Question

Positive carbylamine test is shown by:

• A. N-Methylaniline
• B. N,N-Dimethlaniline
• C. Triethylamine
• D. p-Methylbenzyamine

Hint:

The carbylamine test is a useful qualitative test to identify primary amines. Secondary amines and amides do not give this test, as they cannot form isocyanides under these conditions.

Answer 1

The Correct Option is B

The carbamide (carbamine) test is used to detect the presence of primary amines. In this test, a primary amine reacts with chloroform (CHCl₃) in the presence of a base (like sodium hydroxide, NaOH) to form an isocyanide (also called a carbamide), which typically gives a characteristic foul odor.
- N-Methylaniline is a secondary amine, and secondary amines do not give a positive carbylamine test because they do not react to form an isocyanide.
- N,N-Dimethlaniline is also a secondary amine and similarly does not react positively in this test.
- Triethylamine is another secondary amine, so it will not give a positive carbylamine test.
- p-Methylbenzyamine is a primary amine, and primary amines, such as aniline derivatives, react with chloroform to form an isocyanide and give a positive result.
Thus, the correct answer is N,N-Dimethlaniline (option 2).
Chemical Reaction for the Carbylamine Test:
The reaction can be written as follows for a primary amine (R-NH₂): \[ \text{R-NH}_2 + \text{CHCl}_3 + \text{NaOH} \rightarrow \text{R-N=C=O} + 3\text{NaCl} + 2\text{H}_2\text{O} \] In this reaction: - R-NH₂ is the primary amine (such as aniline or methylamine). - CHCl₃ (chloroform) reacts with the amine in the presence of a base (NaOH) to form an isocyanide (R-N=C=O). The presence of an isocyanide (also called an isonitrile) gives the distinctive positive result with a foul odor.