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

(d) benzylamine is formed.

Step 1: Identify the Reaction Components

The first reaction involves benzyl chloride and sodium cyanide. We know that sodium cyanide (NaCN) acts as a nucleophile which will replace the chloride ion (Cl-) in benzyl chloride, forming benzyl cyanide. In chemical terms, this is a nucleophilic substitution reaction.

Step 2: Understand the Product of First Reaction

The product from the nucleophilic substitution is benzyl cyanide, also known as phenylacetonitrile. This compound contains a phenyl group attached to a two-carbon chain with the functional group -CN (nitrile group) at the end.

Step 3: Identify the Conditions of the Second Reaction

The second part of the question states that benzyl cyanide undergoes reduction with hydrogen in the presence of nickel. Hydrogen in the presence of a nickel catalyst is commonly used for adding hydrogen to unsaturated bonds, a process known as hydrogenation.

Step 4: Understand the Reduction Reaction

The nitrile group (-CN) in benzyl cyanide is hydrogenated to form an amine group (-CH2NH2). This specific reduction transforms the benzyl cyanide into benzylamine. This transformation happens because the reduction of a nitrile typically leads to the corresponding primary amine.

Step 5: Determine the Correct Answer

Considering both parts of the reaction, the initial substitution followed by the reduction will ultimately yield benzylamine (an aromatic primary amine). Thus, the correct answer is option (d).

Key Concepts

Nucleophilic Substitution Reaction

A nucleophilic substitution reaction is a fundamental concept in organic chemistry, where a nucleophile replaces a leaving group attached to a carbon atom. In this case, we have benzyl chloride as our starting material. Here, the chloride ion (Cl-) is the leaving group, and it's replaced by a new group thanks to a nucleophile, which is sodium cyanide (NaCN).

• The nucleophile, cyanide ion (CN-), is attracted to the positively polarized carbon atom in benzyl chloride.
• In this reaction, the cyanide ion takes the place of the chloride ion, forming benzyl cyanide.

This type of reaction is crucial because it allows us to introduce new functional groups into molecules, providing a pathway to synthesize more complex compounds with specific properties.

Reduction with Hydrogen

Reduction with hydrogen involves the addition of hydrogen (H2) to a molecule. This process is known as hydrogenation and is often employed to convert unsaturated compounds into their saturated forms. In the context of our exercise, hydrogen is used to reduce the nitrile group (-CN) in benzyl cyanide to an amine group (-CH2NH2).

• The use of hydrogen helps in breaking down double or triple bonds, usually converting them into single bonds.
• This reduction leads to the formation of a primary amine, highlighting the significance of hydrogen as a reducing agent in organic synthesis.

These reactions are vital in the transformation of compounds, making hydrogen indispensable in organic synthesis.

Catalyst (Nickel)

A catalyst speeds up the chemical reaction without undergoing permanent change. In the reduction process, nickel serves as an effective catalyst. Nickel can absorb hydrogen gas, which facilitates the reduction of nitrile to amine.

• Nickel, being a metal, provides a surface for the reaction to occur more efficiently.
• The catalyst helps to lower the energy required for the reaction, making the process quicker and more efficient.

Catalysts, like nickel, are crucial in catalysis, as they enable the production of products on a larger scale and under milder conditions.

Primary Amine Formation

Primary amines are organic compounds characterized by the presence of an amine group connected to one alkyl or aryl group. In the context of benzyl cyanide, its reduction leads to the formation of benzylamine, a primary amine.

• The nitrile group undergoes complete hydrogenation, resulting in a transformation to a primary amine.
• In this process, the triple bond between carbon and nitrogen is reduced to a single bond with the addition of hydrogen atoms.

The formation of primary amines is essential for synthesizing various pharmaceuticals and agricultural chemicals, displaying the utility of such functional groups in organic synthesis.