Explain Coupling reaction.

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Q. Explain Coupling reaction.

Punjab PsebPSEB Punjab Class 12 Board 2026Subjective· 2mImportance★★★★★

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Concept understanding — Diazonium Salt Reactions

Diazonium Salt Reactions – A First Look

Imagine you have a benzene ring, and you want to attach a new group — say a chlorine, a bromine, a cyano group, or even a hydroxyl — directly onto the ring. The benzene ring is stubborn; it doesn't easily let go of its hydrogen atoms for simple substitution. But there is a clever trick: first convert the ring into a diazonium salt, a highly reactive intermediate that will let you swap in almost any group you want.

That is the core idea. A diazonium salt is a temporary, energetic handle on the benzene ring that you can then replace with a wide variety of substituents. It is one of the most powerful tools in aromatic synthesis.

What is a Diazonium Salt?

A diazonium salt has the general formula Ar–N₂⁺ X⁻, where Ar is an aryl group (like phenyl, C₆H₅–), N₂⁺ is a diazonium cation (two nitrogen atoms triple-bonded, with a positive charge on the terminal nitrogen), and X⁻ is a counterion like chloride, bromide, or hydrogensulfate.

The key structural feature: the –N₂⁺ group is attached directly to the benzene ring. This group is unstable — it wants to leave as N₂ gas. That instability is exactly what makes it useful: when the N₂ leaves, the ring is left with a highly reactive carbocation-like intermediate that can be attacked by a nucleophile.

Watch out

Diazonium salts are thermally unstable and can explode if dried. They are almost always prepared and used in cold solution (0–5 °C) without isolation.

How Do You Make One? (Diazotization)

You start with a primary aromatic amine (Ar–NH₂). Treat it with nitrous acid (HNO₂) at low temperature (0–5 °C). The reaction is:

\text{Ar–NH}_2 + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0-5^\circ\text{C}} \text{Ar–N}_2^+ \text{Cl}^- + \text{NaCl} + 2\text{H}_2\text{O}

The nitrous acid is generated in situ from sodium nitrite and a mineral acid. The amine gets converted into the diazonium salt almost instantly. You must keep the solution cold; if it warms up, the diazonium salt decomposes and you get phenol and nitrogen gas.

Two Major Classes of Reactions

Once you have the diazonium salt in solution, you can do two fundamentally different things with it:

1. Substitution Reactions (N₂ leaves)

Here the –N₂⁺ group is replaced by another group. The nitrogen gas bubbles away, and the ring gets a new substituent. This is called dediazoniation. The leaving group is N₂, which is extremely stable, so the reaction is thermodynamically driven.

The most common substitutions:

Reagent/Condition	Product	Name
CuCl / HCl, heat	Ar–Cl	Sandmeyer reaction
CuBr / HBr, heat	Ar–Br	Sandmeyer reaction
CuCN / KCN, heat	Ar–CN	Sandmeyer reaction
KI, heat	Ar–I	Direct substitution
H₂O, heat	Ar–OH	Hydrolysis
H₃PO₂ (hypophosphorous acid)	Ar–H	Reduction (replaces N₂ with H)
Cu₂O, Cu(NO₃)₂, H₂O	Ar–NO₂	Replacement with nitro group

Tip

The Sandmeyer reaction uses copper(I) halide or cyanide as a catalyst. The copper helps transfer the halide or cyanide to the ring. Without copper, the reaction is much slower or gives different products.

The mechanism for Sandmeyer: the diazonium salt accepts an electron from Cu⁺, forming an aryl radical, which then abstracts a halogen from CuX₂. The N₂ leaves as a gas.

2. Coupling Reactions (N₂ stays)

Here the diazonium salt keeps its N₂ group and attacks another aromatic ring (usually an activated one like phenol or aniline). The result is an azo compound with the general structure Ar–N=N–Ar'. These compounds are intensely coloured — many are used as dyes.

The reaction is an electrophilic aromatic substitution. The diazonium cation is a weak electrophile, so it only attacks rings that are strongly activated (with –OH, –NH₂, –NHR, –NR₂ groups). The coupling occurs at the para position if available; otherwise ortho.

Example: coupling with phenol in alkaline medium:

\text{C}_6\text{H}_5\text{–N}_2^+ \text{Cl}^- + \text{C}_6\text{H}_5\text{–OH} \xrightarrow{\text{NaOH, 0–5}^\circ\text{C}} \text{C}_6\text{H}_5\text{–N=N–C}_6\text{H}_4\text{–OH} \text{ (p-hydroxyazobenzene, orange dye)}

Important

Coupling requires the coupling component (phenol or aniline) to be in its reactive form: phenol is used in alkaline solution (phenoxide ion is more activating), aniline is used in slightly acidic or neutral solution (to avoid protonation of the amino group).

Why Are Diazonium Salts So Versatile? …

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A coupling reaction joins a diazonium salt to an activated aromatic ring (phenol/amine) through an azo (-N=N-) linkage, giving coloured azo dyes.

Aromatic diazonium salts, $Ar-N_2^+X^-$ , act as weak electrophiles. When reacted at low temperature ( $0$ – $5°C$ ) with electron-rich aromatic compounds such as phenols or aromatic amines, the diazonium ion attacks the ring (usually at the position para to the activating $-OH$ / $-NH_2$ group) in an electrophilic aromatic substitution, forming an azo compound:

$C_6H_5-N_2^+Cl^- + C_6H_5OH \xrightarrow{cold,\ mild\ base} C_6H_5-N=N-C_6H_4-OH\ (p\text{-hydroxyazobenzene})$ …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 35 on this concept.

CBSE 2026Set V11 markMCQ
$Q. p-Hydroxyazobenzene is formed by the reaction of benzene diazonium chloride with phenol. It is$
(a) an electrophilic substitution reaction
(b) a nucleophilic substitution reaction
(c) a hydrogenation reaction
(d) a halogenation reaction
›Reveal solutionSolution

Formation of p-hydroxyazobenzene by coupling of benzene diazonium chloride with phenol is an electrophilic aromatic substitution.

Benzene diazonium chloride, $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$ , reacts with phenol in mildly alkaline medium to give the orange azo dye p-hydroxyazobenzene:

$\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + \text{C}_6\text{H}_5\text{OH} \rightarrow p\text{-}\text{HO-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_5 + \text{HCl}$ …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Aqueous solution of benzene diazonium chloride in presence of dil. H2SO4 is heated to produce:$
(a) Aniline
(b) Benzene
(c) Chlorobenzene
(d) Phenol
›Reveal solutionSolution

Heating an aqueous benzenediazonium salt solution hydrolyses it to phenol, releasing nitrogen gas.

Benzenediazonium chloride, $C_6H_5-N_2^+Cl^-$ , is thermally unstable in aqueous acidic solution. When its aqueous solution is warmed (with dilute $H_2SO_4$ or simply with water), the diazonium group is displaced by a water molecule in a nucleophilic substitution, with loss of nitrogen gas:

$C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2\uparrow + HCl$

…

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CBSE 2026Set ANNUAL1 mark
$Q. Write general formula of diazonium salt.$
›Reveal solutionSolution

Diazonium salts have the general formula R-N≡N+ X−, formed by diazotisation of a primary aromatic amine with NaNO2/HCl at 0-5°C.

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The reagent X in the reaction C₆H₅NH₂ (aniline) —X / 273K\to C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride) is -$
(a)
(i) HNO₃
(b)
(ii) NaNO₂ and HCl
(c)
(iii) NaCl and HNO₃
(d)
(iv) NaNO₂ and H₂SO₄
›Reveal solutionSolution

The conversion of aniline to benzenediazonium chloride at 273 K (diazotisation) uses nitrous acid generated from $\text{NaNO}_2 + \text{HCl}$ . Correct option: (ii).

Concept. Diazotisation is the reaction of a primary aromatic amine with nitrous acid ( $\text{HNO}_2$ ) at low temperature (273–278 K) to form a diazonium salt. Nitrous acid is unstable, so it is prepared in situ by adding sodium nitrite ( $\text{NaNO}_2$ ) to a mineral acid.

Why HCl specifically (option ii not iv). The anion of the diazonium salt required here is chloride ( $\text{C}_6\text{H}_5\text{N}_2^{+}\text{Cl}^{-}$ ), so HCl must be the acid supplying $\text{Cl}^{-}$ ; $\text{H}_2\text{SO}_4$ would give the sulphate salt.

Reaction. …

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CBSE 2025Set 56/4/11 markMCQ
$C_6H_5NH_2$
›Reveal solutionSolution
Aniline undergoes diazotization to form a benzenediazonium salt (X), which then reacts with ethanol to undergo reductive dediazoniation, replacing the diazonium group with hydrogen to yield benzene (Y). The correct option is (B).

This problem tests your understanding of two classic reactions in aromatic chemistry: diazotization and the replacement of a diazonium group. The key is to recognize that ethanol here acts as a reducing agent, not as a nucleophile.

Let’s walk through the sequence step by step.

Step 1: Diazotization of aniline Aniline ( $C_6H_5NH_2$ ) is treated with sodium nitrite ( $NaNO_2$ ) and hydrochloric acid ( $HCl$ ) at a low temperature ( $0-5^\circ C$ ). This is the standard condition for forming a diazonium salt. The reaction proceeds as:

$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$
The product X is benzenediazonium chloride ( $C_6H_5N_2^+Cl^-$ ). This is a highly reactive intermediate, stable only in cold solution.

Watch out

A common mistake is to think that the diazonium salt itself is the final product Y. But the reaction sequence continues — X is just an intermediate.

Step 2: Reaction of the diazonium salt with ethanol When benzenediazonium chloride (X) is treated with ethanol ( $C_2H_5OH$ ), a reductive dediazoniation occurs. Ethanol acts as a reducing agent, donating a hydride ion ( $H^-$ ) or a hydrogen atom to replace the diazonium group. The overall transformation is: $C_6H_5N_2^+Cl^- + C_2H_5OH \longrightarrow C_6H_6 + N_2 + CH_3CHO + HCl$ …
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CBSE 2025Set ANNUAL1 markMCQ
$Q. When an aqueous solution of benzenediazonium chloride is boiled with water or steam distilled the product formed is:$
(a) Benzene
(b) Diphenyl
(c) Chlorobenzene
(d) Phenol.
›Reveal solutionSolution

Aqueous benzenediazonium chloride is thermally unstable; warming or steam-distilling it causes hydrolysis, replacing the $-N_2^+Cl^-$ group with $-OH$ and releasing nitrogen gas.

Benzenediazonium chloride ( $C_6H_5-N_2^+Cl^-$ ), when its aqueous solution is warmed/boiled (or steam distilled), undergoes hydrolysis. The diazonium group is displaced by a hydroxyl group from water, with loss of nitrogen gas and hydrochloric acid:

$C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2\uparrow + HCl$

…

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CBSE 2025Set ANNUAL1 mark
$C_6H_5-N_2^+X^-$
›Reveal solutionSolution

Treating a benzenediazonium salt with KI substitutes the diazonium group directly by iodide, giving iodobenzene and nitrogen gas — no copper catalyst is required for this particular halogen (unlike Cl/Br via Sandmeyer).

Aromatic diazonium salts undergo nucleophilic substitution at the carbon bearing the $-N_2^+$ group. Iodide is a good enough nucleophile/reducing species that this substitution proceeds simply on warming with aqueous KI, without needing Cu(I)/Cu(III) catalysis (which is required to introduce –Cl or –Br via the Sandmeyer reaction):

$C_6H_5-N_2^+X^- + KI \rightarrow C_6H_5-I + N_2\uparrow + KX$

…

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CBSE 2025Set ANNUAL1 mark
$Q. Give one chemical test to distinguish between an aromatic primary amine from an aliphatic primary amine.$
›Reveal solutionSolution

Bromine water reacts instantly with an aromatic primary amine like aniline to give a white precipitate of 2,4,6-tribromoaniline, because the –NH $_2$ group strongly activates the benzene ring; an aliphatic primary amine has no such ring to brominate and gives no precipitate.

In an aromatic primary amine such as aniline ( $C_6H_5NH_2$ ), the lone pair on nitrogen is delocalised into the benzene ring by resonance, making the ring extremely electron-rich and strongly activating it towards electrophilic aromatic substitution — so strongly that even mild aqueous bromine (no catalyst needed) substitutes at all three free ortho/para positions at once:

$C_6H_5NH_2 + 3Br_2(aq) \rightarrow 2,4,6\text{-tribromoaniline}\downarrow (\text{white ppt}) + 3HBr$

…

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CBSE 2025Set ANNUAL1 mark
$Q. Diazonium salts of aromatic amines are more stable than those of aliphatic amines. Give reason.$
›Reveal solutionSolution

The aromatic ring in benzenediazonium salts can share the positive charge by resonance, giving extra stability that keeps the salt intact at low temperature; aliphatic diazonium ions have no ring to delocalise into and lose $N_2$ almost as soon as they form.

A diazonium ion has the general structure $R-\overset{+}{N}\equiv N$ (or the resonance form $R-N=\overset{+}{N}$ ). In an aryl diazonium salt, $C_6H_5-N_2^+$ , the ring's π system can conjugate with the $-N\equiv N$ group, so the positive charge is not confined to the terminal nitrogen but is delocalised over the ring and the nitrogen atoms through resonance structures (e.g. with positive charge shifted onto an ortho or para ring carbon).

This delocalisation lowers the energy of the ion and makes the C–N bond to the ring stronger, so benzenediazonium chloride is reasonably stable and can be stored/handled in solution at 273–278 K (0–5 °C) and used in coupling/Sandmeyer reactions.

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Benzene diazonium chloride on hydrolysis gives$
(a) phenol
(b) chlorobenzene
(c) benzene
(d) aniline
›Reveal solutionSolution

Warming a diazonium salt in water lets water act as the nucleophile that displaces N2, an SN1-type substitution via the aryl cation-like transition state.

When an aqueous solution of benzenediazonium chloride is warmed, water acts as a nucleophile, displacing nitrogen gas from the diazonium group: C₆H₅N₂⁺Cl⁻ + H₂O --(Δ)--> C₆H₅OH (phenol) + N₂↑ + HCl. This is one of the most important synthetic uses of diazonium …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Aniline (C6H5-NH2, shown protonated as C6H5-NH2 with a positive-charge notation) is treated with NaNO2/dil. HCl at 0°-5°C to give A, and A is then treated with C6H5OH/OH⊖ to give B (major product). The major product B of the reaction is$
(a) Diphenyl ether, C6H5-O-C6H5
(b) C6H5-N=N-C6H4-OH (the azo-coupling product with -OH at the para position, drawn in-line)
(c) C6H5-N=N-C6H4-OH (the azo-coupling product with -OH shown branching off the second ring)
(d) C6H5-N=N-O-C6H5
›Reveal solutionSolution

Aniline first forms a diazonium salt (A) with $NaNO_2$ /HCl at 0-5°C; this then undergoes azo coupling with phenol to give p-hydroxyazobenzene (B) as the major product.

Step 1 (formation of A): Aniline reacts with $NaNO_2$ and dilute HCl at 0-5°C (diazotisation). At this low temperature the unstable diazonium salt survives without decomposing: $C_6H_5NH_2 + NaNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + NaCl + 2H_2O$ . So A = benzenediazonium chloride.

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. Aromatic primary amines and aliphatic primary amines can be distinguished by -$
(i) Hinsberg's test
(ii) Tollen's test
(iii) Azo dye test
(iv) Litmus test
›Reveal solutionSolution
Aromatic primary amines form a stable diazonium salt on treatment with HNO₂ at low temperature, which couples with a phenol (e.g. β-naphthol) to give a brightly coloured azo dye; aliphatic primary amines cannot do this because their diazonium salts are unstable and decompose instantly.

When a primary amine is treated with nitrous acid (NaNO₂ + HCl, 0–5°C):

Aromatic primary amines (e.g. aniline) form a reasonably stable diazonium salt (ArN₂⁺Cl⁻) at low temperature. This diazonium salt can be coupled with an alkaline solution of β-naphthol (or a similar phenol) to give a bright orange/red azo dye. This visible colour formation is a positive azo dye test. …
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Q.Explain Coupling reaction.

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