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Q6.19(xx):
How the following conversions can be carried out? (xx) Aniline to phenylisocyanide

Solution :

Step 1: Chemical Transformation Identification

The conversion of aniline ($C_6H_5NH_2$) to phenylisocyanide ($C_6H_5NC$) is classified as a Carbylamine Reaction (or Isocyanide test). This reaction is specific to primary amines, where the amine reacts with chloroform in the presence of a strong base to produce an isocyanide, characterized by an offensive odor.

Step 2: Balanced Chemical Equation

The reaction involves the substitution of the hydrogen atoms on the nitrogen atom by a carbon atom derived from chloroform ($CHCl_3$). The overall balanced equation is:

$C_6H_5NH_2 + CHCl_3 + 3KOH_{(alc)} \rightarrow C_6H_5NC + 3KCl + 3H_2O$

Step 3: Reaction Mechanism Analysis

The reaction proceeds via the generation of an electrophilic intermediate, dichlorocarbene ($:CCl_2$).

  • Step A (Formation of Carbene): Potassium hydroxide ($KOH$) abstracts a proton from chloroform to form a trichloromethyl anion, which subsequently loses a chloride ion to generate dichlorocarbene.
    $CHCl_3 + OH^- \rightarrow :CCl_2 + H_2O + Cl^-$ [Per the Alpha-elimination mechanism]
  • Step B (Nucleophilic Attack): The lone pair on the nitrogen of aniline attacks the electron-deficient dichlorocarbene to form an intermediate species.
  • Step C (Elimination): Further treatment with base leads to the elimination of two molecules of $HCl$ (neutralized by $KOH$), resulting in the formation of the isocyanide linkage ($-NC$).

Step 4: Visualizing the Structural Change

Aniline ($C_6H_5NH_2$) NH₂ → $CHCl_3 / KOH$ Phenylisocyanide ($C_6H_5NC$) NC

Step 5: Experimental Conditions

Reagent Purpose
Chloroform ($CHCl_3$) Source of the carbon atom for the isocyanide functional group.
Alcoholic $KOH$ Provides the basic medium to facilitate dehydrohalogenation.
Heat Required to overcome the activation energy for the formation of the carbene intermediate.

Final Solution: Aniline reacts with chloroform and alcoholic potassium hydroxide to yield phenylisocyanide and potassium chloride as a byproduct, according to the Carbylamine reaction: $C_6H_5NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} C_6H_5NC + 3KCl + 3H_2O$.


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