Sandmeyer reaction

The Sandmeyer reaction is a subset of these reactions of diazonium salts, in cases where a copper(I) salt is used as a catalyst. It is an example of a radical-nucleophilic aromatic substitution. The Sandmeyer reaction provides a method through which one can perform unique transformations on benzene, such as halogenation, cyanation, trifluoromethylation.

The reaction was discovered in 1884 by Swiss chemist Traugott Sandmeyer, when he synthesized phenylacetylene from benzenediazonium chloride and copper(I) acetylide. The reaction is a method for substitution of an aromatic amino group via preparation of its diazonium salt followed by its displacement with a nucleophile, catalyzed by copper(I) salts. The nucleophile can include halide anions, cyanide, thiols, water, and others. The reaction does not proceed well with the fluoride anion, but fluorination can be carried out using tetrafluoroborate anions (see below).

Reaction mechanism

The nitrous acid is typically prepared in situ from sodium nitrite and acid. Following two protonation steps, one equivalent of water is lost to form the nitrosonium ion. The nitrosonium ion then acts as an electrophile in a reaction with an aromatic (or heterocyclic) amine, such as aniline, to form a diazonium salt, proceeding through a nitrosamine intermediate.^[4] The substitution of the aromatic diazo group with a halogen or pseudohalogen is initiated by a one-electron transfer mechanism catalyzed by copper(I) to form an aryl radical with loss of nitrogen gas. The substituted arene is formed through a radical mechanism with regeneration of the copper(I) catalyst. This reaction is known as the Sandmeyer reaction and is an example of a radical-nucleophilic aromatic substitution. The radical mechanism of the Sandmeyer reaction was resolved through the detection of biaryl byproducts. It proceeds through the following mechanism.

Formation of the nitrosonium ion

The nitrosonium ion (NO⁺) is formed from nitrous acid (HNO₂) by a mechanism analogous to formation of NO2+ from HNO₃. The mechanism here can occur in water, because HNO₂ is more easily protonated (the first step) than is HNO₃.

Formation of the benzenediazonium ion

The mechanism for formation of the benzenediazonium is beyond the scope of this class, but the reaction can be regarded overall as a dehydration:

ArNH₂ + NO⁺  —->  ArN₂+  +  H₂O