Problem 122 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Identify the produet \((Z)\) formed \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl} \stackrel{\mathrm{H}_{3} \mathrm{PO}_{2}}{\longrightarrow} Z\) (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{OH}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

The product \(Z\) is (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\).

Step 1: Understand the Reaction

The provided reaction involves \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}\) and \(\mathrm{H}_{3} \mathrm{PO}_{2}\). This type of reaction generally pertains to the reduction of aryl diazonium salts by hypophosphorous acid.

Step 2: Identify the Mechanism

In this context, an aryl diazonium salt \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}\) is reduced by \(\mathrm{H}_{3} \mathrm{PO}_{2}\), which is known to result in the replacement of the diazonium group \(\mathrm{-N}_2^+\) with a hydrogen atom.

Step 3: Predict the Product

The \(\mathrm{N}_2^+\) group in \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~N}_{2} \mathrm{Cl}\) would be replaced by a hydrogen atom as a result of the reaction with \(\mathrm{H}_{3} \mathrm{PO}_{2}\). This reaction ultimately forms the compound \(\mathrm{C}_{6} \mathrm{H}_{6}\) (benzene).

Step 4: Choose the Correct Answer

Based on the deduction, the product \(Z\) is \(\mathrm{C}_{6} \mathrm{H}_{6}\). By comparing the options provided: (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\), (b) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{OH}\), (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}\), and (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{OH}\), the correct choice is (a) \(\mathrm{C}_{6} \mathrm{H}_{6}\).

Key Concepts

Aryl Diazonium Reduction

Aryl diazonium salts, such as \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{~N}_{2}\mathrm{Cl}\), are organic compounds that contain a diazonium group \(-\mathrm{N}_2^+\). These salts undergo various chemical reactions due to the reactive nature of the \(-\mathrm{N}_2^+\) group, making them versatile intermediates in organic synthesis.
In the context of reduction reactions, the diazonium group \(-\mathrm{N}_2^+\) is usually replaced by a simple hydrogen atom. This happens because substances like hypophosphorous acid \(\mathrm{H}_{3}\mathrm{PO}_{2}\) act as reducing agents. They donate electrons to the diazonium salt, eliminating the \(-\mathrm{N}_2^+\) group and replacing it with a hydrogen. The overall change involves

• the loss of nitrogen gas \(\mathrm{N}_2\)
• formation of a new C-H bond.

This type of reaction is significant because it allows the transformation of complex aryl diazonium compounds into simpler hydrocarbons, broadening the scope for further chemical manipulation.

Benzene Formation

The reduction of aryl diazonium salts often results in the formation of benzene. Benzene, a simple aromatic hydrocarbon, has the chemical formula \(\mathrm{C}_{6}\mathrm{H}_{6}\). It is characterized by a six-carbon ring with alternating single and double bonds.
During the \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{~N}_{2}\mathrm{Cl}\) reaction with \(\mathrm{H}_{3}\mathrm{PO}_{2}\), the diazonium group \(-\mathrm{N}_2^+\) is removed and replaced by hydrogen, leading to the formation of benzene. Key points of the process include:

• The diazonium group detaches as nitrogen gas, a stable and inert byproduct.
• The remaining aromatic structure stabilizes by forming a \(\mathrm{C}-\mathrm{H}\) bond.

The significance of this transformation lies in its role in organic synthesis, particularly in producing benzene from more complex molecular structures. Benzene is not only a central building block in synthetic chemistry but also a model compound for understanding aromaticity and related chemical properties.

Hypophosphorous Acid Reaction

The role of hypophosphorous acid \(\mathrm{H}_{3}\mathrm{PO}_{2}\) in the reduction of aryl diazonium salts is crucial. It acts as a reducing agent, meaning it donates electrons during the chemical reaction. This attribute makes hypophosphorous acid particularly effective at reducing diazonium salts to form hydrocarbons like benzene.
This reaction occurs in the following way:

• Hypophosphorous acid interacts with the diazonium salt, leading to the breakdown of the diazonium group to nitrogen gas \(\mathrm{N}_2\).
• Electrons supplied by \(\mathrm{H}_{3}\mathrm{PO}_{2}\) are used to form a new bond, specifically converting the diazonium group into a \(\mathrm{C}-\mathrm{H}\) bond.

The practical outcome of this reaction is the simplification of the aryl diazonium compound, often converting it to a basic aromatic compound like benzene. Interestingly, \(\mathrm{H}_{3}\mathrm{PO}_{2}\) is selective, ensuring that only specific reactions occur, which is highly beneficial in organic chemistry where precise synthesis routes are valued. This selectivity makes hypophosphorous acid a valuable tool in both laboratory and industrial chemical processes.