Carbylamine Reaction: The Isocyanide Test
The Carbylamine Reaction, also known as the Isocyanide Test, is a chemical test for the detection of primary amines. When a primary amine (aliphatic or aromatic) reacts with chloroform and alcoholic potassium hydroxide, it produces a foul-smelling isocyanide (carbylamine).
1. General Reaction
A primary amine ($R-NH_2$) reacts with Chloroform ($CHCl_3$) and alcoholic KOH upon heating to form an Isocyanide.
Observation: The formation of a very offensive, unpleasant, foul smell indicates a positive test.
Scope:
- Positive: Primary Amines (e.g., Methylamine, Aniline).
- Negative: Secondary ($2^\circ$) and Tertiary ($3^\circ$) Amines.
2. Detailed Mechanism
The reaction proceeds via the formation of a highly reactive neutral intermediate called Dichlorocarbene.
Step 1: Formation of Dichlorocarbene ($:CCl_2$)
The base ($OH^-$) removes the acidic proton from chloroform ($CHCl_3$) to form the trichloromethyl anion, which loses a chloride ion to generate dichlorocarbene.
Step 2: Nucleophilic Attack
The nitrogen atom of the primary amine acts as a nucleophile and attacks the electron-deficient carbon of the dichlorocarbene.
After a series of proton transfers and elimination of two molecules of HCl (neutralized by KOH), the final product Alkyl Isocyanide ($R-N \equiv C$) is formed.
3. Examples
A. Reaction with Methylamine (Aliphatic)
$$ CH_3NH_2 + CHCl_3 + 3KOH \rightarrow CH_3NC \text{ (Methyl Isocyanide)} + 3KCl + 3H_2O $$B. Reaction with Aniline (Aromatic)
$$ C_6H_5NH_2 + CHCl_3 + 3KOH \rightarrow C_6H_5NC \text{ (Phenyl Isocyanide)} + 3KCl + 3H_2O $$4. Key Limitations & Applications
Distinguishing Test
This reaction is the standard laboratory test to distinguish Primary Amines from Secondary and Tertiary amines. Only $1^\circ$ amines give the foul smell.
- Secondary ($R_2NH$) and Tertiary ($R_3N$) amines do NOT undergo this reaction because they lack the necessary hydrogen atoms on nitrogen to eliminate HCl and form the triple bond.
- It acts as a test for Chloroform as well (using a known primary amine).
Carbylamine Quiz
Test your understanding of the Isocyanide Test. 10 MCQs with explanations.
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