See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: The reaction of NaI in acetone is the Finkelstein reaction, which proceeds via an SN2 mechanism. Two factors govern relative SN2 reactivity here: (1) the nature of the leaving group and (2) steric accessibility of the carbon bearing the leaving group. Pair-A – Comparing leaving groups at the same type of carbon (both secondary): 2-Chloropropane has Cl as leaving group; 2-Bromopropane has Br as leaving group. In SN2 reactions with NaI/acetone, the driving force is the precipitation of NaX (NaCl or NaBr) from acetone solution. NaCl is less soluble in acetone than NaBr, so the equilibrium is pulled further for chlorides, but more importantly, I⁻ is a better nucleophile than Br in polar aprotic solvents, and Br⁻ is a better leaving group than Cl⁻. The key point: Br⁻ is a better leaving group than Cl⁻ (C–Br bond is weaker/longer), so 2-Bromopropane reacts faster than 2-Chloropropane with NaI. Therefore, compound (2) 2-Bromopropane is selected from Pair-A. Pair-B – Comparing steric environment with the same leaving group (both bromides): 1-Bromobutane is a primary alkyl bromide; 2-Bromobutane is a secondary alkyl bromide. SN2 reactions are strongly disfavored by steric hindrance. A primary carbon is far less hindered than a secondary carbon, so the nucleophile (I⁻) can attack the backside much more easily at the primary center. Therefore, 1-Bromobutane (primary) reacts faster than 2-Bromobutane (secondary). Compound (3) 1-Bromobutane is selected from Pair-B. Why other options fail: - Option (a) 1, 3: Selects 2-Chloropropane over 2-Bromopropane, but Br⁻ is a better leaving group than Cl⁻, so this is incorrect for Pair-A. - Option (b) 1, 4: Wrong on both pairs – picks the poorer leaving group and the more hindered substrate. - Option (d) 2, 4: Correctly picks 2-Bromopropane but incorrectly picks 2-Bromobutane (secondary) over 1-Bromobutane (primary). The correct selections are (2) 2-Bromopropane and (3) 1-Bromobutane. Therefore, the correct answer is C.