Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will give a positive carbylamine test? 1\. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\) 2\. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}-\mathrm{CH}_{3}\) 3\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) Select the correct answer using the codes given below. (a) 1 and 3 (b) 2 and 4 (c) 1 and 4 (d) 3 and 4

Step-by-Step Solution

Answer

Option (c) 1 and 4 will give a positive carbylamine test.

Step 1: Understanding the Carbylamine Test

The carbylamine test is used to detect the presence of primary amines. In this test, a primary amine reacts with chloroform and a base (usually KOH) to form an isocyanide (carbylamine), which has a very foul odor. This test is specific only to primary amines.

Step 2: Identify Primary Amines from the Options

Examine each option to determine if it contains a primary amine:1. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\) is a primary amine.2. \(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}-\mathrm{CH}_{3}\) is a secondary amine.3. \((\mathrm{CH}_{3})_{3} \mathrm{~N}\) is a tertiary amine.4. \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\) is a primary amine.

Step 3: Select Options with Primary Amines

From the identification:- Option 1 (\(\mathrm{H}_{3} \mathrm{C}-\mathrm{NH}_{2}\)) is a primary amine.- Option 4 (\(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{NH}_{2}\)) is a primary amine.Thus, the amines that will give a positive carbylamine test are options 1 and 4.

Key Concepts

Primary Amines

Primary amines are a class of compounds in organic chemistry that consist of a nitrogen atom bonded to at least one alkyl or aryl group and two hydrogen atoms. The general formula for a primary amine is \(-RNH_2\), where \(-R\) represents the alkyl or aryl group.
Primary amines are crucial in various biochemical processes and industrial applications.
Their unique structure allows them to participate in specific reactions, such as the carbylamine test.

• The nitrogen atom in primary amines has a lone pair of electrons, making it nucleophilic and allowing it to react with electrophiles.
• They can form hydrogen bonds, which affects their physical properties like boiling point and solubility.

In the context of the carbylamine test, primary amines like \( ext{CH}_3- ext{NH}_2\) and \( ext{C}_6 ext{H}_5- ext{NH}_2\) can be identified because they react with chloroform and a base to produce isocyanides, which have a distinct, unpleasant odor.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with the study of compounds primarily made of carbon and hydrogen, often containing elements such as oxygen, nitrogen, sulfur, and halogens. This scientific field explores the structure, properties, composition, and reactions of organic compounds.
Understanding organic chemistry is essential for recognizing the behavior of functional groups like amines in reactions.

• Amines are classified based on the number of carbon-containing groups attached to the nitrogen atom: primary (one group), secondary (two groups), and tertiary (three groups).
• The carbylamine test highlights the principle that chemical reactions can selectively detect functional groups in organic molecules.

In the case of the carbylamine test, it specifically detects primary amines by their ability to form isocyanides upon reaction with chloroform and a base, a unique feature leveraged in practical organic analysis.

Isocyanides

Isocyanides, also known as isonitriles, are a class of organic compounds characterized by the functional group \(-NC\). They are structurally isomers of cyanides but exhibit very different chemical and physical properties.
Isocyanides are known for their strong, unfavorable smell, which is a key aspect of the carbylamine test.

• They are formed when a primary amine undergoes a reaction with chloroform and a strong base, such as potassium hydroxide.
• The reaction sequence involves the formation of dichlorocarbene, which reacts with the primary amine to form the isocyanide.

The distinct odor of isocyanides serves as an indicator in the carbylamine test for the presence of primary amines, making this reaction a valuable tool in identifying these functional groups in organic material. Despite their less favorable smell, isocyanides play a significant role in certain synthetic pathways and as intermediates in organic synthesis.