Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reagent can be used to convert benzenediazonium chloride into benzene? (a) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (b) \(\mathrm{LiAlH}_{4}\) (c) \(\mathrm{Br}_{2}-\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CH}_{3} \mathrm{OH}\)

Step-by-Step Solution

Answer

Use \(H_3PO_2\) to convert benzenediazonium chloride into benzene.

Step 1: Understanding Benzenediazonium Chloride

Benzenediazonium chloride is a diazonium salt with the formula \(C_6H_5N_2^+Cl^-\). It is often used in substitution reactions to form various aromatic compounds due to its ability to replace the diazo group with other atoms or groups.

Step 2: Identify Possible Reagents

We'll consider the ability of each reagent to reduce the diazonium group to produce benzene by removing the diazo group \(N_2^+\) and replacing it with a hydrogen atom, thereby forming benzene \(C_6H_6\).

Step 3: Analyze Each Reagent

(a) \(H_3PO_2\) (Hypophosphorous Acid): Known to convert diazonium salts into their respective hydrocarbons by reducing the diazo group to hydrogen.(b) \(LiAlH_4\) (Lithium Aluminium Hydride): Primarily used for reducing carbonyl compounds and not effective for reducing diazonium groups.(c) \(Br_2-H_2O\): Utilized mainly for bromination reactions, not for reducing diazonium groups.(d) \(CH_3OH\) (Methanol): Primarily a solvent or mild nucleophile, not used for reducing diazonium groups.

Step 4: Conclusion From the Analysis

Among the given reagents, \(H_3PO_2\) is the one commonly used to reduce diazonium salts to hydrocarbons by replacing the \(N_2^+\) group with hydrogen, thereby successfully converting benzenediazonium chloride to benzene.

Key Concepts

Diazonium Salt Reduction

In organic chemistry, diazonium salts like benzenediazonium chloride play a crucial role in the transformation of functional groups. A common process involving these salts is the conversion of the diazo group ( 2^+ ) into a simpler functional group. Reduction is one such reaction, where the diazo group is replaced by a hydrogen atom, forming benzene from benzenediazonium chloride. This process typically involves specific reducing agents. In this case, we are focusing on hypophosphorous acid ( H_3PO_2 ), which is a reliable reagent for reducing diazonium salts to hydrocarbons. By donating electrons, H_3PO_2 assists in cleaving the nitrogen molecule from the salt and in replacing it with hydrogen, effectively reducing the benzenediazonium chloride to benzene. This transformation is essential for creating simpler aromatic hydrocarbons from more complex compounds.

Aromatic Compounds

Aromatic compounds are a fascinating class of organic molecules known for their ring-like stability and distinct smell. Benzene is the simplest aromatic compound, characterized by a six-carbon ring with delocalized electrons that contribute to its resonance stability. Aromaticity arises when compounds follow Huckel's rule, having 4n + 2 pi electrons (where n is an integer) in the ring structure.

• These compounds are highly stable due to the electron distribution around the ring.
• The unique stability of benzene makes it a central figure in organic chemistry reactions, serving as both a building block and a reaction partner.

Transformations involving aromatic compounds often involve maintaining the integrity of the ring while substituting different functional groups. In reactions such as the reduction of diazonium salts, the aromatic core remains unchanged, illustrating the non-reactive nature of the aromatic pi-bond system in these exchanges.

Organic Chemistry Reactions

Organic chemistry is filled with diverse reactions where functional groups are transformed, enabling the synthesis of complex molecules. Specifically, reactions involving diazonium salts are instrumental in organic synthesis due to their versatile reactivity.

• These salts can undergo a range of reactions such as coupling reactions, Sandmeyer reactions, and reduction reactions.
• The diazonium group provides an excellent leaving group, often replaced by other nucleophiles, leading to a variety of functionalized aromatic compounds.

In the reduction scenario, a diazonium salt loses its diazo group, forming a simpler hydrocarbon - a fundamental transformation in organic reactions. Understanding the conditions under which these transformations occur and the choice of reagents is pivotal for successfully conducting organic syntheses. Each transformation adds to the chemist's toolkit for creating more complex structures from simpler starting materials.