Question
An aromatic primary amine is treated with NaNO₂ + HCl at 0-5°C to form a diazonium salt. This salt is then treated with phenol in alkaline medium. Identify the reactions involved and write the products at each stage.
(JEE Main / NEET pattern)
Solution — Step by Step
flowchart TD
A["ArNH₂\n(Aromatic 1° Amine)"] -->|"NaNO₂ + HCl\n0-5°C"| B["ArN₂⁺Cl⁻\n(Diazonium salt)"]
A -->|"CH₃COCl or\n(CH₃CO)₂O"| C["ArNHCOCH₃\n(Acylation)"]
A -->|"CHCl₃ + KOH"| D["ArNC\n(Carbylamine test)"]
B -->|"CuCl / CuBr\n(Sandmeyer)"| E["ArCl / ArBr"]
B -->|"H₃PO₂"| F["ArH\n(Deamination)"]
B -->|"ArOH / NaOH"| G["Ar-N=N-ArOH\n(Azo dye)"]
B -->|"KI"| H["ArI"]Aniline reacts with NaNO₂ and HCl at 0-5°C:
The temperature must be kept below 5°C because diazonium salts are unstable and decompose above this temperature (they lose N₂ gas).
The diazonium salt couples with phenol in alkaline medium at the para position:
The product is p-hydroxyazobenzene, an orange-yellow azo dye. Coupling occurs at the para position because phenoxide ion is a strong activating group.
In alkaline medium, phenol converts to phenoxide ion (), which is a much stronger nucleophile and activator of the ring. Without NaOH, the coupling reaction is extremely slow or does not occur.
Why This Works
Diazonium salts are one of the most versatile intermediates in organic chemistry. The group is an excellent leaving group — it leaves as gas, making the reaction irreversible. This is why Sandmeyer reactions work so cleanly.
In coupling reactions, the diazonium cation acts as a weak electrophile that attacks electron-rich aromatic rings (phenols and amines). The bridge in the product is the chromophore responsible for the intense colour of azo dyes. This is genuinely practical chemistry — most synthetic dyes in the textile industry use this reaction.
Alternative Method — Hofmann Bromamide Reaction
If the question asks about converting an amide to an amine with one fewer carbon:
This is the Hofmann degradation — a chain-shortening reaction. The product amine has one carbon less than the starting amide.
For NEET, diazonium salt reactions are a favourite. Remember this hierarchy: Sandmeyer (CuX → ArX), Gattermann (Cu/HX → ArX, similar but without CuX salt), and Balz-Schiemann (ArN₂⁺BF₄⁻ → ArF). JEE often asks about the coupling reaction for dye formation.
Common Mistake
The most frequent error: performing diazotization at room temperature instead of 0-5°C. Students write the reaction correctly but forget the temperature condition — and lose marks. Aliphatic diazonium salts are so unstable they cannot even be isolated; they decompose instantly. Aromatic diazonium salts survive only at low temperatures. Always mention 0-5°C in your answer.