Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of benzyl chloride with sodium cyanide followed by reduction with hydrogen in the presence of nickel gives (a) n-isobutylaniline (b) \(\beta\)-phenylethylamine (c) aniline (d) benzylamine

Step-by-Step Solution

Answer

The reaction yields benzylamine, option (d).

Step 1: Identify the Starting Material

The starting material for this reaction is benzyl chloride. This compound has the chemical formula \( C_6H_5CH_2Cl \).

Step 2: First Reaction - Nucleophilic Substitution

In the first step, benzyl chloride reacts with sodium cyanide (NaCN) to undergo a nucleophilic substitution reaction. The chlorine atom in benzyl chloride is replaced by the cyanide ion (CN\(^-\)), forming benzyl cyanide \( C_6H_5CH_2CN \).

Step 3: Second Reaction - Reduction

The benzyl cyanide is then subjected to catalytic hydrogenation in the presence of nickel as a catalyst. This reduction process converts the cyano group (\(-CN\)) into an amine group (\(-CH_2NH_2\)). The product of this reaction is benzylamine \( C_6H_5CH_2NH_2 \).

Step 4: Identify the Final Product

After both the substitution and reduction steps are completed, the final product obtained is benzylamine. This corresponds to option (d) in the given list.

Key Concepts

Nucleophilic Substitution

In organic chemistry, nucleophilic substitution is a fundamental reaction mechanism. This process occurs when a nucleophile, a molecule or ion with a free pair of electrons, attacks a target molecule, usually an electrophile. In our case, sodium cyanide ( NaCN ) acts as the nucleophile.

The reaction begins with benzyl chloride, with the chemical structure C_6H_5CH_2Cl . The chlorine atom, which is bonded to the benzyl group, acts as a leaving group. When sodium cyanide is introduced, the cyanide ion ( CN^- ) replaces the chlorine. This is called a nucleophilic attack.

• The cyanide ion ( CN^- ) has a lone pair of electrons, which it donates to form a bond with the carbon previously connected to chlorine.
• The chlorine ion is displaced and leaves as Cl^-, resulting in benzyl cyanide C_6H_5CH_2CN .

Nucleophilic substitution reactions like this are crucial for transforming molecules in chemistry, opening pathways to synthesize varied compounds.

Catalytic Hydrogenation

Catalytic hydrogenation is a key reaction in transforming unsaturated organic molecules to saturated ones. This process requires a catalyst – often a metal like nickel – and hydrogen gas ( H_2 ).

In our exercise, catalytic hydrogenation is used to reduce benzyl cyanide to benzylamine. Here, nickel acts as the catalyst, facilitating the reaction and speeding up the conversion without being consumed. The cyano group ( -CN ) turns into an amine group ( -CH_2NH_2 ) in the presence of hydrogen and the nickel catalyst.

• The catalyst adsorbs hydrogen gas, breaking the H_2 molecule into two hydrogen atoms on its surface.
• These hydrogen atoms then add across the triple bond of the cyano group, converting it into an amine group, which is less stable without the catalyst.

This reaction is not only central in producing amines from nitriles but also exemplifies the broader category of reduction reactions which are widely used in chemical synthesis.

Benzyl Chloride

Benzyl chloride is a significant reagent in organic synthesis. Its structure, C_6H_5CH_2Cl , comprises a benzene ring (an aromatic group) attached to a methylene group ( -CH_2- ) and a chlorine atom.

It serves prominently in chemistry as a starting material for various reactions due to its reactive chloride group. The presence of the benzyl group is crucial because it stabilizes the transition states and intermediates during reactions, allowing for cleaner transformations.

• This compound is highly reactive with nucleophiles due to the polar carbon-chlorine bond, making it suitable for nucleophilic substitution reactions.
• In practical applications, benzyl chloride is often used to introduce benzyl groups to other molecules and synthesize synthetic intermediates like benzylamine.

Despite its usefulness, handling benzyl chloride requires caution as it is an irritant and needs proper protective equipment during use. Overall, benzyl chloride remains essential to creating more complex organic molecules.