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Question 52

Which of the following amine(s) show(s) positive carbylamines test?

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Choose the correct answer from the options given below :

Solution

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The carbylamine test (also known as Hofmann's carbylamine test) is a specific chemical test used exclusively for the detection of primary ($$1^\circ$$) amines.

When a primary amine is heated with chloroform ($$\text{CHCl}_3$$) and alcoholic potassium hydroxide ($$\text{KOH}$$), it forms an isocyanide (carbylamine), which has an incredibly foul, offensive smell. Secondary ($$2^\circ$$) and tertiary ($$3^\circ$$) amines do not show this reaction.

$$\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH} \xrightarrow{\Delta} \text{R-NC} \ (\text{Isocyanide}) + 3\text{KCl} + 3\text{H}_2\text{O}$$

  • A. Aniline ($$\text{C}_6\text{H}_5\text{NH}_2$$): This is a primary aromatic amine. Since the nitrogen atom is attached to only one carbon skeleton, it will give a positive test.
  • B. $$(\text{CH}_3)_2\text{NH}$$: This is a secondary ($$2^\circ$$) aliphatic amine. It will not give the test.
  • C. $$\text{CH}_3\text{NH}_2$$: This is a primary ($$1^\circ$$) aliphatic amine. It will give a positive test.
  • D. $$(\text{CH}_3)_3\text{N}$$: This is a tertiary ($$3^\circ$$) aliphatic amine. It will not give the test.
  • E. N-methylaniline ($$\text{C}_6\text{H}_5\text{NHCH}_3$$): This is a secondary ($$2^\circ$$) aromatic amine. It will not give the test.

    • Because only A and C are primary amines, they are the only ones that show a positive carbylamine test.

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