Question

The major product of carbylamine reaction is :

• A. Carboxylic acid
• B. Aldehyde
• C. Cyanide
• D. Isocyanide

Hint:

Isocyanides are also called carbylamines, giving the reaction its name.
Secondary and tertiary amines do not undergo this reaction because they lack the required two hydrogens on nitrogen.
The foul smell of the product makes it an excellent laboratory test for primary amines.

Answer 1

The Correct Option is D

Concept:

• The Carbylamine reaction is a diagnostic test used exclusively for primary amines (both aliphatic and aromatic).

• It involves the reaction of a primary amine with chloroform and ethanolic potassium hydroxide.

• The reaction leads to the synthesis of isocyanides, which are characterized by an extremely offensive and pungent odor.
Step 1: Generation of the reactive intermediate, Dichlorocarbene
In the presence of a strong base like KOH, chloroform \( (\text{CHCl}_3) \) undergoes \( \alpha \)-elimination.
This process removes a proton and a chloride ion to generate the highly reactive neutral intermediate known as dichlorocarbene \( (:\text{CCl}_2) \).
\[ \text{CHCl}_3 + \text{OH}^- \rightarrow :\text{CCl}_2 + \text{H}_2\text{O} + \text{Cl}^- \]

Step 2: Nucleophilic attack by the primary amine and subsequent eliminations
The nitrogen lone pair of the primary amine \( (\text{R}-\text{NH}_2) \) attacks the electron-deficient dichlorocarbene.
This is followed by the elimination of two molecules of HCl (neutralized by KOH).
The overall sequence leads to the formation of the alkyl/aryl isocyanide.
\[ \text{R}-\text{NH}_2 + :\text{CCl}_2 \xrightarrow{-2\text{HCl}} \text{R}-\text{N} \equiv \text{C} \]

Step 3: Final reaction summary
The balanced chemical equation for the process is:
\[ \text{R}-\text{NH}_2 + \text{CHCl}_3 + 3\text{KOH (alc.)} \xrightarrow{\Delta} \text{R}-\text{NC} + 3\text{KCl} + 3\text{H}_2\text{O} \]
The product \( \text{R}-\text{NC} \) is called an isocyanide or carbylamine.