See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: In nucleophilic substitution (SN2) vs elimination (E2) competition, the nature of the base/nucleophile and the substrate determine the outcome. For a primary alkyl halide like CH3-CH2-Br, SN2 is generally favored because there is little steric hindrance at the carbon bearing the leaving group. However, the choice of base also matters: a bulky base favors elimination (E2) over substitution (SN2) because steric hindrance prevents it from attacking the carbon backside, directing it instead to abstract a proton. Step 1: Identify the substrate. CH3-CH2-Br is a primary alkyl bromide (ethyl bromide). Primary substrates strongly favor SN2 over E2 because the carbon is unhindered. Step 2: Evaluate reagent (a): CH3CH2OK (sodium/potassium ethoxide) in DMSO. Ethoxide is a relatively small, non-bulky nucleophile/base. DMSO is a polar aprotic solvent that enhances nucleophilicity by not solvating the nucleophile. With a small nucleophile and a primary substrate, SN2 substitution is strongly favored. This combination will yield a greater amount of substitution product (CH3CH2OCH2CH3, diethyl ether). Step 3: Evaluate reagent (b): (CH3)3COK (potassium tert-butoxide) in DMSO. Tert-butoxide is a very bulky base. Even though DMSO is a polar aprotic solvent that would normally favor nucleophilic attack, the extreme steric bulk of tert-butoxide prevents efficient backside attack on carbon. Instead, it preferentially abstracts a beta-proton, favoring E2 elimination to give ethylene (CH2=CH2). Thus, reagent (b) gives more elimination than substitution. Step 4: Compare. Reagent (a) with the smaller, less hindered ethoxide gives significantly more SN2 substitution than reagent (b) with bulky tert-butoxide. Why other options fail: - Option (b) alone: tert-butoxide is too bulky and favors elimination with the primary substrate, giving less substitution. - Option (c): The two reagents do NOT give comparable amounts of substitution; ethoxide gives much more substitution than tert-butoxide. - Option (d): Ethoxide in DMSO clearly gives substitution, so this is incorrect. Therefore, the correct answer is A.