Chapter 18 — Exercises

Forty-five problems on radical reactions. Drawing required wherever a structure or mechanism is asked for. ∗ marks problems with full worked solutions in Appendix Answers to Selected Exercises.

Section A — Radical fundamentals

18.1∗ (routine) Define radical, homolysis, heterolysis. Compare ionic and radical mechanisms with their respective arrow conventions.

18.2 (routine) Why are radicals usually drawn as planar (sp²)? What's in the perpendicular p orbital?

18.3∗ (routine) Rank these radicals by stability: methyl, ethyl, isopropyl, tert-butyl, allyl, benzyl.

18.4 (routine) Why is benzyl radical more stable than allyl, even though both are resonance-stabilized?

18.5 (moderate) Bond dissociation energy (BDE) determines homolysis ease. Rank these C-H bonds by ease of homolysis: - methane C-H (105 kcal/mol) - ethane C-H (101) - propane 2°C-H (98) - isobutane 3°C-H (96) - benzyl C-H (89) - allyl C-H (88)

18.6 (challenge) Why does the benzyl C-H BDE drop more than the allyl C-H BDE? Connect to the number of resonance structures of the radical.

Section B — Chain reactions

18.7∗ (routine) For radical halogenation of methane (CH₄ + Cl₂ → CH₃Cl + HCl), write: (a) the initiation step. (b) the propagation steps. (c) one possible termination step.

18.8 (routine) Why does each propagation step have a radical reactant and a radical product (no net change in radical count)?

18.9 (moderate) Why is the chain length of typical radical reactions ~10⁴-10⁶?

18.10 (challenge) A common termination step is "radical-radical coupling." But there's also "disproportionation." What's the difference?

Section C — Radical halogenation

18.11∗ (routine) Predict the major product of: (a) propane + Br₂ + heat → ? (b) isobutane + Br₂ + heat → ? (c) cyclohexane + Br₂ + heat → ?

18.12 (routine) Why is bromination more selective than chlorination? Connect to the Hammond postulate.

18.13 (moderate) Predict the products of: 2,3-dimethylbutane + Cl₂ + light. List the positions chlorinated and the rough selectivity.

18.14 (challenge) A 1° H is brominated 1600× slower than a 3° H. A 1° H is chlorinated only 5× slower than a 3° H. Calculate the percent of 1° vs 3° product for each reagent (assume 12 1° H and 1 3° H in 2,3-dimethylbutane).

Section D — Anti-Markovnikov HBr

18.15∗ (routine) Predict the product: (a) 1-pentene + HBr + peroxide (radical) → ? (b) 1-pentene + HBr (no peroxide; ionic) → ?

18.16 (routine) Sketch the radical chain mechanism for: propene + HBr + peroxide → 1-bromopropane.

18.17 (moderate) Why does only HBr give the peroxide effect (anti-Markovnikov)? Why don't HCl and HI?

18.18 (challenge) A reaction works for HBr at moderate temperature with peroxide initiator. Predict whether it would work without the peroxide (just on heat alone). Justify.

Section E — NBS allylic bromination

18.19∗ (routine) Predict the product: (a) cyclohexene + NBS + light → ? (b) cyclopentene + NBS + light → ? (c) 1-butene + NBS + light → ?

18.20 (routine) Why does NBS brominate allylic position rather than adding across C=C?

18.21 (moderate) Sketch the mechanism of NBS + cyclohexene → 3-bromocyclohexene. Identify how Br₂ is generated in low concentration.

18.22 (challenge) Predict the product of: 1-methylcyclopentene + NBS. Identify which allylic H is preferentially abstracted.

Section F — Selectivity arguments

18.23∗ (routine) Predict the major product of: toluene + Br₂ + light → ? (a) ortho-bromination on the ring (electrophilic aromatic substitution). (b) Benzylic bromination of the methyl group (radical chain). Why does the radical pathway dominate?

18.24 (routine) Predict the product of: cumene (isopropylbenzene) + O₂ + heat. (Industrial cumene autoxidation; benzylic radical.)

18.25 (moderate) Industrial cumene oxidation gives cumene hydroperoxide; this is hydrolyzed to phenol + acetone (the Hock process). Sketch the chemistry. This is the major industrial route to phenol.

18.26 (challenge) Predict the products of bromination at different positions in: 2-pentene. Connect to allylic vs vinyl vs alkyl positions.

Section G — Industrial polymerization

18.27∗ (routine) Sketch the radical polymerization of ethylene: peroxide initiator + ethylene → polyethylene. Show propagation step.

18.28 (routine) Compare LDPE (radical) vs HDPE (Ziegler-Natta). Why does LDPE have branched chains?

18.29 (moderate) What initiators are commonly used in industrial radical polymerization?

18.30 (challenge) A "living radical polymerization" (e.g., ATRP, RAFT) gives controlled molecular weight and dispersity. Sketch the principle.

Section H — Combustion

18.31 (routine) Combustion of CH₄ + O₂: write the radical chain. Identify a few key propagation steps.

18.32 (moderate) Why is combustion exponential once it starts? Connect to chain length and concentration of radicals.

18.33 (challenge) Why does a flame need oxygen? What happens at the flame's surface mechanistically?

Section I — Lipid peroxidation

18.34∗ (routine) Sketch the radical chain of lipid peroxidation: - Initiation: ROS + lipid → lipid radical. - Propagation: lipid radical + O₂ → lipid peroxyl radical; peroxyl + lipid → peroxide + lipid radical. - Termination: vitamin E donates H.

18.35 (routine) Why are polyunsaturated fatty acids (PUFAs) particularly vulnerable to lipid peroxidation?

18.36 (moderate) How does vitamin E (α-tocopherol) terminate the lipid peroxidation chain? Sketch the chemistry.

18.37 (challenge) Why is vitamin E recycled by vitamin C? Sketch the redox cycle.

Section J — Modern radical chemistry

18.38∗ (routine) What is photoredox catalysis? How does it generate radicals at room temperature?

18.39 (moderate) Compare a Ru(bpy)₃²⁺ photocatalyst with a thermal peroxide initiator. What are the advantages of photoredox?

18.40 (challenge) Modern photoredox + chiral catalyst gives asymmetric radical reactions. Sketch the principle.

Section K — Multistep synthesis

18.41∗ (routine) Design a synthesis of 1-bromopentane from 1-pentene using radical HBr addition.

18.42 (routine) Design a synthesis of 3-bromocyclohexene from cyclohexene using NBS.

18.43 (moderate) Design a synthesis of an allylic ether by combining NBS bromination + Williamson ether synthesis (SN2 with alkoxide).

18.44 (challenge) Design a 4-step synthesis using radical chemistry to install an allyl group at a specific position.

18.45 (challenge) Modern photoredox-catalyzed reaction: design a hypothetical photoredox-catalyzed radical addition or coupling.

Notes for instructors: Common stumbling blocks for Chapter 18: (1) Confusing fish-hook (single-electron) and double-barb (two-electron) arrows. (2) Mismatching ionic and radical Markovnikov outcomes. (3) Forgetting that NBS keeps [Br₂] low. (4) Not recognizing combustion as radical. Computational exercises: estimate radical stability via DFT-calculated bond dissociation energies; compare to experimental BDEs.