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

The 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, also known as the isocyanide test, is a chemical test used to detect primary amines. In the presence of chloroform and a base, primary amines produce isocyanides, which are identified by their foul smell. This test does not work for secondary or tertiary amines.

Step 2: Identify Primary Amines Among Options

Look at each compound given:- (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is a primary amine.- (b) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{NH}_{2}\) is a primary amine.- (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) is a primary amine (aniline).- (d) \(\mathrm{C}_{2} \mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2} \mathrm{H}_{5}\) is a secondary amine.

Step 3: Determine Which Compound Fails the Test

The carbylamine test only works for primary amines. Since option (d) \(\mathrm{C}_{2}\mathrm{H}_{5}-\mathrm{NH}-\mathrm{C}_{2}\mathrm{H}_{5}\) is a secondary amine, it does not give a positive carbylamine test.

Key Concepts

Chemical Identification

The carbylamine test is a fascinating method used in chemistry to identify primary amines in a compound. It involves mixing the substance with chloroform and a strong base, usually an alkali such as sodium or potassium hydroxide. This reaction specifically detects primary amines by transforming them into isocyanides, which are easily recognizable due to their distinctly unpleasant odor. This bad smell serves as the key indicator that a primary amine is present.

The power of the carbylamine test lies in its simplicity and specificity. It remains a staple in chemical laboratories for quickly determining if a primary amine is present without requiring advanced equipment. However, it does have limitations. It cannot be used with secondary or tertiary amines, as they do not produce isocyanides in this reaction. Hence, when using this test, it’s essential to understand its scope to avoid mismatches in identification.

Primary Amines

Primary amines are organic compounds that feature an amino group \((\text{\text{NH}}_{2})\) attached to a carbon atom. This structure equips primary amines with unique chemical properties that make them suitable candidates for the carbylamine test. Each amine’s behavior in chemical reactions can vary, depending on factors like the nature of the carbon backbone it is attached to.

In the context of the exercise, examples like \(\text{CH}_3 \text{CH}_2 \text{NH}_2\) (ethylamine), \(\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{NH}_2\) (propylamine), and \(\text{C}_6 \text{H}_5 \text{NH}_2\) (aniline) showcase primary amines' versatility. Each possesses the defining \([\text{NH}_2]\) group, allowing them to undergo the carbylamine reaction successfully. When subjected to the conditions of the carbylamine test, these molecules will produce isocyanides with a distinctive odor, signifying the presence of a primary amine.

Secondary Amines

Secondary amines differ from primary amines because they contain two carbon groups attached to the nitrogen atom instead of one. This structural change dramatically affects their chemical behavior, including their interaction with tests like the carbylamine test.

In the exercise, \(\text{C}_2 \text{H}_5-\text{NH}-\text{C}_2 \text{H}_5\) (diethylamine) is an example of a secondary amine. Unlike primary amines, secondary amines do not yield isocyanides when treated with chloroform and an alkali. Thus, they do not release the foul odor indicative of the carbylamine reaction.

Understanding the structural and functional differences between primary and secondary amines is crucial in chemical identification tasks. While both belong to the broader class of amines, their distinct behavior highlights the necessity of choosing the right chemical tests for accurate identification.