Question

Benzenediazonium chloride on reaction with phenol in weakly basic medium gives (a) diphenyl ether (b) p-hydroxyazobenzene (c) chlorobenzene (d) benzene

Short answer

The reaction gives p-hydroxyazobenzene.

Step-by-step solution

Understanding the Reaction Participants

Identify the reactants in the reaction: Benzenediazonium chloride (an aryl diazonium salt) and phenol.

Understanding Reaction Conditions

The reaction occurs in a weakly basic medium. This is crucial as it influences the reaction pathway and the products formed when these compounds interact.

Considering the Mechanism

In weakly basic conditions, phenol acts as a nucleophile due to the oxygen in the hydroxyl group, which can participate in azo coupling reactions with the diazonium group present in benzenediazonium chloride.

Predicting the Reaction Product

Azo coupling occurs, where the benzenediazonium chloride reacts with phenol to form an azo compound. This specifically forms a p-hydroxyazobenzene due to the para-position preference of the azo coupling reaction.

Key concepts

Benzenediazonium Chloride

Benzenediazonium chloride is an important chemical compound in organic synthesis. It is classified as an aryl diazonium salt, characterized by the formula \( C_6H_5N_2^+Cl^- \). This compound is highly reactive and is commonly used in a variety of chemical transformations due to its ability to undergo azo coupling reactions.

One of the important aspects of benzenediazonium chloride is its instability, especially in water, where it can decompose to release nitrogen gas, forming phenols. However, under controlled conditions, such as those in a weakly basic medium, it acts as a robust reagent forming stable azo compounds.

• Highly reactive in nature, making it ideal for synthesis reactions.
• Functions as the starting material for azo coupling.
• Used in the preparation of aromatic compounds by substitution reactions.

Phenol

Phenol is a crucial reactant in many chemical reactions, including the azo coupling reaction with benzenediazonium chloride. Its chemical structure comprises a benzene ring bonded to a hydroxyl group \( -OH \), which renders it slightly acidic.

In mild conditions, such as a weakly basic medium, phenol can act as a nucleophile. The oxygen in the hydroxyl group has a lone pair of electrons that can be donated to an electrophile, such as the diazonium ion in benzenediazonium chloride. This property is what makes phenol an effective participant in azo coupling reactions.

• Contains a reactive hydroxyl group \( -OH \).
• Acts as a nucleophile in reactions, capable of attacking electrophiles.
• Undergoes reactions preferentially at the para- position due to resonance stabilization.

Weakly Basic Medium

The use of a weakly basic medium is a crucial condition to facilitate the coupling reaction between benzenediazonium chloride and phenol. In such a medium, phenol gets slightly deprotonated, enhancing its nucleophilicity, which is pivotal for the reaction to proceed effectively.

The weakly basic conditions are typically ensured by the use of substances like sodium carbonate or dilute sodium hydroxide. This careful control prevents unwanted side reactions and promotes the selective formation of specific products.

• Increases the nucleophilicity of phenol, making the oxygen more reactive.
• Avoids the complete deprotonation that could disrupt the desired reaction path.
• Ensures a stable environment for the coupling reaction to occur smoothly.

p-Hydroxyazobenzene

p-Hydroxyazobenzene is the major product of the azo coupling reaction between benzenediazonium chloride and phenol, occurring specifically in a weakly basic medium. This compound is characterized by the azo group \( -N=N- \) linking the phenolic and diazonium components.

The reason it forms specifically at the para-position can be attributed to the resonance stabilization of the phenolic structure. This makes the para-position more accessible and reactive for the incoming diazonium ion, thus favoring the formation of p-hydroxyazobenzene over other possible isomers.

• Produced by the interaction of a diazonium ion and phenol.
• Features a vibrant color, often used in dyeing industries.
• Exhibits structural stability due to resonance effects.