See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

The SN2 reaction proceeds via a backside attack by a nucleophile on the carbon bearing the leaving group. The rate depends critically on steric accessibility at the electrophilic carbon. Concept: SN2 reactivity order: methyl > primary > secondary >> tertiary (essentially zero). Additionally, special steric environments (bridgehead, aryl, neopentyl) can completely prevent SN2 even at formally primary or secondary centers. Step-by-step analysis: (a) 1-Bromobutane: Primary alkyl halide, Br at C1. No branching adjacent to the reaction site. SN2 proceeds readily. → YES (b) 1-Chlorobutane: Primary alkyl halide, Cl at C1. Similar to (a); chloride is a leaving group and the carbon is unhindered primary. SN2 proceeds at a reasonable rate. → YES (c) 2-Bromobutane: Secondary alkyl halide, Br at C2. Secondary carbons are more hindered than primary but SN2 can still occur at a reasonable rate with a good nucleophile. → YES (d) 2-Chlorobutane: Secondary alkyl halide, Cl at C2. Same reasoning as (c). SN2 occurs at a reasonable rate. → YES (e) 2-Chloro-2-methylpropane (tert-butyl chloride): Tertiary alkyl halide. The carbon bearing Cl has three methyl groups, creating severe steric hindrance. Backside attack is essentially impossible. SN2 does NOT occur at a reasonable rate. → NO (f) Bromocyclohexane: Secondary cyclic alkyl halide. The ring carbon bearing Br is secondary. Although cyclic, SN2 can still occur at secondary cyclic carbons (the ring does not fully block backside attack for cyclohexyl). → YES (g) Bromobenzene: Aryl halide. The C-Br bond is part of the aromatic ring; the carbon is sp2-hybridized. Backside attack is geometrically impossible for SN2 (the ring blocks the back lobe). → NO (h) Benzyl bromide: Primary benzylic carbon (Ph-CH2-Br). The carbon is primary and unhindered; in fact benzylic position is activated toward SN2 due to partial stabilization in the transition state. → YES (i) 1-Bromo-2,2-dimethylpropane (neopentyl bromide): Although the carbon bearing Br is formally primary (Br-CH2-), the adjacent carbon is quaternary C(CH3)3, creating severe beta-branching steric hindrance. The bulky tert-butyl group blocks backside approach. SN2 is extremely slow. → NO (j) Bicyclo compound (norbornane-type bridgehead bromide): The Br is at the bridgehead carbon of the bicyclic system. Bridgehead carbons cannot undergo backside attack because the ring system physically prevents the nucleophile from approaching from the back (Bredt's rule context for SN2). → NO (k) 1-Bromotriptycene: Triptycene is a highly rigid tricyclic framework; the Br is at the bridgehead carbon surrounded by three aromatic rings. This is the most sterically congested bridgehead possible—backside attack is completely blocked. → NO Summary: - YES (SN2 at reasonable rate): (a) primary, (b) primary, (c) secondary, (d) secondary, (f) secondary cyclic, (h) primary benzylic - NO (SN2 not at reasonable rate): (e) tertiary, (g) aryl, (i) neopentyl (primary but beta-hindered), (j) bridgehead bicyclic, (k) bridgehead triptycene Therefore, the correct answer is {"Yes": ["A", "B", "C", "D", "F", "H"], "No": ["E", "G", "I", "J", "K"]}.