Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

When a primary amine reacts with chloroform in ethanolic \(\mathrm{KOH}\), then the product formed is (a) an aldehyde (b) an isocyanide (c) an cyanide (d) an alcohol

Step-by-Step Solution

Answer

The product formed is an isocyanide.

Step 1: Identifying the Reaction

When a primary amine reacts with chloroform (CHCl₃) in the presence of ethanolic potassium hydroxide (KOH), this is known as the carbylamine reaction or isocyanide test.

Step 2: Understanding the Mechanism

In the carbylamine reaction, the amine deprotonates the KOH forming a reactive intermediate, which subsequently reacts with chloroform. The KOH helps in forming dichlorocarbene (:CCl₂) from chloroform.

Step 3: Formation of Intermediate

The dichlorocarbene reacts with the primary amine to form an isocyanide. The reaction sequence leads to the creation of an intermediate known as an isocyanide (R-NC).

Step 4: Determining the Product

In conclusion, when a primary amine reacts with chloroform in ethanolic KOH, the final stable product of the reaction is an isocyanide (also known as a carbylamine).

Key Concepts

Understanding Primary Amines

Primary amines are organic compounds that contain an amino group attached to a single alkyl or aryl group. They are characterized by the presence of the functional group \(-NH_2\). This amine group is bonded to a carbon atom that is part of a larger organic structure. Because of their structure, primary amines are capable of participating in a variety of chemical reactions. They are known to be nucleophilic, meaning they can donate a pair of electrons to form a new chemical bond. This property plays a crucial role in the carbylamine reaction.

• Structure: Contains an \(-NH_2\) group bonded to a carbon atom.
• Properties: Nucleophilic and able to participate in a variety of reactions.
• Examples: Methylamine (\(CH_3NH_2\)), aniline (\(C_6H_5NH_2\)).

This makes primary amines suitable candidates for reactions like the carbylamine reaction, where their nucleophilic nature is utilized.

The Role of Chloroform

Chloroform, or trichloromethane (CHCl₃), is a simple organic compound with chloroform as its common name. It is a colorless liquid with a slightly sweet smell and was once widely used as an anesthetic. In chemical reactions, chloroform can act as a source of dichlorocarbene, particularly when treated with a strong base like potassium hydroxide (KOH).

• Formula: \(CHCl_3\).
• Properties: Colorless, sweet-smelling liquid.
• Use in Reactions: Generates dichlorocarbene in the presence of KOH.

For the carbylamine reaction, chloroform’s role is crucial as it provides the dichlorocarbene necessary to interact with the primary amine and form isocyanides.

Dichlorocarbene Generation

Dichlorocarbene is a reactive intermediate with the formula \(:CCl_2\). It is generated from chloroform during the carbylamine reaction in the presence of a strong base like KOH. The process involves the deprotonation of chloroform, followed by the elimination of hydrochloride ions, resulting in the formation of dichlorocarbene.

• Formation: Occurs under basic conditions (using KOH).
• Structure: Reactive species with two chlorine atoms bonded to a carbon atom (A\(:CCl_2\)).

Dichlorocarbene is highly reactive due to its electron-deficient center. This makes it capable of bonding with other nucleophiles, in this case, the primary amine, to produce isocyanides.

Isocyanide Formation

The formation of isocyanides, or carbylamines, occurs in the final step of the carbylamine reaction. Once dichlorocarbene is formed, it quickly reacts with the primary amine. The nucleophilic nitrogen in the primary amine attacks the electron-deficient carbon of dichlorocarbene, leading to the formation of an isocyanide.

• Reaction: Primary amine reacts with dichlorocarbene.
• Product: Isocyanide (R-NC), characterized by a distinct unpleasant odor.

Isocyanides have a unique structure and are known for their strong, pungent smell. This smell is actually utilized in the isocyanide test, a qualitative test to determine the presence of primary amines. The production of isocyanides during the reaction not only confirms the occurrence of the carbylamine reaction but also the presence of a primary amine in the test compound.