Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

On heating benzyl amine with chloroform and ethanolic KOH, the product obtained is [2002] (a) benzyl alcohol (b) benzaldehyde (c) benzonitrile (d) benzyl isocyanide

Step-by-Step Solution

Answer

The product obtained is benzyl isocyanide.

Step 1: Identify the Reaction

The reaction of benzyl amine with chloroform and ethanolic KOH is known as the Hofmann's carbylamine reaction or the carbylamine reaction. In this reaction, primary amines react with chloroform and a base to form isocyanides (also called carbylamines).

Step 2: Understand the Reactants

In this reaction, the primary amine used is benzylamine ( C6H5CH2NH2). Chloroform (CHCl3) acts as the source of the dichlorocarbene intermediate, and ethanolic KOH (potassium hydroxide in ethanol) provides the strong basic medium required for the reaction.

Step 3: Reaction Mechanism

Under the reaction conditions, KOH deprotonates chloroform to form the dichlorocarbene intermediate (:CCl2). This intermediate then reacts with the benzylamine to form benzyl isocyanide. Thus, the mechanism involves the formation of a carbylamine due to the interaction of the dichlorocarbene and the primary amine.

Step 4: Determine the Product

According to the carbylamine reaction, the final product formed when benzylamine reacts with chloroform and ethanolic KOH is benzyl isocyanide (C6H5CH2NC). This is a characteristic product of a carbylamine reaction when starting with a primary amine.

Key Concepts

Primary Amines

Primary amines are fundamental organic compounds characterized by the presence of the amine group, \(-NH_2\), attached to a carbon chain. These compounds serve as essential building blocks in organic chemistry due to their ability to react with various reagents. When considering reactions, the primary amine, such as benzylamine (\(C_6H_5CH_2NH_2\)), is crucial because of its lone pair of electrons on nitrogen.

The amine group can participate in nucleophilic reactions, making it a key player in the formation of other compounds.

• The presence of the amine group enables primary amines to react specifically in the Hofmann's carbylamine reaction to yield isocyanides.
• They are distinct from secondary and tertiary amines, which have different reactivities due to the number of alkyl groups bonded to the nitrogen atom.

Understanding the structure and reactivity of primary amines is vital for predicting the outcomes of chemical processes involving these groups.

Dichlorocarbene Intermediate

In reactions involving chloroform (\(CHCl_3\)), such as the Hofmann's carbylamine reaction, a key intermediate called dichlorocarbene (\(:CCl_2\)) is formed. This occurs when chloroform is deprotonated by a strong base, such as ethanolic potassium hydroxide (KOH).

The formation of the dichlorocarbene intermediate is pivotal because it acts as a highly reactive species that facilitates further chemical transformations.

• Dichlorocarbene is a reactive carbene with two chlorine atoms attached, making it highly electrophilic.
• Its generation is a two-step process involving deprotonation of chloroform followed by elimination of chloride.

The presence of dichlorocarbene in the reaction environment enables the conversion of primary amines into isocyanides, underscoring its importance in this specific reaction mechanism.

Benzyl Isocyanide

Benzyl isocyanide is the product of the Hofmann's carbylamine reaction when benzylamine reacts with chloroform and KOH. As an isocyanide, it is characterized by the \(-N\equiv\ C\) group, different from common amines or nitriles.

The formation of benzyl isocyanide is a hallmark of the carbylamine reaction due to the involvement of primary amines and dichlorocarbene.

• Isocyanides have a distinct and pungent smell, often considered unpleasant, and are less common compared to other functional groups in organic chemistry.
• They serve as intermediates in various organic synthesis processes and exhibit unique reactivity due to the connectivity of nitrogen and carbon atoms.

The formation of benzyl isocyanide from benzylamine highlights the utility and specificity of Hofmann's carbylamine reaction in producing isocyanides from primary amines.

Reaction Mechanism

Understanding the reaction mechanism of the Hofmann's carbylamine reaction provides insight into how primary amines like benzylamine are transformed into isocyanides. The mechanism is a multi-step process involving several key stages:

• Deprotonation of chloroform by the strong base KOH to generate the dichlorocarbene intermediate.
• Formation of dichlorocarbene, a highly reactive species that acts as an electrophile in the reaction.
• Interaction between the dichlorocarbene and the primary amine, where the lone pair of electrons on nitrogen plays a crucial role.

This stepwise transformation is critical for forming benzyl isocyanide, the final product.

Each stage in this reaction mechanism highlights the importance of intermediates and electron pair interactions, providing a deeper understanding of how seemingly simple starting materials like benzylamine and chloroform can undergo transformation into more complex compounds.