See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Comparing basicity and nucleophilicity of sulfur-based anions versus oxygen-based anions. Step 1 - Basicity comparison: Basicity is determined by how readily a species accepts a proton, which correlates with the pKa of the conjugate acid. The conjugate acid of CH3CH2S- is CH3CH2SH (ethanethiol, pKa ≈ 10.5), while the conjugate acid of CH3CH2O- is CH3CH2OH (ethanol, pKa ≈ 16). A lower pKa of the conjugate acid means a weaker base. Since ethanethiol (pKa ~10.5) is a much stronger acid than ethanol (pKa ~16), CH3CH2S- is a WEAKER base than CH3CH2O-. Sulfur's larger atomic radius and greater polarizability allow it to stabilize negative charge better, making the thiolate anion more stable and thus a weaker base. Step 2 - Nucleophilicity comparison: Nucleophilicity in polar protic or aprotic solvents follows polarizability trends down a group. Sulfur (3rd period) is much more polarizable than oxygen (2nd period). Greater polarizability means the electron cloud of sulfur can be more easily distorted toward an electrophile, making CH3CH2S- a BETTER nucleophile than CH3CH2O- despite being a weaker base. This is a classic case where nucleophilicity and basicity diverge when comparing atoms in different periods. Step 3 - Evaluating options: (a) Incorrect - CH3CH2S- is NOT a stronger base than CH3CH2O-. (b) Incorrect - CH3CH2S- is NOT a stronger base, and it is actually more nucleophilic, not less. (c) Correct - CH3CH2S- is a weaker base (thiolate is more stable, pKa of conjugate acid ~10.5 vs ~16) but is more nucleophilic (greater polarizability of sulfur) than CH3CH2O-. (d) Incorrect - CH3CH2S- is indeed a weaker base, but it is NOT less nucleophilic; it is more nucleophilic. Therefore, the correct answer is C.