Question

Which of the following compounds gives carbylamine test?
A. N-methyl-o-methyl aniline
B. N, N-dimethyl aniline
C. 2, 4-diethyl aniline
D. p-methyl-N-methyl benzyl amine

Answer 1

Hint: Carbylamine test is given by aliphatic or aromatic primary amines only. The secondary, tertiary amines give a negative result for this test.

Complete answer:
Isopropyl amine is a primary amine. It can give positive carbylamine test. In the carbylamine test or Hoffmann's carbylamine test (also known as Isocyanide test), the aliphatic or aromatic primary amine of consideration is heated with chloroform in presence of alcoholic potassium hydroxide to give foul smelling alkyl/aryl isocyanide or carbylamine.

From the structure we can say that N-methyl-o-methyl aniline is a secondary amine.

From the structure we can say that N, N-dimethyl aniline is a tertiary amine.

 From the structure we can say that 2, 4-diethyl aniline is a primary amine.

From the structure we can say that p-methyl-N-methyl benzyl amine is a secondary amine.
Hence, from all the above 2, 4-diethyl aniline is a primary amine. Therefore, C option is correct.
Additional Information: Aliphatic or aromatic primary amines in heating with chloroform and alcoholic KOH give foul smelling alkyl isocyanides or carbylamines.
The reaction is as follows:
$\text{R-N}{{\text{H}}_{\text{2}}}\text{+CHC}{{\text{l}}_{\text{3}}}\text{+3KOH}\xrightarrow{\text{ }\!\!\Delta\!\!\text{ }}\text{R-NC+3KCl+3}{{\text{H}}_{\text{2}}}\text{O}$
Only the ${{1}^{\circ }}$amines give the positive result for the isocyanide test, hence the reaction can be used as a chemical test for the distinguishing purposes between${{1}^{\circ }}$, ${{2}^{\circ }}$and ${{3}^{\circ }}$amines. Sometimes the reaction is also known as Saytzeff's isocyanides test. The mechanism involves the addition of amine to dichlorocarbene, a reactive intermediate which is generated by the dehydrohalogenation of chloroform. If a primary amine is present, and undergoes the carbylamine reaction then the product formed is isocyanide which is also known as carbylamine, with the presence of foul smell.

Note:
The Carbylamine test is not given by secondary and tertiary amines. Hence we cannot make isocyanides from ${{2}^{\circ }}$and ${{3}^{\circ }}$amines as C have to form triple bond with N to form the isocyanide. In this reaction the chloroform not only acts as a solvent but also as a working reagent.