN only; F and I come by other reagents.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 2 · Q42Moderate

Example 2 · Amines · Diazonium Salts: Stability and Replacement Reactions

Consider the following organic reaction sequence. Choose the final product (X) from the following (consider the major product in all intermediate reactions)

Ethanol reduces, it does not make an ether

Benzenediazonium chloride with ethanol gives benzene and ethanal. A reaction claiming phenetole,

\mathrm{C_6H_5OC_2H_5}

, as the product is wrong.

Fluoro- and iodobenzene are not Sandmeyer products

Copper(I) salts deliver chloride, bromide and cyanide. Iodobenzene comes from KI; fluorobenzene comes from the fluoroborate on heating.

Drill 11 more on replacement reactions of arenediazonium salts

Concept 3 of 3: Synthesis planning with diazonium salts: the amino group as a temporary director

The amino group, and the nitro group it comes from, can direct a substitution to a place no other group would, and then be replaced or removed through the diazonium salt. So the order of steps is the whole question: ask which group is on the ring at the moment each substitution happens, and what it directs.

Definition

$\mathrm{NO_2}$ directs meta; $\mathrm{NH_2}$ (or $\mathrm{NHCOCH_3}$ ) directs ortho and para. Reducing $\mathrm{NO_2}$ to $\mathrm{NH_2}$ switches the director.
Brominate a nitro compound first to put Br meta; reduce, diazotise, then replace the $\mathrm{N_2^+}$ .
Brominate the amine to put Br ortho and para to it; then remove the $\mathrm{N_2^+}$ with $\mathrm{H_3PO_2}$ , leaving a pattern that no direct bromination could give.
Protect the amine (acetylate) when only one bromine is wanted.
A side-chain $\mathrm{CH_3}$ can be oxidised to COOH by $\mathrm{KMnO_4}$ , usually as a late step, because COOH directs meta.

Removing the amino group after it has directed

\mathrm{ArNH_2 \xrightarrow{NaNO_2,\ HCl,\ 273\ K} ArN_2^+Cl^- \xrightarrow{H_3PO_2,\ H_2O} ArH + N_2}

Worked example

Give the reagents, in order, to make 3-bromophenol from nitrobenzene.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 13 April 2023 · Q129Moderate

Example 3 · Amines · Diazonium Salts: Stability and Replacement Reactions

Compound

A

from the following reaction sequence is:

The director is whatever is on the ring at that step

The same nitrogen directs meta as

\mathrm{NO_2}

and ortho-para as

\mathrm{NH_2}

. A sequence that reduces before brominating puts the bromine in a different place from one that brominates first.

Removing NH₂ leaves the pattern it created

After

\mathrm{H_3PO_2}

removes the diazo group, the substituents stay where the amino group sent them. Two bromines placed ortho and para to an

\mathrm{NH_2}

are meta to each other once the

\mathrm{NH_2}

is gone.

Drill 12 more on synthesis planning with diazonium salts: the amino group as a temporary director

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Diazotisation and the stability of diazonium salts
Diazotisation of aniline
$\mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273{-}278\ K} C_6H_5N_2^+Cl^- + NaCl + 2H_2O}$
Synthesis planning with diazonium salts: the amino group as a temporary director
Removing the amino group after it has directed
$\mathrm{ArNH_2 \xrightarrow{NaNO_2,\ HCl,\ 273\ K} ArN_2^+Cl^- \xrightarrow{H_3PO_2,\ H_2O} ArH + N_2}$

Reference tables (1)

Replacement reactions of arenediazonium salts9 rows

Reagent	Product from ArN₂⁺	Name or note
$\mathrm{CuCl/HCl}$	ArCl	Sandmeyer
$\mathrm{CuBr/HBr}$	ArBr	Sandmeyer
$\mathrm{CuCN/KCN}$	ArCN	Sandmeyer
Cu powder with HCl or HBr	ArCl or ArBr	Gattermann
KI	ArI	No copper needed
$\mathrm{HBF_4}$ , then heat	ArF, with $\mathrm{BF_3}$ and $\mathrm{N_2}$	Balz–Schiemann
$\mathrm{H_2O}$ , warm	ArOH	Phenol and $\mathrm{N_2}$
$\mathrm{H_3PO_2}$ and $\mathrm{H_2O}$	ArH	Reductive removal; $\mathrm{H_3PO_3}$ forms
$\mathrm{CH_3CH_2OH}$	ArH	Ethanol is oxidised to ethanal

Sandmeyer delivers Cl, Br and CN only; F and I come by other reagents.

Watch out for (6)

Electron-withdrawing groups destabilise the diazonium salt
→ Diazotisation and the stability of diazonium salts
Only a nitrogen on the ring gives a stable salt
→ Diazotisation and the stability of diazonium salts
Ethanol reduces, it does not make an ether
→ Replacement reactions of arenediazonium salts
Fluoro- and iodobenzene are not Sandmeyer products
→ Replacement reactions of arenediazonium salts
The director is whatever is on the ring at that step
→ Synthesis planning with diazonium salts: the amino group as a temporary director
Removing NH₂ leaves the pattern it created
→ Synthesis planning with diazonium salts: the amino group as a temporary director

Test yourself on Amines

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Amines chapter test Drill all 29 questions