SN2 Mechanism: Nucleophilic Attack by Cyanide Carbon

Steps:

• Identify SN2 as a concerted, bimolecular substitution where the nucleophile attacks the substrate carbon from the back, inverting configuration.
• Recognize cyanide ion (CN^-) as the nucleophile, with the carbon atom bearing the lone pair that attacks electrophiles.
• Note bromoethane (CH3CH2Br) has a primary carbon, favoring SN2 over SN1.
• Confirm the reaction forms propanenitrile (CH3CH2CN) via direct displacement of Br^- without intermediates.

Why A is correct:

• In SN2, the nucleophilic carbon of CN^- donates its lone pair to the electrophilic C-Br carbon, forming the new C-C bond in a single step, per the mechanism definition.

Why the others are wrong:

• B: Nitrogen in CN^- has its lone pair involved in resonance, making carbon the primary nucleophilic site.
• C: SN2 is concerted with no carbocation; attack occurs on intact C-Br bond.
• D: Combines incorrect nucleophile (C of CN) with nonexistent carbocation in SN2.

Final answer: A