Problem 45 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds cannot by identified by carbylamine test? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

Compound (d) \\( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5} \) cannot be identified by the carbylamine test.

Step 1: Understand the Carbylamine Test

The carbylamine test is used to identify primary amines. When a primary amine is heated with chloroform and alcoholic potassium hydroxide, it produces isocyanides, which have a characteristic foul smell. This test does not work for secondary or tertiary amines.

Step 2: Analyze the Given Compounds

We need to determine whether the given compounds are primary, secondary, or aromatic amines. - Compound (a) \\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \) is a primary amine.- Compound (b) \\( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) is also a primary amine.- Compound (c) \\( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) is an aromatic primary amine.- Compound (d) \\( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a secondary amine.

Step 3: Identify the Compound that Cannot be Tested with Carbylamine Test

Since the carbylamine test is applicable only to primary amines and not to secondary or tertiary amines, Compound (d) \\( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5} \) cannot be identified using this test because it is a secondary amine.

Key Concepts

Understanding Primary Amines

Primary amines are organic compounds characterized by the presence of an amino group attached to a single alkyl or aryl group. The amino group is denoted as \(-\mathrm{NH}_2\), and is the distinctive feature of primary amines. Primary amines are key because they can be identified using the carbylamine test.

• Structure: They possess the general formula \(R-\mathrm{NH}_2\), where \(R\) can be any alkyl or aryl group.
• Reactivity: They can undergo various chemical reactions, making them significant in synthetic chemistry.
• Distinguishing Trait: The presence of only one hydrocarbon group attached to nitrogen differentiates primary amines from secondary and tertiary amines.

This distinction is vital in organic chemistry, as it determines how amines react under certain conditions, such as in the carbylamine test.

Exploring Secondary Amines

Secondary amines contain two organic substituents bonded to the nitrogen atom, making them different from primary amines. They are not reactive in the carbylamine test due to their structure.

• Structure: Secondary amines have the general formula \(R_2\mathrm{NH}\), where each \(R\) represents an alkyl or aryl group.
• Properties: Secondary amines exhibit different chemical behaviors compared to primary and tertiary amines, marked by their inability to form isocyanides with the carbylamine test.

In the context of organic chemistry, understanding these differences helps predict reactivity and recognize structural variations among amines.

Isocyanides and Their Significance

Isocyanides, or carbylamines, are compounds with the functional group \(-\mathrm{NC}\). They are known for their extremely pungent odor, which is why their formation is a good indicator in the carbylamine test.

• Formation: Formed when a primary amine is treated with chloroform and an alkaline solution, like potassium hydroxide.
• Characteristic: Their foul smell provides a simple way to identify the presence of primary amines in a mixture.
• Reactivity: While isocyanides themselves are generally used in chemical synthesis, their formation marks a specific type of primary amine reaction.

The pivotal role of isocyanides in the carbylamine test emphasizes the practical applications of basic organic chemistry experimentation.

Role of Chloroform and Potassium Hydroxide in the Carbylamine Test

Chloroform and potassium hydroxide are essential chemicals in conducting the carbylamine test. Their role is to facilitate the transformation of a primary amine into an isocyanide.

• Chloroform: Acts as the carbon donor in the synthesis of isocyanides when heated with primary amines under basic conditions.
• Potassium Hydroxide: Provides the alkaline environment necessary for the reaction. It deprotonates the amine, making it more reactive towards chloroform.

Together, these reagents create the conditions necessary for identifying primary amines through the distinctive formation of isocyanides, turning theoretical chemistry into measurable, observable results.