Unlike haloalkane, which can be made from alcohol, haloarene cannot be made from alcohol. Because the –OH group of phenol cannot be easily substituted due to resonance, the C–O bond in phenols has a partial double bond character and is thus stronger and more difficult to leave. As a consequence, phenols cannot be converted into haloarenes. But there are several ways to make haloarene, including diazonium salts, Sandmeyer's reaction, and Gattermann's reaction. We will discuss Sandmeyer's reaction and Gattermann's reaction in-depth in this article.
What is Sandmeyer's Reaction? A type of substitution reaction. Sandmeyers reaction is used to prepare aryl halides from aryl diazonium salts. Copper salts like chloride, bromide, or iodide ions are used as catalysts in Sandmeyer's reaction. Diazonium salts are highly reactive compounds that are used in the synthesis of arene derivatives. When diazonium salt is treated with copper (I) chloride (Cu2Cl2) or copper (I) bromide (Cu2Br2), the corresponding haloarene is formed. This is referred to as the Sandmeyer reaction. Sandmeyer reaction is used to prepare haloarenes.
Example of Sandmeyer Reaction
Mechanism of Sandmeyer’s Reaction
A free radical mechanism governs the Sandmeyer reaction. The production of diazonium salts and the transformation of diazo intermediates into aryl halides are two steps in the synthesis of aryl halides from primary aryl amines (i.e., displacement with a nucleophile).
Step 1: Nitrosonium ion formation: Protonation occurs twice, with sodium nitrite reacting with hydrogen halide to form nitrosonium and halide ions.
Formation of Nitrosonium Ion
Step 2: Formation of benzene diazonium ion: The electrophilic nitrosonium converts an amine into a diazonium salt.
Formation of Benzene Diazonium Ion
Step 3: A single electron transfer from the copper to diazonium and from halide to copper (I) halide ion results in the formation of a neutral diazo radical and copper (II) halide.
Transfer of an Electron from Copper to Diazonium
Step 4: The diazo radical releases nitrogen gas to form an aryl radical, which reacts with the copper (II) halide to regenerate the copper (I) halide catalyst and form the final aryl halide product.
Formation of Aryl Halide
The Gattermann reaction method is almost similar to the Sandmeyer reaction. It is a formulation method for aromatic ring compounds. The Friedel-Crafts reaction is also similar to the Gattermann reaction. By treating benzene diazonium chloride with Cu/HCl or Cu/HBr, the Gattermann reaction is used to obtain chlorobenzene or bromobenzene. Aromatic diazonium salts are typically made by adding a cold aqueous solution of sodium nitrite to a solution or suspension of a primary aromatic amine in acid at a temperature of 273-278 K.
Example of Gattermann’s Reaction
Preparation of Diazonium Salt
Diazotization is the process of converting a primary aromatic amine into its diazonium salt. Let us discuss the process for the formation of diazonium salts. The primary aromatic amine is dissolved or suspended in a dilute acid like HCL, H2SO4 and so on. (In general, three moles of acid are used for every mole of amine to be diazotized; one mole to form the amine salt, one mole to liberate nitrous acid from sodium nitrite, and one mole to keep the reaction mixture acidic enough to suppress undesirable side reactions such as coupling of the diazonium salt thus formed with the free amine.)
The amine solution is cooled, and a cold aqueous solution of sodium nitrite is slowly added to it at such a rate that the reaction mixture's temperature does not rise above 278 K. When a few drops of the reaction mixture produce a blue colour with starch-potassium iodide paper, indicating the presence of unreacted nitrous acid in the reaction mixture, the addition of sodium nitrite solution is stopped. Excess nitrous acid is generally avoided because it interferes with the reactions of diazonium salts. The excess nitrous acid can be destroyed by adding urea.
Preparation of Diazonium Salt
Haloarene cannot be produced using alcohol, in contrast to haloalkane, which can. Phenols are not capable of being transformed into haloarenes. However, there are numerous processes for producing haloarene, including Gattermann's reaction, Sandmeyer's reaction, and diazonium salts. Aryl diazonium salts are converted into aryl halides using the Sandmeyers reaction. In Sandmeyer's reaction, catalysts such as chloride, bromide, or iodide ions are utilised to speed up the process. The Gattermann reaction method and the Sandmeyer reaction are essentially identical. The Gatterman reaction is a technique for creating compounds with aromatic rings. It is possible to produce chlorobenzene or bromobenzene via the Gattermann reaction by reacting benzene diazonium chloride with Cu/HCl or Cu/HBr.