Nucleophilic substitution with cyanide to form nitrile for chain extension

Steps:

• Recognize butanoic acid (C3H7CO2H) requires adding one carbon to propyl chain from C3H7I.
• First stage must replace iodide with cyanide via SN2 to yield propanenitrile (C3H7CN).
• Second stage hydrolyzes nitrile to carboxylic acid, extending chain by one carbon.
• Select reagent enabling clean SN2 on primary alkyl halide without side reactions.

Why C is correct:

• Ethanolic KCN reacts with primary alkyl iodide in SN2 mechanism to form alkyl nitrile (R-I + CN⁻ → R-CN + I⁻), which hydrolyzes to RCO2H.

Why the others are wrong:

• A: Aqueous NaOH substitutes to form propan-1-ol (alcohol), not extending chain.
• B: Ethanolic ammonia substitutes to form propylamine, yielding amine not acid precursor.
• D: Ethanolic NaOH promotes E2 elimination to propene, not substitution for chain extension.

Final answer: C