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 as it is not a primary amine.

Step 1: Understanding the Carbylamine Test

The carbylamine test is used in organic chemistry to identify primary amines. When a primary amine is heated with chloroform and alcoholic potassium hydroxide, an isocyanide is formed which has a very foul smell. This test is specific for primary amines, both aliphatic and aromatic.

Step 2: Identifying Primary Amines

Let's examine each compound: (a) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \) is a primary amine, hence it will respond to the carbylamine test. (b) \( \mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2} \) is also a primary amine and will respond to the test. (c) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) is an aromatic primary amine and will also respond to the carbylamine test. (d) \( \mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5} \) is a secondary amine, so it will not respond to the carbylamine test.

Step 3: Determining the Compound Not Identified

Since the carbylamine test only identifies primary amines, compound (d) which is a secondary amine cannot be identified by this test.

Key Concepts

Primary Amines

Primary amines are fundamental compounds in organic chemistry. They contain an amino group (-NH\(_2\)) linked to a carbon atom that is not part of an aromatic ring. These compounds are distinguished by their ability to form hydrogen bonds due to the presence of the hydrogen atoms attached to the nitrogen.They are characterized by their structure, where the nitrogen atom is bonded to one alkyl or aryl group.

• Simple examples include methylamine (CH\(_3\)NH\(_2\)) and ethylamine (CH\(_3\)CH\(_2\)NH\(_2\)).
• They can be aliphatic or aromatic, with the aromatic type having the nitrogen attached to an aromatic ring, such as in aniline (C\(_6\)H\(_5\)NH\(_2\)).

Primary amines are versatile and react with different reagents. This reactivity makes them useful in identifying tests, such as the Carbylamine Test, to detect their presence.

Isocyanide Formation

Isocyanide formation is an essential reaction for identifying primary amines through the Carbylamine Test. When a primary amine is treated with chloroform (CHCl\(_3\)) and a strong base like alcoholic potassium hydroxide (KOH), it forms an isocyanide (also called isonitrile).This compound has a distinctively unpleasant smell, often described as harsh or offensive, which provides a clear indication of the reaction’s occurrence.The reaction proceeds as follows:- The primary amine reacts with chloroform and KOH.- The intermediate product eliminates HCl.- An isocyanide forms as a result.This particular test is exclusive to primary amines, helping to differentiate them from other amines based on the distinctive odor emitted. Secondary and tertiary amines do not engage in this reaction, which makes this test very specific.

Secondary Amines

Secondary amines have a different structural framework compared to primary amines. In these compounds, the nitrogen atom is bonded to two carbon-containing groups, which could be either alkyl or aryl groups.Unlike primary amines, secondary amines cannot participate in the Carbylamine Test because they do not possess the free hydrogen atom necessary to form the isocyanide intermediate. This is why they are omitted in tests specific for primary amines.

• Common examples include diethylamine (C\(_2\)H\(_5\)-NH-C\(_2\)H\(_5\)).
• They have stronger basicity due to the electron-releasing effect of the attached alkyl groups.

Despite not being detectable by the Carbylamine Test, secondary amines have significant applications in synthesis and industry due to their unique chemical characteristics and reactivity patterns.