See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: SN1 and SN2 reactions have opposite dependences on substrate substitution. SN2 is fastest for methyl substrates and slows with increasing substitution (steric hindrance increases). SN1 is slowest for methyl/primary substrates and fastest for tertiary substrates (carbocation stability increases). Step 2 - Interpreting curve B: Curve B starts at a high log(rate) for CH3-X and decreases as substitution increases (minimum around secondary). This matches SN2 behavior - fastest at methyl, slowest at more substituted substrates. Step 3 - Interpreting curve A: Curve A starts lower and increases sharply as substitution increases, reaching a maximum at tertiary (Me3CH-X). This matches SN1 behavior - rate increases with substitution because more substituted carbocations are more stable. Step 4 - Matching to options: Curve A (increases with substitution) → SN1; Curve B (decreases with substitution) → SN2. This corresponds to option (a): A → SN1, B → SN2. Step 5 - Why other options fail: - Option (b) reverses the assignments incorrectly. - Options (c) and (d) assign both curves to E1 or E2 elimination, but the curves clearly show the classic opposite substitution-rate trends of SN1 vs SN2, not elimination reactions. Therefore, the correct answer is A.