Problem 62 - A Complete Resource Book in Chemistry for JEE Main

Question

Benzylamine reacts with nitrous acid to form (a) benzene (b) benzyl alcohol (c) phenol (d) azobenzene

Step-by-Step Solution

Answer

The reaction forms benzyl alcohol (b).

Step 1: Understand the Reaction

Benzylamine (C6H5CH2NH2) reacts with nitrous acid (HNO2). This type of reaction generally involves the formation of a diazonium ion followed by hydrolysis or decomposition, depending on the structure of the amine.

Step 2: Reaction Pathway Analysis

When benzylamine reacts with nitrous acid, due to the primary aliphatic nature of the amine group, it forms an unstable diazonium salt. The initial product decomposes to form nitrogen gas and benzyl cation.

Step 3: Identify the Final Products

The benzyl cation generated can quickly react with water to form benzyl alcohol (C6H5CH2OH). Thus, the reaction primarily leads to the formation of benzyl alcohol.

Key Concepts

Nitrous Acid Reaction

The nitrous acid reaction is an interesting process that involves the reaction between a primary amine, such as benzylamine, and nitrous acid. This reaction is notable because it typically leads to the formation of a diazonium salt, an intermediate compound. In the case of benzylamine, when it reacts with the nitrous acid, a diazonium salt is temporarily formed.

• Benzylamine, with the formula \( \text{C}_6\text{H}_5\text{CH}_2\text{NH}_2 \), contains an amino group that can react with HNO2.
• The reaction starts with the formation of nitrous acid, which is often generated in situ from sodium nitrite (NaNO2) and a mineral acid, like hydrochloric acid (HCl).
• This initial reaction with the amine group sets the stage for the upcoming formation of a diazonium salt.

Understanding this initial phase is essential as it creates the foundation for subsequent transformations, such as the formation of diazonium salts and eventually, benzyl cations.

Diazonium Salt Formation

Diazonium salts are crucial intermediates in organic chemistry, known for their involvement in various chemical reactions. When benzylamine undergoes reaction with nitrous acid, it doesn’t initially yield a stable diazonium salt due to the aliphatic nature of the amine.

• The resulting diazonium ion, however, is generally unstable in aliphatic systems, unlike in aromatic amines where they tend to be more stable.
• This instability of aliphatic diazonium salts often results in rapid decomposition, leading to the release of nitrogen gas (\( \text{N}_2 \)).
• The decomposition ultimately forms a benzyl cation, which is a positively charged intermediate.

The importance of this step rests in the transient nature of the diazonium salt in this context, which leads quickly to further reaction stages, highlighting the swift evolution from amine to resultant alcohol through intermediary stages.

Benzyl Cation Formation

The formation of a benzyl cation is a pivotal moment in the reaction process. This positively charged ion arises after the diazonium salt decomposes and marks the transition to the final stages of the reaction.

• Once formed, benzyl cations are highly reactive due to their positive charge and unstable nature.
• In this reaction, the benzyl cation doesn't exist for long before reacting with other available molecules.
• In a typical reaction setup, water molecules from the solution quickly interact with the benzyl cation to form benzyl alcohol \( \left( \text{C}_6\text{H}_5\text{CH}_2\text{OH} \right) \).

This step is essential as it drives the conversion towards the final organic product, benzyl alcohol, demonstrating the interconnectedness of reaction steps from initial amine to final alcohol through a cationic intermediate.