Question

A compound \(C_3H_5N\) undergoes reduction to form a primary amine. The amine gives compounds [B] and [C] in the following reactions. Identify [B] and [C].

• A. [B]: alcohol,[C]: nitrile
• B. [B]: ketone,[C]: amide
• C. [B]: amide,[C]: isocyanide
• D. [B]: aldehyde,[C]: nitro compound

Hint:

Only primary amines give the carbylamine test and produce foul-smelling isocyanides. Nitriles on reduction always form primary amines having one extra carbon atom in the chain.

Answer 1

The Correct Option is C

Concept: This question is based on the chemistry of nitriles and primary amines. Nitriles undergo reduction to produce primary amines. Primary amines further undergo several characteristic reactions such as:
• Carbylamine reaction giving isocyanides
• Acylation reactions producing amides Only primary amines respond positively to the carbylamine test, which is an important identification reaction in organic chemistry.

Step 1: Identification of the compound \(C_3H_5N\). The molecular formula given is: \[ C_3H_5N \] A compound having this molecular formula and capable of producing a primary amine on reduction is a nitrile. Hence the compound is: \[ \mathrm{CH_3CH_2CN} \] which is known as propanenitrile or ethyl cyanide. The structure can be written as: \[ \mathrm{CH_3-CH_2-C \equiv N} \] The presence of the nitrile group: \[ \mathrm{-C \equiv N} \] is responsible for its reduction to a primary amine.

Step 2: Reduction of nitrile to primary amine. Nitriles undergo catalytic hydrogenation in the presence of nickel catalyst or can also be reduced using reducing agents such as: \[ \mathrm{LiAlH_4} \] During reduction, the triple bond between carbon and nitrogen gets converted into a single bond and the nitrile carbon becomes attached to: \[ \mathrm{-CH_2NH_2} \] Thus: \[ \mathrm{CH_3CH_2CN \xrightarrow{H_2/Ni} CH_3CH_2CH_2NH_2} \] The product formed is: \[ \mathrm{CH_3CH_2CH_2NH_2} \] which is propylamine. Propylamine is a primary amine because the amino group: \[ \mathrm{-NH_2} \] is attached to only one alkyl group.

Step 3: Formation of isocyanide by carbylamine reaction. Primary amines undergo the carbylamine reaction when heated with chloroform and alcoholic potassium hydroxide. The general reaction is: \[ \mathrm{RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O} \] In this reaction:
• \(RNH_2\) is a primary amine
• \(RNC\) is an isocyanide or carbylamine Since propylamine is a primary amine, it gives this reaction successfully. Hence compound [C] is: \[ [C] = \text{isocyanide} \] This reaction is extremely important because secondary and tertiary amines do not give the carbylamine test.

Step 4: Formation of amide from primary amine. Primary amines react with acid chlorides such as acetyl chloride to produce amides. The reaction is: \[ \mathrm{RNH_2 + CH_3COCl \rightarrow RNHCOCH_3 + HCl} \] In this process:
• The hydrogen atom of the amino group gets replaced
• An acyl group gets attached to nitrogen
• An amide linkage is formed Therefore the product [B] formed is an amide. Hence: \[ [B] = \text{amide} \]

Step 5: Final conclusion. From the above reactions: \[ [B] = \text{amide} \] and \[ [C] = \text{isocyanide} \] Therefore the correct option is: \[ \boxed{(3)} \]