See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: SN2 reactivity at a benzylic carbon depends on the electrophilicity of the carbon bearing the leaving group. Electron-withdrawing groups (EWGs) on the aromatic ring decrease electron density at the benzylic carbon, making it more electrophilic and thus more reactive toward nucleophilic attack in an SN2 reaction. Conversely, electron-donating groups (EDGs) increase electron density at the benzylic carbon, reducing its electrophilicity and slowing SN2 reactions. Step 1: Identify the substituents on the aromatic ring for each option. (a) para-CH3 (methyl): EDG via hyperconjugation/induction — increases electron density at the benzylic carbon, decreasing SN2 reactivity. (b) No substituent (H): baseline benzyl chloride reactivity. (c) para-Cl: weakly electron-withdrawing by induction, but also weakly electron-donating by resonance; net effect is slightly deactivating compared to H, giving modest increase in electrophilicity. (d) para-NO2 (nitro): strong EWG by both induction and resonance — significantly decreases electron density at the benzylic carbon through the ring, making the benzylic carbon most electrophilic. Step 2: Rank SN2 reactivity. The more electron-poor the benzylic carbon, the faster the SN2 reaction. The nitro group is the strongest EWG among the substituents listed, so option (d) has the most electrophilic benzylic carbon. Step 3: Why other options fail. - (a) para-CH3 is electron-donating, reducing electrophilicity — least reactive. - (b) Unsubstituted benzyl chloride is intermediate in reactivity. - (c) para-Cl is a weaker EWG than NO2, so less reactive than (d). Conclusion: 4-nitrobenzyl chloride (option d) has the most electron-deficient benzylic carbon due to the strong electron-withdrawing nitro group, making it most reactive toward SN2. Therefore, the correct answer is D.