Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following cannot be identified by carbyl amine test? 1\. \(\mathrm{C}_{3} \mathrm{H}_{7} \mathrm{NH}_{2}\) 2\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) 3\. \(\mathrm{C}_{6}{\underline{\phantom{xx}}}^{3} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{6} \mathrm{H}_{5}\) 4\. \(\left(\mathrm{C}_{3} \mathrm{H}_{7}\right)_{3} \mathrm{~N}\) (a) 2,3 (b) \(1,2,4\) (c) 3,4 (d) \(1,3,4\)

Step-by-Step Solution

Answer

Option (c) 3,4

Step 1: Understand the Carbylamine Test

The carbylamine test is a chemical test for detecting primary amines. When a primary amine reacts with chloroform and alcoholic potassium hydroxide, an isocyanide (carbylamine) is formed, which has a foul smell. Secondary and tertiary amines do not undergo this test and will not form carbylamines.

Step 2: Identify the Functional Groups

Examine each compound to determine if it contains a primary amine group:1. \( \mathrm{C}_3 \mathrm{H}_7 \mathrm{NH}_2 \) is a primary amine (aliphatic).2. \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \) is a primary amine (aromatic).3. \( \mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}-\mathrm{C}_6 \mathrm{H}_5 \) is a secondary amine.4. \( (\mathrm{C}_3 \mathrm{H}_7)_3 \mathrm{~N} \) is a tertiary amine.

Step 3: Apply the Carbylamine Test Criteria

Based on the test criteria, determine which compounds cannot undergo the carbylamine test:- Compound 3 (\( \mathrm{C}_6 \mathrm{H}_5-\mathrm{NH}-\mathrm{C}_6 \mathrm{H}_5 \)), being a secondary amine, cannot be identified by the test.- Compound 4 (\( (\mathrm{C}_3 \mathrm{H}_7)_3 \mathrm{~N} \)), being a tertiary amine, also cannot be identified by the test.

Step 4: Select the Correct Answer

Considering the compounds that cannot be identified by the carbylamine test (secondary and tertiary amines), the correct options that match are 3 and 4. This corresponds to option (c) \((3,4)\).

Key Concepts

Primary Amines

Primary amines are the simplest type of amine. They have one alkyl or aryl group attached to the nitrogen atom. This structure can be represented as R-NH₂, where "R" stands for an alkyl or aryl group.

These amines are highly reactive, which makes them easily identifiable using chemical tests, such as the carbylamine test.

• In primary amines, the presence of only one carbon-containing group allows them to interact significantly with other molecules.
• They can form hydrogen bonds, which contribute to their solubility in water and influence their boiling points.

The most notable feature of primary amines in the context of chemical testing is their ability to undergo the carbylamine test.

During this test, primary amines react with chloroform in the presence of alcoholic potassium hydroxide, forming isocyanide, which is recognizable by its strong, unpleasant odor.

Because secondary and tertiary amines do not produce isocyanides, this test specifically identifies primary amines.

Secondary Amines

Secondary amines contain two organic groups attached to the nitrogen atom, which can be represented as R₂NH, where "R" represents alkyl or aryl groups. Secondary amines differ from primary amines in both structure and chemical reactivity.

• The additional organic group leads to different chemical behavior and physical properties compared to primary amines.
• They have moderate ability to form hydrogen bonds due to one of the hydrogen atoms being replaced with an organic group.

While they can participate in many reactions that involve amines, secondary amines do not undergo the carbylamine test.

The carbylamine test only works with primary amines. Secondary amines will not react with chloroform and potassium hydroxide to form isocyanides, hence they aren't detected using this test.

This makes secondary amines fall outside the scope of identification through this traditional method. Knowing this distinction helps chemists to isolate and study different types of amines more effectively.

Tertiary Amines

Tertiary amines are characterized by the presence of three organic groups attached to the nitrogen atom, and their structure is often denoted as R₃N. These groups can either be alkyl or aryl.

• Unlike primary and secondary amines, tertiary amines lack any hydrogen atoms bound directly to the nitrogen, which affects their ability to participate in reactions.
• This structural feature significantly alters their solubility and affects how they engage in hydrogen bonding.

Tertiary amines are notable because they do not react in the carbylamine test.

Without the hydrogen atoms to enable the reaction, tertiary amines will not produce isocyanides in the presence of chloroform and alcoholic potassium hydroxide.

Hence, they cannot be detected using the carbylamine test. This limitation is crucial for distinguishing between primary, secondary, and tertiary amines during chemical analysis.