See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

We analyze each part using fundamental mechanistic principles: (a) SN2 reaction rate — SN2 is favored by less steric hindrance at the electrophilic carbon. - Cyclopentyl-CH2-Br: the carbon bearing Br is a PRIMARY carbon (–CH2–Br) with only one substituent (the cyclopentyl group). This is a primary alkyl halide — very accessible to backside attack. - 1-Methylcyclopentyl bromide: Br is on a TERTIARY carbon (the ring carbon also bearing CH3 and two ring carbons). Tertiary carbons are extremely hindered toward SN2. - Therefore, cyclopentylmethyl bromide (the primary substrate) undergoes SN2 far more rapidly. (b) E1 reaction rate — E1 proceeds via carbocation intermediate; rate depends on carbocation stability. - Ph-CH(CH3)-CH2-CH2-Br: Br is on a PRIMARY carbon (–CH2–Br, three carbons away from Ph). The carbocation formed after ionization would be primary/secondary with limited stabilization. - Ph-C(CH3)(Br)-CH2-CH3: Br is on a TERTIARY carbon that is also BENZYLIC (directly attached to the phenyl ring and bearing CH3 and ethyl). Ionization gives a tertiary benzylic carbocation, which is highly stabilized by both the tertiary nature and resonance with the aromatic ring. - Therefore, Ph-C(CH3)(Br)-CH2-CH3 (2-bromo-2-phenylbutane) undergoes E1 more rapidly. (c) SN1 reaction rate — SN1 is favored by more stable carbocations upon ionization. - 3-bromocyclopent-1-ene (Br at C3, vinyl double bond at C1–C2): C3 is ALLYLIC to the double bond. Loss of Br gives an allylic carbocation stabilized by resonance with the double bond — this is a secondary allylic carbocation. - The other isomer shown has Br at C4 (labeled as the lower Br in the image, also drawn as a cyclopentene with Br further from the double bond): C4 is homoallylic or simply secondary but NOT allylic — no resonance stabilization of the carbocation. - Therefore, 3-bromocyclopent-1-ene (allylic bromide, Br at C3) undergoes SN1 more rapidly. (d) SN2 reaction rate — again, steric accessibility is key. - Both substrates are chloronorbornanes (bicyclo[2.2.1]heptane framework). - The first structure has Cl at the C7 position (the one-carbon bridge, the methylene bridge at the top of norbornane). C7 is a secondary carbon but it sits in an EXO-accessible position with relatively less steric crowding compared to the bridgehead-adjacent endo position. - The second structure has Cl at the C2 bridgehead-adjacent position on the two-carbon bridge, which is more sterically hindered within the rigid bicyclic cage. - In norbornane systems, the C7 (bridge methylene) position allows better backside approach for SN2 than the more cage-congested C2 position. - Therefore, the norbornane with Cl on the one-carbon bridge (C7) undergoes SN2 more rapidly. (e) E2 reaction rate — E2 requires an anti-periplanar arrangement of the H and leaving group. - Both are chloronorbornanes. - For the C7 chloride (one-carbon bridge): the rigid bicyclic framework makes it geometrically very difficult to achieve the required anti-periplanar H–C–C–Cl arrangement for E2 elimination because of the bridged ring constraints at C7. - For the C2 chloride (ring carbon adjacent to bridgehead): although the norbornane framework is rigid, the C2 position can more readily achieve an anti-periplanar relationship with adjacent H atoms for E2 elimination. - Therefore, the norbornane with Cl on C2 (ring carbon adjacent to bridgehead) undergoes E2 more rapidly. Therefore, the correct answer is {"a": "cyclopentylmethyl bromide (cyclopentane-CH2-Br)", "b": "2-bromo-2-phenylbutane (Ph-C(CH3)(Br)-CH2CH3)", "c": "3-bromocyclopent-1-ene (allylic bromide)", "d": "2-chloronorbornane, Cl on the one-carbon bridge (C7) position", "e": "2-chloronorbornane, Cl on a ring carbon adjacent to the bridgehead (C2)"}.