Q.Predict the missing reagents (labelled 1, 3 and 5) and the missing products (labelled 2 and 4) in the following reaction sequence. p-nitrotoluene (a benzene ring with –CH3 and –NO2 para to each other) is treated with reagent 1 to give p-toluidine (–CH3 and –NH2 para). p-Toluidine is treated with (CH3CO)2O / pyridine to give p-methylacetanilide (–CH3 and –NHCOCH3 para). p-Methylacetanilide is treated with HNO3/H2SO4 to give product 2. Product 2 is treated with reagent 3 to give 4-methyl-2-nitroaniline (–CH3 para to –NH2, with –NO2 ortho to the –NH2). 4-methyl-2-nitroaniline is treated with NaNO2/HCl to give compound 4. Compound 4 is treated with reagent 5 to give m-nitrotoluene (a benzene ring with –CH3 and –NO2 meta to each other).
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).
Reagent 1 reduces –NO2 to –NH2; the acetamido group directs nitration ortho, giving product 2; reagent 3 hydrolyses off the acetyl group; NaNO2/HCl makes the diazonium salt (compound 4); reagent 5 removes it by deamination. …
The sequence reduces the nitro group to an amine, protects it as the acetanilide, nitrates ortho to nitrogen, hydrolyses the protecting group, diazotises the amine, and finally removes it by deamination — shifting the substitution pattern to give m-nitrotoluene.
Reagent 1 – reduction
p-Nitrotoluene → p-toluidine requires reduction of –NO2 to –NH2. Reagent 1 = Sn/HCl (equivalently Fe/HCl or H2/catalyst).
Product 2 – nitration of p-methylacetanilide
p-Methylacetanilide has –CH3 and –NHCOCH3 para. On nitration (HNO3/H2SO4), the strongly directing acetamido group sends –NO2 to the position ortho to itself. Product 2 = 4-methyl-2-nitroacetanilide (–CH3, –NHCOCH3 para, –NO2 ortho to –NHCOCH3).
Reagent 3 – hydrolysis
Product 2 → 4-methyl-2-nitroaniline means the acetyl protecting group is removed. Reagent 3 = H3O+ (dilute acid hydrolysis; aqueous base then acidification also works).
Method: Reverse-Engineering a Multistep Reagent/Product Sequence
Core Concept
Each blank in a reaction sequence can be filled by comparing the structures immediately before and after it, identifying which named reaction type explains that specific change, and then recalling the standard reagent/conditions (or product) for that reaction.
Steps
Look at each adjacent pair of given/missing structures in turn and note exactly which functional group changed.
Match that change to a known reaction type: -NO2 to -NH2 is reduction; -NH2 to -NHCOCH3 is acetylation/protection; a new -NO2 appearing on a protected amine is nitration; -NHCOCH3 to -NH2 is hydrolysis; -NH2 to -N2+ is diazotisation; -N2+ to -H is deamination.
Recall the standard reagents/conditions for that reaction type.
Fill each blank (reagent or product) in sequence, checking that the resulting structure is a valid input for the next step. …