Problem 143 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The product formed is an isocyanide.

Step 1: Understanding the Reaction Context

The reaction described involves a primary amine reacting with chloroform and ethanolic KOH. This is known as the "carbylamine reaction" or "isocyanide test," which is used to identify primary amines by forming isocyanides.

Step 2: Identifying Reactants and Reagents

The key reactants in this reaction are a primary amine (R-NH_2) and chloroform (CHCl_3) in the presence of ethanolic KOH. Ethanolic means we use ethanol as a solvent, and KOH is the base used in the reaction.

Step 3: Analyzing the Mechanism

The reaction mechanism involves a series of steps: 1. Chloroform reacts with KOH to form dichlorocarbene (:CCl_2). 2. The dichlorocarbene then reacts with the primary amine, leading to the formation of an isocyanide.

Step 4: Chemical Equation Representation

The balanced chemical equation for the carbylamine reaction is: RNH_2 + CHCl_3 + 3KOH → RNC + 3KCl + 3H_2O This shows the formation of an isocyanide (is a notable feature of this reaction).

Step 5: Determining the Product

Based on the chemical equation and the mechanism, the primary product of this reaction is an isocyanide. Therefore, the correct answer is option (d), an isocyanide.

Key Concepts

Isocyanide Test

The isocyanide test is a distinctive chemical test used for the identification of primary amines. When a primary amine reacts with chloroform in the presence of ethanolic potassium hydroxide (KOH), it results in the formation of an isocyanide. This type of reaction is characteristic of the carbylamine reaction.
One of the unique features of this test is the production of an isocyanide, which often has a very pungent and unpleasant smell. This strong odor serves as an indicator of the reaction's progress and confirms the presence of primary amines in the test sample.
By understanding the isocyanide test, chemists can selectively identify compounds that contain primary amines even when mixed with other functional groups. This test plays a crucial role in organic synthesis and educational laboratories.

Primary Amines

Primary amines are organic compounds characterized by having one alkyl or aryl group attached to the nitrogen atom, represented by the general formula: R-NH_2. These compounds are fundamental building blocks in organic chemistry, participating extensively in diverse reactions because of their high reactivity.
Primary amines are pivotal in the isocyanide test because they are the functional groups that react with chloroform under basic conditions to produce isocyanides.
Some important characteristics of primary amines include:

• Polar Nature: Due to the nitrogen atom's electronegativity, primary amines display polar characteristics, affecting their solubility and boiling point.
• Reactivity: The lone pair of electrons on nitrogen makes primary amines highly reactive, leading to their use in various organic reactions.
• Odor: Like isocyanides, many primary amines have distinct odors, which can help in their identification.

Understanding primary amines' structure and reactivity is crucial for effectively utilizing the isocyanide test in detecting them.

Chemical Reaction Mechanism

A chemical reaction mechanism provides a detailed step-by-step account of a chemical change, demonstrating the transformation of reactants into products. In the carbylamine reaction, the mechanism unveils how a primary amine reacts with chloroform in the presence of ethanolic KOH, resulting in an isocyanide.
The carbylamine reaction mechanism involves the following stages:

• Formation of Dichlorocarbene: The reaction begins with chloroform reacting with KOH to create dichlorocarbene (:CCl_2). This intermediate is critical because it acts as a reactive species that will interact with the primary amine.
• Formation of Isocyanide: The dichlorocarbene subsequently reacts with the primary amine to produce an isocyanide, represented in the chemical equation as RNC.

The balanced chemical equation can be written as: \[ RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O \] This mechanism emphasizes the importance of intermediates like dichlorocarbene in facilitating reactions and forming the final product. Through this mechanism, chemists gain insights into how and why specific reactions occur, assisting in controlling and manipulating various chemical processes.