CBSE Class 12 Chemistry: Step-by-Step Marking Scheme

Part (a)

Step 1: Nucleophilic Substitution and Reduction [0.5 Mark]

Reaction of alkyl halide with KCN follows $S_N2$ mechanism to form nitrile, followed by reduction with $\text{LiAlH}_4$ to form a primary amine.

$$ \text{CH}_3\text{CH}_2\text{Cl} \xrightarrow{\text{KCN}} \underbrace{\text{CH}_3\text{CH}_2\text{CN}}_{\text{A}} \xrightarrow{\text{LiAlH}_4} \underbrace{\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2}_{\text{B}} $$

Step 2: Diazotization [0.5 Mark]

Reaction of primary aliphatic amine with nitrous acid ($\text{HNO}_2$) at $0^\circ\text{C}$ forms an unstable diazonium salt which decomposes to form an alcohol.

$$ \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2 \xrightarrow{\text{HNO}_2, 0^\circ\text{C}} \underbrace{\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}}_{\text{C}} $$

Step 3: Final Identification [0.5 Mark]

• A: $\text{CH}_3\text{CH}_2\text{CN}$ (Propanenitrile)
• B: $\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$ (Propan-1-amine)
• C: $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ (Propan-1-ol)

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Part (b)

Step 1: Reduction of Nitro group [0.5 Mark]

Reduction of nitrobenzene using $\text{Fe/HCl}$ yields aniline.

$$ \text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{Fe/HCl}} \underbrace{\text{C}_6\text{H}_5\text{NH}_2}_{\text{A}} $$

Step 2: Diazotization and Coupling [0.5 Mark]

Aniline reacts with $\text{NaNO}_2 + \text{HCl}$ at $273\text{ K}$ to form benzene diazonium chloride, which undergoes electrophilic substitution (coupling) with phenol.

$$ \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2 + \text{HCl}, 273\text{ K}} \underbrace{\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-}_{\text{B}} \xrightarrow{\text{C}_6\text{H}_5\text{OH}} \underbrace{\text{C}_6\text{H}_5\text{N}=\text{N}-\text{C}_6\text{H}_4\text{OH}}_{\text{C}} $$

Step 3: Final Identification [0.5 Mark]

• A: $\text{C}_6\text{H}_5\text{NH}_2$ (Aniline)
• B: $\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$ (Benz