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

The correct reagent is H₃PO₂ (Option a).

Step 1: Identify the Compound

Benzenediazonium chloride is a diazonium salt with the formula \( C_6H_5N_2^+Cl^- \). It is commonly used in organic synthesis for the substitution reactions to replace the diazonium group.

Step 2: Understand the Transformation

The task is to convert benzenediazonium chloride, \( C_6H_5N_2^+Cl^- \), into benzene, \( C_6H_6 \). This transformation requires replacing the diazonium group with a hydrogen atom.

Step 3: Identify Suitable Reagent

- \( ext{H}_3 ext{PO}_2 \) (Hypophosphorous acid) is known to reduce benzenediazonium salts to benzene by replacing the diazonium group with hydrogen.- \( ext{LiAlH}_4 \) is a reducing agent mainly used for reducing carbonyl groups.- \( ext{Br}_2- ext{H}_2 ext{O} \) is used for bromination, and \( ext{CH}_3 ext{OH} \) is a solvent/alcohol, neither are suitable for this reaction.

Step 4: Select the Correct Reagent

Since \( ext{H}_3 ext{PO}_2 \) is specifically designed to convert diazonium salts to hydrocarbons like benzene through reduction, it is the correct choice for this transformation.

Step 5: Check the Reaction Equation

The reaction for the transformation can be represented as: \\[C_6H_5N_2^+Cl^- + H_3PO_2 + H_2O ightarrow C_6H_6 + N_2 + H_3PO_3 + HCl\]This confirms that \( ext{H}_3 ext{PO}_2 \) effectively converts benzenediazonium chloride to benzene.

Key Concepts

Benzenediazonium Chloride

Benzenediazonium chloride is a special type of compound known as a diazonium salt. Its chemical formula is \( C_6H_5N_2^+Cl^- \). One key property of diazonium salts is their ability to participate in various chemical reactions, which makes them highly useful in organic chemistry. These reactions often involve replacing the diazonium group (\( -N_2^+ \)) with other atoms or groups.In the case of benzenediazonium chloride, it is primarily used for reactions where the diazonium group needs to be replaced. For example:

• Transforming into different aromatic compounds by substituting the diazonium group with a new substituent.
• Utilized for introducing electrophilic substituents into aromatic rings, like introducing halogens or cyanide groups.

Understanding the nature and behavior of benzenediazonium chloride provides insight into its role and techniques for manipulating aromatic compounds. It highlights its importance in synthetic organic chemistry.

Reduction Reaction

A reduction reaction is a chemical process in which a molecule, atom, or ion gains electrons, often involving a decrease in oxidation state. For benzenediazonium chloride, the reduction process is essential to convert it to benzene \((C_6H_6)\). This transformation involves replacing the diazonium group \((N_2^+)\) with a hydrogen atom.During this reduction:

• The diazonium group is removed as nitrogen gas (\(N_2\)), which is a stable byproduct and drives the reaction forward.
• A hydrogen atom is introduced in its place, forming benzene.
• The reaction mechanism involves transferring electrons to the diazonium salt to facilitate the removal of nitrogen.

Reduction reactions are not only about gaining electrons but also about simplifying complex molecules into simpler ones, often releasing gas as a byproduct, which is beneficial for driving the reaction to completion.

Hypophosphorous Acid

Hypophosphorous acid \((H_3PO_2)\) is a crucial reagent for the reduction of benzenediazonium chloride to benzene. Recognized for its ability to donate electrons, \(H_3PO_2\) acts as a reducing agent. It helps replace the diazonium group with hydrogen through a straightforward method that accelerates the reaction.In the particular scenario of converting benzenediazonium chloride:

• \(H_3PO_2\) donates hydrogen atoms and helps facilitate the release of nitrogen as \(N_2\).
• It plays a balancing role in the reaction, ensuring all components reach a stable state, such as forming \(H_3PO_3\) as a byproduct.
• This reduction pathway using \(H_3PO_2\) is well-known for its efficiency and specificity in transforming diazonium salts.

Hypophosphorous acid's application in this context underscores its significance as a reducer, especially in reactions aiming to remove functional groups and produce simpler aromatic compounds like benzene.