Nucleophilic substitution with cyanide followed by nitrile hydrolysis

Steps:

• 1-Bromopropane undergoes SN2 reaction with KCN in ethanol to form 1-cyanopropane (compound Q), adding one carbon.
• Compound Q (nitrile) is hydrolyzed with HCl(aq) in ethanol to butanoic acid via protonation and water addition.
• This two-step process extends the chain from C3 to C4 and converts the nitrile to carboxylic acid.

Why A is correct:

• KCN replaces Br with CN (R-Br + CN⁻ → R-CN + Br⁻), and HCl hydrolyzes nitrile to carboxylic acid (R-CN + 2H₂O + HCl → R-COOH + NH₄Cl).

Why the others are wrong:

• B: NaOH hydrolyzes nitrile to carboxylate salt (R-COO⁻ Na⁺), not the acid.
• C: NH₃ forms primary amine (propylamine), and HNO₃ nitration doesn't yield carboxylic acid.
• D: NaOH gives propan-1-ol via substitution, and H⁺ alone doesn't extend chain or form acid.

Final answer: A