Carbon chain extension via nitrile formation

Steps:

• Target butanoic acid (CH₃CH₂CH₂COOH, C4) from 1-bromopropane (CH₃CH₂CH₂Br, C3 halide).
• First step: SN2 nucleophilic substitution with CN⁻ (e.g., KCN) replaces Br, adding one carbon to form butanenitrile (CH₃CH₂CH₂CN).
• Second step: Acid or base hydrolysis of nitrile converts -CN to -COOH, yielding butanoic acid.

Why D is correct:

• Butanenitrile forms directly via SN2 reaction (CH₃CH₂CH₂Br + CN⁻ → CH₃CH₂CH₂CN + Br⁻), introducing the carbon atom needed for the carboxyl group in standard alkyl halide to carboxylic acid synthesis.

Why the others are wrong:

• A: 1-Aminopropane from NH₃ substitution; produces amine, not precursor to carboxylic acid.
• B: Propan-1-ol from OH⁻ substitution; keeps C3 chain, insufficient for C4 acid without further extension.
• C: Butanal from nitrile reduction; not first-step product of alkyl halide, requires prior nitrile formation.

Final answer: D