32. SANDMEYERS REACTION

32. Sandmeyer’s Reaction

The Sandmeyer’s Reaction is a very important method in organic chemistry used to convert an aromatic amine into an aryl halide. It is especially useful when direct halogenation of benzene does not give a clean or controlled result.

Why Sandmeyer’s Reaction is Needed?

  • Aryl halides cannot be easily prepared from phenols like alkyl halides are prepared from alcohols.
  • Direct halogenation of benzene gives a mixture of isomers, making purification difficult.
  • Sandmeyer’s Reaction allows us to place the halogen exactly where the amino group was present.

Basic Principle

First, the aromatic amine (like aniline) is converted into an aryl diazonium salt using nitrous acid in the presence of a mineral acid such as HCl or H2SO4, at a low temperature (0–5°C). This diazonium salt then reacts with a cuprous halide (CuCl, CuBr, etc.) to give the corresponding aryl halide.

General Reaction

Aryl amine → Aryl diazonium salt → Aryl chloride/bromide/iodide/fluoride

Mechanism (Simple Explanation)

The mechanism mainly involves the formation of an aryl free radical. Cuprous ions help in this process:

  1. Cuprous ions (Cu⁺) reduce the diazonium ion to produce an aryl radical.
  2. The aryl radical quickly reacts with halide ions to form the aryl halide.
  3. Copper is first oxidized and then reduced during the process, acting as both reducing and oxidizing agent.

Special Cases

Preparation of Aryl Iodides

Iodide salts (like KI) alone can decompose the diazonium salt—no copper catalyst is required. This still proceeds through oxidation–reduction, and iodine is often released.

Preparation of Aryl Fluorides – Schiemann Reaction

Aryl diazonium salts react with fluoroboric acid to form diazonium fluoroborate (a solid). On heating, the salt decomposes to produce the aryl fluoride. This process is known as the Schiemann Reaction.

Gattermann Modification

Copper powder or copper bronze can also be used in place of cuprous chloride. These produce the needed cuprous ions on the metal surface.

Applications of Sandmeyer’s Reaction

Sandmeyer’s Reaction is more useful than direct halogenation for many reasons:

  • Only one product is formed (no mixture of isomers).
  • Aryl iodides and fluorides, which are difficult to obtain by direct halogenation, can be easily prepared.
  • Certain positions on the aromatic ring can be substituted where direct halogenation is difficult due to electronic effects.

Important Applications

1. Preparation of m-Dichlorobenzene

Direct chlorination does not give this product in high purity, but Sandmeyer’s method does.

2. Preparation of 2-Chloronaphthalene

Again, easier through the diazonium route rather than direct halogenation.

3. Preparation of Aromatic Acids

Aryl diazonium salts can react with cuprous cyanide to form nitriles, which on hydrolysis give aromatic acids.

4. Removal of Amino Group from Aromatic Ring

Diazonium salts treated with hypophosphorous acid remove the –NH2 group. This is useful for preparing compounds that cannot be made by direct substitution on benzene.

Example: m-Nitrotoluene can be prepared from toluene using this strategy.

Detailed Notes:

For PDF style full-color notes, open the complete study material below:

PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ SANDMEYERS REACTION.

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