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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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

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.

Memory hook

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

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

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.

Avoid common mistakes

Common confusion

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.

Common wrong answer

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

Exam tip

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

Quick check

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

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

Answer writing and exam use

1-mark answer

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

2-mark answer

Diazonium salts are compounds containing the Ar-N2+ X- group, commonly formed from primary aromatic amines using nitrous acid at low temperature. 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. Aniline is converted to benzene diazonium chloride at low temperature, then treated with cuprous chloride to give chlorobenzene by Sandmeyer reaction.

3-mark answer

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. 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. Convert aniline to phenol. First diazotise aniline with NaNO2 and HCl at 273-278 K to form benzene diazonium chloride. Then warm with water to replace the diazonium group by OH, giving phenol. It is asked in conversion sequences, reagent completion, named reaction identification, coupling product questions, and reason-based questions on low temperature stability. Using CuCl directly on aniline to make chlorobenzene is wrong; aniline must first be converted to the diazonium salt.
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