Q.Give the structure of 'A' in the following reaction sequence. 4-methyl-2-nitroaniline (an aniline bearing a –CH3 group para to the –NH2 and an –NO2 group ortho to the –NH2) is treated with
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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.
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 |
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:
C6H5–N2+Cl−+C6H5–OHNaOH, 0–5∘CC6H5–N=N–C6H4–OH (p-hydroxyazobenzene, orange dye)
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? …
NaNO2/HCl at 273–278 K diazotises the –NH2 group; H3PO2 (hypophosphorous acid) then reductively removes the diazonium group, replacing it with –H (deamination). The –CH3 and –NO2 that remain are meta to each other. …
The amino group is turned into a diazonium salt and then replaced by hydrogen. Removing –NH2 from 4-methyl-2-nitroaniline leaves –CH3 and –NO2 meta to each other, so A is m-nitrotoluene.
Step 1 – Diazotisation
4-Methyl-2-nitroaniline + NaNO2 + HCl at 273–278 K converts the –NH2 into a diazonium group:
Ar–NH2 → Ar–N2+ Cl− (4-methyl-2-nitrobenzenediazonium chloride).
Step 2 – Deamination with H3PO2
Hypophosphorous acid (H3PO2) in water reduces the diazonium group, replacing –N2+ by –H and releasing N2:
Ar–N2+ + H3PO2 + H2O → Ar–H + N2 + H3PO3 + HCl.
Result …
Method: Diazotisation Followed by Reductive Deamination (H3PO2)
Core Concept
An aromatic -NH2 group can be converted into a diazonium salt and then reductively replaced by -H using hypophosphorous acid (H3PO2) - a two-step "deamination" used to remove an amino group after it has served its purpose (e.g. as a positional director), leaving the remaining substituents in a new relative arrangement.
Steps
- Diazotise the aromatic amine: ArNH2 + NaNO2/HCl at 273-278 K gives ArN2+ Cl-. The low temperature is essential - the diazonium salt decomposes above about 283 K.
- Treat the diazonium salt with H3PO2 and H2O: this reducing agent replaces -N2+ with -H, releasing N2 gas: ArN2+ + H3PO2 + H2O gives ArH + N2 + H3PO3 + HCl.
- Renumber the ring in the product based on the substituents that remain, since the position that carried -NH2 is now simply -H. …
- AHSEC Higher Secondary (HS) Final Examination 2024Set ANNUAL2 marksQ.Diazonium salts of aromatic amines are more stable than those of aliphatic amines. Why? OR Why do primary amines have higher boiling point than tertiary amines?
›Reveal solutionSolution
Ring conjugation stabilises the aryl diazonium cation (main question); alternatively, N–H hydrogen bonding explains why 1° amines boil higher than 3° amines (OR part).
Main answer — stability of diazonium salts:
In an aromatic diazonium salt, Ar–N2⁺, the positive charge on the terminal nitrogen can be delocalised into the aromatic ring through resonance (the –N≡N⁺ group is conjugated with the π-system of the ring), spreading the charge over several atoms and lowering the overall energy of the cation. This resonance stabilisation allows aryl diazonium salts (e.g. C6H5N2⁺Cl⁻) to be isolated and used in solution at low temperature (0–5 °C).
Aliphatic diazonium salts (R–N2⁺, R = alkyl) have no adjacent π-system to delocalise the charge into. They are extremely unstable and decompose the instant they form, expelling N2 gas and generating a highly reactive alkyl carbocation (R⁺), which is why aliphatic diazonium salts cannot be isolated.
OR — boiling points of 1° vs 3° amines:
A primary amine, R–NH2, has two N–H bonds, so its molecules can form extensive intermolecular hydrogen bonds with each other (N–H···N), requiring more energy to separate the molecules on boiling. …
- AHSEC Higher Secondary (HS) Final Examination 2022Set ANNUAL2 marksQ.Identify A and B in the following reaction scheme: Aniline (benzene ring with -NH2) --NaNO2/HCl, 0-5°C--> A --OH- --> B, where B is shown as a benzene ring bearing an -OH group.
›Reveal solutionSolution
Aniline is diazotised at 0-5°C to give benzenediazonium chloride (A), which is then hydrolysed in aqueous acid/base to give phenol (B).
Step 1 — Diazotisation (formation of A):
Aniline reacts with NaNO2 and HCl at a low temperature (0-5°C, necessary because diazonium salts are unstable and decompose above this range):
C6H5NH2 + NaNO2 + 2HCl —0-5°C→ C6H5N2⁺Cl⁻ (A, Benzenediazonium chloride) + NaCl + 2H2O
Step 2 — Hydrolysis (formation of B): …
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