medium difficulty8 min

Diazonium Salts and Azo Coupling

Diazonium salts are compounds containing the Ar-N2+ X- group, commonly formed from primary aromatic amines using nitrous acid at low temperature.

In one line: Diazonium is the aromatic switch point: first make Ar-N2+, then replace or couple.

Practice This Concept

Understand it

Benzene diazonium chloride is prepared from aniline using sodium nitrite and hydrochloric acid at 273-278 K.
Diazonium salts are important synthetic intermediates because the diazonium group can be replaced by Cl, Br, CN, I, F, OH, or H under suitable conditions.
Sandmeyer and Gattermann reactions introduce halo or cyano groups.
Diazonium salts also undergo coupling with activated aromatic compounds such as phenols and anilines to form coloured azo compounds.

Example

Aniline is converted to benzene diazonium chloride at low temperature, then treated with cuprous chloride to give chlorobenzene by Sandmeyer reaction.

Rule to remember

Diazotisation pattern: ArNH₂ + NaNO₂ + HCl at 273-278 K gives ArN2+Cl-.

Sandmeyer pattern: ArN2+Cl- with CuCl/HCl gives ArCl; with CuBr/HBr gives ArBr; with CuCN/KCN gives ArCN.

Coupling forms Ar-N=N-Ar type azo compounds.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Convert aniline to phenol.

1First diazotise aniline with NaNO₂ and HCl at 273-278 K to form benzene diazonium chloride.
2Then warm with water to replace the diazonium group by OH, giving phenol.

Method you can reuse

1Start from primary aromatic amine.
2Form diazonium salt using NaNO2/HCl at 273-278 K.
3Then choose the replacement reagent or coupling partner based on the required final product.

Visual tip

A reaction map from aniline to diazonium salt and then to replacement or azo coupling products is highly useful.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is diazotisation of aniline carried out at 273-278 K?

Think first, then show answer

The low temperature helps keep the benzene diazonium salt stable enough for further reactions.

Common wrong answer

Using CuCl directly on aniline to make chlorobenzene is wrong; aniline must first be converted to the diazonium salt.

Don't mix this up

Students often omit the low temperature condition during diazotisation or apply diazonium salt formation to aliphatic primary amines in the same stable way as aromatic amines.

Exam tip

Always write 273-278 K for diazotisation of aniline and identify whether the next step is replacement or coupling.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

It is asked in conversion sequences, reagent completion, named reaction identification, coupling product questions, and reason-based questions on low temperature stability.

1
Enough for 1 mark
Diazonium salts are compounds containing the Ar-N2+ X- group, commonly formed from primary aromatic amines using nitrous acid at low temperature.
2
Add for 2 marks
Example: Aniline is converted to benzene diazonium chloride at low temperature, then treated with cuprous chloride to give chlorobenzene by Sandmeyer reaction.
3
Add for 3 marks
Benzene diazonium chloride is prepared from aniline using sodium nitrite and hydrochloric acid at 273-278 K.

MCQ Quiz

Practice this concept with focused MCQs

Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.

10 MCQs5 MinutesInstant Results