Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct sequence of reagents used for conversion of aniline into benzylamine is (a) \(\mathrm{NaNO}_{2} / \mathrm{HCl}, \mathrm{CuCN}, \mathrm{Sn} / \mathrm{HCl}\) (b) \(\mathrm{NaNO}_{2} / \mathrm{HCl}, \mathrm{CuCN}, \mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{NaNO}_{2} / \mathrm{HCl}, \mathrm{Cu}^{+} / \mathrm{H}_{3} \mathrm{PO}_{2}, \mathrm{CH}_{3} \mathrm{NH}_{2}\) (d) None

Step-by-Step Solution

Answer

Option (a) is the correct sequence of reagents.

Step 1: Aniline Diazotization

The first step in converting aniline (C6H5NH2) into benzylamine involves diazotization of aniline. In this step, aniline is treated with sodium nitrite (NaNO2) and hydrochloric acid (HCl) at a low temperature (0-5°C) to form a diazonium salt (C6H5N2+Cl−). This step is common in the Sandmeyer reaction.

Step 2: Cyanation

Next, the diazonium salt formed is reacted with cuprous cyanide (CuCN). This Sandmeyer reaction leads to the replacement of the diazo group (N2+) with a cyano group (CN), forming benzonitrile (C6H5CN).

Step 3: Reduction

Finally, benzonitrile is reduced to benzylamine (C6H5CH2NH2) using either Sn/HCl or H3O+ as the reducing agent. This reduction converts the nitrile group into an amine group.

Step 4: Conclusion

Following these steps, the correct sequence of reagents is detailed as NaNO2/HCl for diazotization, CuCN for cyanation, and finally the reduction of the nitrile to amine. Therefore, option (a) matches this sequence because it uses NaNO2/HCl, CuCN, and then a reducing agent (Sn/HCl) to achieve the end product.

Key Concepts

Diazotization

Diazotization is a chemical process crucial for transforming aniline into a reactive intermediate known as a diazonium salt. Aniline is a simple aromatic amine, and when it undergoes diazotization, it reacts with sodium nitrite ( NaNO_2 ) and hydrochloric acid ( HCl ) in cold conditions, usually between 0-5°C. This process transforms aniline into a diazonium salt ( C_6H_5N_2^+Cl^− ). The low temperature is important to prevent the diazonium compound from decomposing too quickly.

• The diazonium salt formation is a preparatory step to introduce other functional groups in place of the amine ( NH_2 ) group.
• It is an integral part of various coupling reactions such as the Sandmeyer reaction.

By forming a diazonium salt, the nitrogen atoms become positively charged, making them easy to replace with other substituents during subsequent reactions.

Sandmeyer Reaction

The Sandmeyer reaction is a technique used to introduce functional groups like cyano ( CN ) into an aromatic ring. In the conversion of aniline to benzylamine, the Sandmeyer reaction is utilized to replace the diazo group with a cyano group.

• The diazonium salt from the diazotization reaction reacts with cuprous cyanide ( CuCN ).
• This chemical interaction replaces the N_2^+ group from the diazonium salt with the CN group, forming benzonitrile ( C_6H_5CN ).

This step is important as it paves the way for further modifications, converting the aromatic compound into nitriles, which are valuable intermediates for producing various chemical compounds.

Reduction of Benzonitrile

The final stage in transforming aniline to benzylamine involves the reduction of benzonitrile. Benzonitrile ( C_6H_5CN ) is subjected to a reduction process, converting it into benzylamine ( C_6H_5CH_2NH_2 ), an aromatic amine with useful applications. Reduction can be performed using either:

• Stannous chloride in hydrochloric acid ( Sn/HCl ).
• Hydronium ions ( H_3O^+ ), effectively reducing the nitrile group to an amine group.

This step is crucial as it completes the conversion process, allowing for the synthesis of benzylamine from benzonitrile. Benzylamine is a simpler form of amine, which can further embark on other synthetic pathways for producing diverse organic compounds.