Nucleophilic substitution to nitrile, then hydrolysis to carboxylic acid

Steps:

• Bromoethane (CH₃CH₂Br) undergoes SN2 substitution with CN⁻ from KCN to form propanenitrile (CH₃CH₂CN).
• Propanenitrile hydrolyzes under acidic conditions with dilute HCl, adding H₂O across the nitrile to yield propanoic acid (CH₃CH₂COOH).
• This two-stage process extends the carbon chain by one atom, matching the required product.
• No oxidation or elimination occurs, preserving the chain length.

Why D is correct:

• KCN in ethanol (implied for substitution) gives the nitrile; dilute HCl provides H⁺ for partial hydrolysis to the acid without decarboxylation.

Why the others are wrong:

• A: Aqueous NaCN risks early hydrolysis to hydroxy acid; K₂Cr₂O₇ oxidizes, not hydrolyzes, nitriles.
• B: Aqueous NaOH substitutes to ethanol (CH₃CH₂OH); Na₂Cr₂O₇ oxidizes to ethanoic acid (one less carbon).
• C: Ethanolic NaOH eliminates to ethene; KMnO₄ cleaves to CO₂, not propanoic acid.

Final answer: D