See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: The SN2 reaction is a bimolecular nucleophilic substitution reaction. Its rate law is: Rate = k[nucleophile][electrophile]. In an SN2 reaction, both the nucleophile and the electrophile (substrate) appear in the rate-determining step, so the reaction is second order overall — first order in each reactant. Reasoning: The 'reaction rate' depends on the concentration of the electrophile (substrate) directly and proportionally. If the concentration of the electrophile is decreased by a factor of 3 (i.e., new concentration = original/3), then since Rate = k[Nu][E], the rate becomes k[Nu][E/3] = (1/3) × k[Nu][E]. This means the rate decreases by a factor of 3. Why other options fail: (a) The rate cannot increase when a reactant concentration is decreased — this contradicts the rate law. (b) A factor of 3^2 change would be expected if the reaction were second order in the electrophile, but SN2 is only first order in the electrophile, so this is incorrect. (d) 'Remain about the same' would apply if the electrophile did not appear in the rate law (as in SN1, which is unimolecular and zero order in nucleophile), but for SN2, the electrophile is explicitly in the rate expression. Therefore, the correct answer is C.