Nucleophilic substitution forms nitrile intermediate for carboxylic acid synthesis

Steps:

• Chloroethane (CH₃CH₂Cl) undergoes nucleophilic substitution to extend the carbon chain.
• Intermediate Q must be propanenitrile (CH₃CH₂CN), which hydrolyzes to propanoic acid, then forms sodium propanoate.
• Cyanide ion (CN⁻) acts as the nucleophile in an SN2 reaction with the primary alkyl halide.
• Ethanol solvent with KCN ensures anhydrous conditions for efficient substitution.

Why D is correct:

• Potassium cyanide in ethanol dissociates to provide CN⁻ nucleophile, yielding CH₃CH₂CN via SN2 mechanism: CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl.

Why the others are wrong:

• A: Dilute sulfuric acid protonates but does not substitute chloride, yielding no chain-extended product.
• B: Hydrogen cyanide in water forms HCN(aq), a weak nucleophile that favors addition reactions over substitution.
• C: Dilute sodium hydroxide performs hydrolysis to ethanol (CH₃CH₂OH), not a carboxylic acid precursor.

Final answer: D