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Sandmeyer Reaction Mechanism

What is Sandmeyer Reaction?

Sandmeyer reaction is a type of substitution reaction that is widely used in the production of aryl halides from aryl diazonium salts. Copper salts like chloride, bromide or iodide ions are used as catalysts in this reaction. Notably, Sandmeyer reaction can be used to perform unique transformations on benzene. The transformations include hydroxylation, trifluoromethylation, cyanation, and halogenation.

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The reaction was first discovered in the year 1884 when Traugott Sandmeyer, a Swiss chemist, was conducting an experiment to synthesize phenylacetylene from benzene diazonium chloride and cuprous acetylide. However, at the end of the experiment, he obtained phenyl chloride as the main product.

Sandmeyer reaction is believed to be a great example of a radical-nucleophilic aromatic substitution. This reaction is a useful tool by which an amino group on an aromatic ring can be replaced with different substituents. During the Sandmeyer reaction, the amino group that is attached to an aromatic ring is converted into a diazonium salt that can be transformed into various functional groups.

Sandmeyer Reaction Mechanism

The Sandmeyer reaction follows a free radical mechanism. The reaction is actually a two-step process where the synthesis of aryl halides from primary aryl amines involves the formation of diazonium salts and the transformation of diazo intermediates into aryl halides (displacement with a nucleophile). Interestingly, the nucleophile can be a halide anion, cyanide, water, etc.

To elucidate further, the Sandmeyer reaction mechanism commences with a transfer of a single electron from the copper to the diazonium. This results in the formation of a non-participating diazo radical as well as copper(II) halide. A molecule of nitrogen gas is then released by the diazo radical to give aryl radical, which then reacts with the copper(II) halide to restore the catalyst [copper(I) halide]. After all this, the final product, aryl halide is obtained.

Sandmeyer reaction

Sandmeyer Reaction Mechanism

Formation of Nitrosonium Ion

We will look at Sandmeyer reaction mechanism during the formation of nitrosonium ion. In the experiment, we take sodium nitrite and acid to form nitrous acid. What follows next is the two protonation steps where one equivalent of water is removed. This results in the generation of the nitrosonium ion (nitrogen monoxide cation).

The ion acts as an electrophile when it is reacted further with an aromatic or heterocyclic amine (for e.g. aniline). Finally, the diazonium salt is formed. The reaction is carried out at temperatures ranging between 25-30oC.

Nitrosonium Ion Formation

Sandmeyer Reaction Mechanism – Formation of Nitrosonium Ion

Benzenediazonium Ion Formation

Students can also go through the below diagram highlighting the Benzenediazonium ion formation to understand the Sandmeyer reaction mechanism more clearly.

Benzenediazonium Ion Formation

Sandmeyer Reaction Mechanism – Formation of Benzenediazonium Ion


Sandmeyer Reaction Example

Importance of Sandmeyer Reaction

  • The Sandmeyer reaction is very important when we are studying aromatic chemistry. The reaction can be used to generate substitution patterns that are sometimes not possible to achieve by direct substitution.
  • In the reaction, a primary arylamine is mainly diazotized to create an aryl diazonium salt which is further reacted with a halide ion to produce the desired aryl halide product.
  • It follows an electron-transfer-mechanism including free radicals.
  • Sandmeyer reaction has wide synthetic applicability and is complementary to electrophilic aromatic substitution.

Applications of Sandmeyer Reaction

Today, a lot of variations of the Sandmeyer reaction have been developed. As a result of these developments, the reaction has multiple synthetic applications. The different adaptations help in the synthesis of aryl thioethers, aryl fluorides (the Schiemann reaction), phenols, and aryl nitriles.

The common uses of the Sandmeyer reaction are:

  • It is used in hydroxylation to convert aryl amines to phenols leading to the formation of an aryl diazonium salt.
  • To create aryl compounds during the process of trifluoromethylation. The compounds inherit unique chemical properties with a wide variety of practical applications.
  • Sandmeyer reaction is used for the formation of benzonitriles through the process of cyanation. The reaction is also found during the synthesis of neoamphimedine, a chemical compound that targets topoisomerase II as an anti-cancer drug.
  • To create aryl halides. Some of the solvents used in this process are diiodomethane to synthesize aryl iodides, bromoform to synthesize aryl bromides, chloroform to synthesize aryl chlorides.

Frequently Asked Questions on Sandmeyer Reaction

Explain the mechanism of the Sandmeyer reaction?

Sandmeyer reaction mechanism follows an electron transfer mechanism including free radicals. The Sandmeyer reaction is a great example of a radical-nucleophilic aromatic substitution.

What type of reaction is Sandmeyer reaction?

The reaction is classified as a substitution reaction and is widely used in the production or synthesis of aryl halides from aryl diazonium salts.

What is the main difference between the Sandmeyer reaction and the Gattermann reaction?

One of the significant difference between the Sandmeyer reaction and the Gattermann reaction is the type of reactant used. In the Sandmeyer reaction the reactant is CuCl and HCl whereas in the Gattermann reaction copper powder with CuCl or CuBr is used to form chlorobenzene and bromobenzene.

Which catalyst is used in the Sandmeyer reaction?

Copper salts are used as catalyst or reagents in the Sandmeyer reaction.

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