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
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).
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:
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. …