Preparation of diazonium salts

How are diazonium salts prepared?

In plain words: keep the mixture ice-cold, add sodium nitrite to aniline in hydrochloric acid, and the –NH₂ group becomes –N₂⁺. The salt is unstable, so it is made just before it is needed.

What diazonium salts are

• The arenediazonium salts studied here have the general formula R–N₂⁺X⁻, where R is an aryl group and X⁻ may be Cl⁻, Br⁻, HSO₄⁻, BF₄⁻ and so on; the N₂⁺ group is the diazonium group (NCERT page 274).
• They are named by adding “diazonium” to the name of the parent hydrocarbon, followed by the anion: C₆H₅N₂⁺Cl⁻ is benzenediazonium chloride and C₆H₅N₂⁺HSO₄⁻ benzenediazonium hydrogensulphate (NCERT page 274).
• Primary aliphatic amines give highly unstable alkyldiazonium salts, which lose nitrogen at once. Primary aromatic amines give arenediazonium salts that last for a short time in solution at 273–278 K; their stability is explained by resonance (NCERT page 274).

Diazotisation of aniline at 273–278 K

• Aniline reacts with nitrous acid at 273–278 K to give benzenediazonium chloride; the nitrous acid is made in the mixture from sodium nitrite and hydrochloric acid (NCERT page 274).
• Converting a primary aromatic amine into a diazonium salt is called diazotisation (NCERT page 274).
• Because it is unstable, the diazonium salt is not usually stored but used immediately after it is made (NCERT page 274).

• For any primary aromatic amine ArNH₂, with Ar an aryl group, the equation is ArNH₂ + NaNO₂ + 2HCl → ArN₂⁺Cl⁻ + NaCl + 2H₂O (reasoned from NCERT page 274).
• 273–278 K is approximately 0–5 °C (exact conversion subtracts 273.15); the reaction mixture temperature must be monitored in a suitably cooled bath. NCERT says that if the temperature of the solution “is allowed to rise upto 283 K, the salt gets hydrolysed to phenol” (NCERT page 276), so keep the mixture in ice. Strictly, 283 K is not a sharp threshold or a guarantee of instant complete conversion (reasoned).
• A note on the heading (page 274): NCERT prints the section heading as “Method of Preparation of Diazoniun Salts”; it means diazonium.

Solved examples

Common mistakes to avoid

Quick revision summary

• Diazonium salt: R–N₂⁺X⁻ (R aryl; X⁻ Cl⁻, Br⁻, HSO₄⁻, BF₄⁻); named benzenediazonium chloride and so on.
• Diazotisation: primary aromatic amine + NaNO₂ + HCl at 273–278 K; the salt is used at once.
• Arenediazonium ions are stabilised by resonance; alkyldiazonium salts are highly unstable.

Questions students ask

What is the mechanism of diazotisation?

Diazotisation is a nitrosation of the amine’s nitrogen, a mechanism beyond the NCERT text: in strong acid, nitrous acid gives the nitrosonium ion, NO⁺, and the lone pair on the amine’s nitrogen bonds to it, so that loss of H⁺ leaves an N-nitrosamine, ArNH–N=O (Wikipedia, “Nitrous acid” and “Nitrosation and nitrosylation”). From a primary amine this goes on in acid to the diazonium ion (Wikipedia): the remaining N–H hydrogen moves to the oxygen, giving Ar–N=N–OH, which takes up H⁺ and loses water as ArN₂⁺ (reasoned). Both hydrogens of –NH₂ are lost on the way, so the amine must be primary (reasoned).

Why is diazotisation carried out at low temperature?

Cold conditions slow competing decomposition of the arenediazonium product. NCERT uses approximately 0–5 °C and illustrates hydrolysis to phenol on warming to 283 K. Reaction lifetime depends on structure, medium and temperature; 283 K is not a sharp universal threshold. Prepare the dilute solution for immediate use under an approved protocol, and do not isolate or dry the chloride.

Why is excess hydrochloric acid used in diazotisation?

Excess hydrochloric acid, more than the two moles per mole of amine that NCERT’s equation uses (page 274), keeps a diazotisation mixture strongly acidic, so that aniline not yet diazotised stays protonated as the anilinium ion (NCERT page 266; reasoned). Beyond the NCERT text, a diazonium ion couples with the nitrogen of a free amine, giving a triazene (Wikipedia, “Azo coupling”): diazoaminobenzene, the precursor of aniline yellow, forms when only part of the aniline is diazotised (Wikipedia, “1,3-Diphenyltriazene”). Strong acid also turns nitrous acid into the diazotising NO⁺ ion (Wikipedia, “Nitrous acid”).

What is the difference between diazotisation and coupling?

Diazotisation makes a diazonium salt, turning the –NH₂ group of a primary aromatic amine into –N₂⁺ with nitrous acid at 273–278 K (NCERT page 274), while coupling uses that salt: the diazonium ion attacks an activated ring, such as that of phenol or aniline, and keeps both its nitrogens, joining the two rings through –N=N– (NCERT page 276; coupling reactions). So the two are the first and second steps in making an azo dye, as in the azo dye test and the preparation of aniline yellow (reasoned).

Can a diazonium salt be prepared from benzene or from phenol?

A diazonium salt cannot be prepared from benzene or phenol directly, because diazotisation needs a primary aromatic amine (NCERT page 274). Benzene is first nitrated to nitrobenzene (NCERT Class 11 Chemistry, Unit 9, page 322), which is reduced to aniline, for example with iron scrap and hydrochloric acid (reducing nitro compounds), and the aniline is then diazotised at 273–278 K (reasoned from NCERT pages 262 and 274). Phenol has no –NH₂ group to diazotise: in diazonium chemistry it is the partner a diazonium salt couples with, or the product of warming one (NCERT page 276).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is benzenediazonium chloride made just before use?

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   It is unstable, so it is not generally stored but used immediately after its preparation (NCERT page 274).

2. Q2. What is diazotisation, and what supplies the nitrous acid?

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   The conversion of a primary aromatic amine into a diazonium salt; the nitrous acid is produced in the reaction mixture from sodium nitrite and hydrochloric acid (NCERT page 274).