Benzenediazonium chloride on reaction with phenol in weakly basic medium gives: (a) Diphenyl ether (b) o- hydroxyazobenzene (c) Chlorobenzene (d) Benzene

Benzenediazonium chloride, often referred to as diazonium salt, is a particularly important compound in organic chemistry due to its reactivity and role in synthesis. The distinct structure of this compound features a benzene ring attached to a diazonium group (—N₂⁺Cl⁻).

Diazonium salts are known for their instability at room temperature and their tendency to undergo decomposition. However, when kept at low temperatures, they can be relatively stable and hence utilized in various chemical reactions. One of the most significant transformations that benzenediazonium chloride undergoes is coupling reactions with phenolic compounds or aromatic amines to produce a class of vividly colored compounds commonly known as azo compounds.

In a laboratory setting, benzenediazonium chloride is prepared by the reaction of aniline with nitrous acid at temperatures below 5 °C. This formation process requires precision to manage the reactive nature of the diazonium group and to avoid undesired side reactions or decomposition.

Phenol, which is also known as carbolic acid, is an organic compound with the formula C₆H₅OH. It consists of a hydroxyl group (-OH) attached to a benzene ring, making it an aromatic alcohol. Due to the phenolic OH group's acidic nature, phenol can undergo ionization in a basic medium to produce a phenoxide ion (C₆H₅O⁻).

The reactivity of phenol in chemical reactions is significantly influenced by its ability to form the phenoxide ion. In a weakly basic medium, phenol is more reactive towards electrophilic substitution reactions because the phenoxide ion is a better nucleophile than the neutral phenol molecule. As a result, reactions such as azo coupling can readily take place, specifically at the ortho (o-) and para (p-) positions of the aromatic ring, due to the increased electron density provided by the phenoxide oxygen.

Azo compound formation is a classic example of a coupling reaction, a type of synthesis where two different molecules are joined together by an azo group (-N=N-). The name 'azo' comes from the French word for nitrogen ('azote'). Azo compounds are characterized by their vivid colors and are often used as dyes, known as azo dyes.

In the context of the reaction between benzenediazonium chloride and phenol, the formation of an azo compound occurs when the phenoxide ion attacks the electrophilic diazonium carbon, leading to the displacement of the nitrogen gas and the formation of a new bond between the benzene ring of phenol and the diazonium group.

Ortho Position Activation

In this specific case, the ortho position to the hydroxyl group on phenol is activated, making it the preferred site for the coupling reaction. This results in the formation of o-hydroxyazobenzene, an azo compound with a distinct chemical structure and properties.

• The azo bond formation is a key step in synthesizing this class of compounds.
• Azo compounds are important in various industries, especially for their role in creating dyes and pigments.
• The reaction is facilitated by a weakly basic medium, which helps generate the phenoxide ion, a crucial intermediate in the coupling process.