Question

An organic compound \(A\) with molecular formula \(C_3H_5N\) on reaction with \(C_6H_5MgBr\) followed by hydrolysis gives compound \(B\). Compound \(B\) forms orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens' or Fehling's reagent nor decolourises bromine water. On drastic oxidation with chromic acid it gives carboxylic acid \(C\) having molecular formula \(C_7H_6O_2\). Identify \(A\), \(B\), and \(C\). Write the reactions.

Hint:

Nitrile plus Grignard reagent followed by hydrolysis gives ketone.

Answer 1

Concept:
Nitriles react with Grignard reagents followed by hydrolysis to give ketones. \[ R-CN+R'MgX \rightarrow \text{imine magnesium salt} \xrightarrow{H_3O^+} R-CO-R' \] The 2,4-DNP test confirms the presence of carbonyl group. No Tollens' or Fehling's test means the compound is not an aldehyde. No iodoform test means it is not a methyl ketone.

Step 1: Identify compound \(A\).
The molecular formula is: \[ C_3H_5N \] A nitrile with this formula is propanenitrile: \[ CH_3CH_2CN \] So: \[ A=CH_3CH_2CN \]

Step 2: Reaction with phenyl magnesium bromide.
Propanenitrile reacts with phenyl magnesium bromide. \[ CH_3CH_2CN+C_6H_5MgBr \rightarrow CH_3CH_2C(=NMgBr)C_6H_5 \] On hydrolysis: \[ CH_3CH_2C(=NMgBr)C_6H_5 \xrightarrow{H_3O^+} C_6H_5COCH_2CH_3 \] Therefore: \[ B=C_6H_5COCH_2CH_3 \] This is propiophenone.

Step 3: Use chemical tests to confirm \(B\).
Compound \(B\) gives 2,4-DNP test. So it contains carbonyl group. It does not give Tollens' or Fehling's test. So it is not an aldehyde. It does not give iodoform test. So it is not methyl ketone. \[ C_6H_5COCH_2CH_3 \] fits all conditions.

Step 4: Oxidation product.
Strong oxidation of alkyl benzene side chain gives benzoic acid. Here \(B\) on drastic oxidation gives: \[ C_6H_5COOH \] Molecular formula: \[ C_7H_6O_2 \] So: \[ C=C_6H_5COOH \] Hence: \[ \boxed{A=CH_3CH_2CN} \] \[ \boxed{B=C_6H_5COCH_2CH_3} \] \[ \boxed{C=C_6H_5COOH} \] \[ \boxed{CH_3CH_2CN+C_6H_5MgBr \rightarrow CH_3CH_2C(=NMgBr)C_6H_5 \xrightarrow{H_3O^+} C_6H_5COCH_2CH_3} \]