Question

Explain the carbylamine test.

Answer 1

Hint: Try to recall the reagents used for carbylamine test. Now write down the reactant and try to add the reagent and see how the reaction goes. Remember that the end product is going to be carbyl amine.
Complete step-by-step solution:
-Carbylamine reaction also known as Hofmann isocyanide synthesis is the synthesis of an isocyanide by the reaction of a primary amine with chloroform in presence of a base.
-The intermediate form is dichlorocarbene.
-Carbylamine test is one of the confirmatory tests for primary amines, as only primary amine answers this test.
-The completion of this test is indicated by the foul smell of the isocyanide.
We will write the reactions involve for a primary amine, say
$C{{H}_{3}}-N{{H}_{2}}$
Chloroform in the presence of base gives dichlorocarbene.
$\text{H-CC}{{\text{l}}_{3}}\text{ }\xrightarrow{O{{H}^{-}}}\text{ CC}{{\text{l}}_{3}}^{-}\text{ }\xrightarrow{-C{{l}^{-}}}\text{ :CC}{{\text{l}}_{2}}$
-Dichlorocarbene then reacts with the primary amine,
$C{{H}_{3}}-N{{H}_{2}}\text{ }\xrightarrow{:CC{{l}_{2}}}\text{ }C{{H}_{3}}-N{{H}_{2}}^{+}-CC{{l}_{2}}^{-}$
To this intermediate, 2 moles of base acts giving,
$C{{H}_{3}}-NH-CHC{{l}_{2}}\text{ }\xrightarrow{O{{H}^{-}}}\text{ C}{{\text{H}}_{3}}-N=CHCl$
$C{{H}_{3}}-N=CHCl\text{ }\xrightarrow{O{{H}^{-}}}\text{ }C{{H}_{3}}-N=C:$
From the above reaction mechanism, we understand the formation of carbyl amine and is confirmed by the foul-smelling odour of the isocyanide.

Note: Delocalize the bonds in the correct order to obtain the final product. Rearrangement occurs in the reaction to maximise the stability of intermediate. Always remember that there is one carbon atom extra in the product as there is the formation of isocyanide.