Q.Complete the following reaction: phenol (C6H5OH) is treated with an aryldiazonium chloride, Ar–N2+ Cl−, in the presence of hydroxide ion (OH−, mildly alkaline medium). Give the product.
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:
Ar–NH2+NaNO2+2HCl0−5∘CAr–N2+Cl−+NaCl+2H2O
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.
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).
This is an azo coupling. The aryldiazonium ion is a weak electrophile that attacks the electron-rich phenol ring, preferentially at the para position, to give a coloured azo compound. …
An aryldiazonium salt couples with phenol in mild alkali by electrophilic aromatic substitution at the position para to –OH, giving a p-hydroxyazobenzene (an orange azo dye).
Concept – Azo coupling
The diazonium cation Ar–N2+ is a weak electrophile. It reacts only with strongly activated rings such as phenols and aromatic amines. In mild alkali the phenol is converted to the phenoxide ion (–O−), which is even more activating, so coupling occurs readily.
Regiochemistry
The –OH (or –O−) group directs the incoming diazonium electrophile ortho/para; the para position is preferred (less steric hindrance). The nitrogen of the diazonium becomes the –N=N– (azo) link.
Method: Azo Coupling of a Diazonium Salt with an Activated Arene
Core Concept
An aryldiazonium ion is a weak electrophile that can only attack very electron-rich (strongly activated) rings such as phenols or aromatic amines; in mild alkali the substrate becomes even more activated, and coupling proceeds by ordinary electrophilic aromatic substitution to give a coloured azo compound.
Steps
Identify the electrophile: Ar-N2+ (weak, because the positive charge is delocalised over the N=N system).
Identify the substrate and check it is strongly activating (phenol/aniline-type ring) - an ordinary benzene ring is too unreactive to couple.
Note the effect of the mild alkaline medium: phenol (C6H5OH) is converted to the more nucleophilic phenoxide ion (C6H5O-), which activates the ring further and drives the coupling.
Apply the directing rule of -OH/-O-: an ortho/para director; predict attack mainly at the position para to -OH (less hindered than ortho). …
(a) Benzene ring with –N2Cl group (benzenediazonium chloride)
(b) Benzene ring with –NO2 group (nitrobenzene)
(c) Benzene ring with –OH group (phenol)
(d) Benzene ring with –NHOH group (N-phenylhydroxylamine)
›Reveal solutionSolution
Step (i) is diazotisation of aniline to benzenediazonium chloride at 0–5°C; step (ii), warming this salt with water, hydrolyses the diazonium group to give phenol, releasing N2 and HCl — exactly matching the byproducts stated in the question.
Q.In the reaction shown (an aryl diazonium salt bearing -NO2 and -CH3 groups is converted, by reagent 'A', into the corresponding compound with the diazonium group replaced by -H), 'A' is –
(a) H+/H2O
(b) Zn, NH4Cl
(c) CuCl/HCl
(d) H3PO2, H2O
›Reveal solutionSolution
Replacing the diazonium group (-N2+) with -H (reductive deamination) is carried out using hypophosphorous acid (H3PO2) and water.
An aryl diazonium salt, Ar-N2+Cl-, can be converted to the corresponding arene Ar-H by treatment with H3PO2 (hypophosphorous acid) and H2O. H3PO2 acts as a mild reducing agent, replacing -N2+ with -H and releasing N2 gas: