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: Identify Initial Compound

Start with aniline ( H₂N−C₆H₅ ), a compound with an amino group attached to a benzene ring.

Step 2: Understand the Target Compound

Benzylamine has the formula C₆H₅−CH₂−NH₂ . Notice the amine group is attached to a different carbon than in aniline.

Step 3: Analyze Option (a)

This option represents diazotization (NaNO₂ / HCl) , cyanation (CuCN) , and reduction (Sn / HCl) . This sequence converts briefly to an intermediate into phenylacetonitrile and then to benzylamine.

Step 4: Analyze Option (b)

This option involves diazotization (NaNO₂ / HCl) , cyanation (CuCN) , and hydrolysis (H₃O⁺) . This will convert the nitrile group into a carboxylic acid, not an amine.

Step 5: Analyze Option (c)

This involves diazotization (NaNO₂ / HCl) , reduction (Cu⁺ / H₃PO₂) , and methylation. However, this does not provide a direct route to benzylamine.

Step 6: Determine which Option Matches

Based on the transformations, option (a) effectively changes the functional groups to transform aniline to benzylamine by forming a nitrile followed by its reduction to the desired primary amine.

Key Concepts

Diazotization

Diazotization is an essential reaction in organic chemistry. It involves converting an amine into a diazonium salt using nitrous acid, typically generated in situ from sodium nitrite (\(\text{NaNO}_2\)) and hydrochloric acid (\(\text{HCl}\)). This process is vital because diazonium salts serve as versatile intermediates in the synthesis of various aromatic compounds.

In the context of converting aniline to benzylamine, diazotization is the first step. It transforms aniline (\(\text{H}_2\text{N}−\text{C}_6\text{H}_5\)) into a diazonium salt. This reaction is highly effective because the diazonium group can be replaced through different nucleophilic substitution reactions, including cyanation.

It's important to note that diazonium salts are generally unstable at high temperatures, necessitating careful control of reaction conditions. Commonly, these reactions are carried out at temperatures below 5°C to maintain stability.

Cyanation

Following diazotization, cyanation is a critical step where the diazonium group is replaced by a cyano group (\(\text{CN}\)). This is typically achieved using a copper(I) cyanide (\(\text{CuCN}\)) reagent.

In this reaction, the diazonium salt derived from aniline becomes phenylacetonitrile after cyanation. This transformation is essential for introducing the cyano group, which is a precursor for further conversions, eventually leading to the desired compound, benzylamine. The cyano group can be manipulated further using reduction to achieve this goal.

Cyanation builds on the versatility of diazonium salts. Moreover, it's a classic demonstration of how a seemingly minute group addition, like a cyano group, can dramatically expand the functional scope of an aromatic compound.

Reduction Reactions

Reduction reactions are a cornerstone of synthetic organic chemistry, providing the means to transform nitriles into amines, among other conversions. In this context, reduction follows cyanation, converting the cyano group (\(\text{CN}\)) to an amine (\(\text{NH}_2\)) group.

When synthesizing benzylamine from phenylacetonitrile, the reduction step is crucial. Typically, this conversion can be achieved using tin (\(\text{Sn}\)) and hydrochloric acid (\(\text{HCl}\)), which effectively reduces the nitrile group to the primary amine.

The importance of reduction reactions lies in their ability to lower bond orders, hence transforming molecules into different functional classes. For students, understanding reduction reactions is fundamental as it opens myriad paths for molecular transformations, broadening the strategy to achieve complex organic syntheses.