Chapter 18 — Quiz
Twenty-five questions on radical reactions. ∗ marks questions answered in the answer key.
Multiple choice
1.∗ A radical has: (a) full octet (b) unpaired electron (c) empty orbital (d) negative charge
2.∗ Homolysis of a covalent bond: (a) one atom gets both electrons (b) each atom gets one electron (giving two radicals) (c) the bond doesn't break (d) random
3.∗ A radical chain reaction has: (a) only initiation (b) initiation + propagation + termination stages (c) only termination (d) random sequence
4.∗ Most stable radical of: (a) methyl (b) primary (c) secondary (d) tertiary (most stable; hyperconjugation)
5.∗ Radical bromination selectivity: (a) 1° > 3° (chlorination is the opposite of this) (b) 3° > 2° > 1° (bromine more selective; ~1600× for 3° vs 1°) (c) all the same (d) only on aromatics
6.∗ Anti-Markovnikov HBr addition requires: (a) light (b) peroxide initiator (radical mechanism via Br• then alkene addition) (c) heat alone (d) Pd catalyst
7.∗ NBS brominates: (a) the allylic C (with low [Br₂] selecting for resonance-stabilized allylic radical) (b) across the C=C (c) the carbonyl (d) random
8.∗ Fish-hook arrow notation indicates: (a) 2-electron movement (heterolytic) (b) 1-electron movement (homolytic; single-electron transfer) (c) lone pair (d) bond formation
9.∗ Polyethylene (LDPE) is made via: (a) ionic polymerization (b) radical polymerization (high T, high pressure, peroxide initiator) (c) Ziegler-Natta (d) anionic polymerization
10.∗ Antioxidants work by: (a) creating radicals (b) quenching radical chains (donating H to peroxyl radicals; terminating the chain) (c) creating ions (d) heating the system
11.∗ Allyl radical is more stable than 3° alkyl radical because: (a) allyl radical's unpaired electron can delocalize via resonance into the C=C (b) it's bigger (c) only 3° is stable (d) random
12.∗ Benzyl radical is even more stable than allyl because: (a) the unpaired electron delocalizes into multiple resonance structures with the benzene ring (b) only because of inductive effects (c) it's photochemical (d) it's ionic
13.∗ Lipid peroxidation in cell membranes is initiated by: (a) reactive oxygen species (superoxide, hydroxyl radical) abstracting allylic H from PUFAs (b) only enzymes (c) only photochemistry (d) only at high temperature
14.∗ Vitamin E (α-tocopherol) quenches the lipid peroxidation chain by: (a) donating its phenolic O-H to a peroxyl radical (LOO• + Vit E-OH → LOOH + Vit E-O•; the new radical is resonance-stabilized and unreactive) (b) absorbing light (c) inhibiting enzymes (d) breaking down the lipid
15.∗ In radical halogenation, why is bromination more selective than chlorination? (a) Br H abstraction is endothermic; late TS reflects substrate radical stability differences (b) Br is bigger (c) Cl is too reactive in solvents (d) only random
16.∗ Cumene autoxidation gives cumene hydroperoxide; this is the basis of: (a) the Hock process (industrial production of phenol + acetone) (b) only laboratory synthesis (c) only with chiral catalysts (d) photochemistry only
17.∗ Why do allyl and benzyl C-H bonds have lower BDE than typical alkyl C-H? (a) the resulting radical is resonance-stabilized; low BDE (b) random (c) hyperconjugation only (d) inductive effects only
18.∗ Why does HBr radical addition to alkene give anti-Markovnikov? (a) the Br• adds to less-substituted C; the resulting alkyl radical at the more-substituted C is stable; H-Br bond breaks at end → Br ends up at less-substituted C (anti-Markovnikov) (b) it's actually Markovnikov (c) random (d) only at high pressure
19.∗ Why does Cl₂ + alkane (radical chlorination) give a less-selective product mixture than Br₂ + alkane? (a) Cl-H bond is weaker than C-H; Cl abstraction is exothermic; less selective (b) the Cl• radical is too unreactive (c) only random (d) only at low T
20.∗ Photoredox catalysis (modern radical chemistry): (a) uses light + photocatalyst (Ru, Ir, organic dyes) for single-electron transfers; mild conditions; asymmetric possible (b) only thermal (c) only with peroxides (d) only random
Short answer
21. Sketch the chain mechanism (initiation, propagation, termination) of: methane + Cl₂ + light → methyl chloride + HCl.
22. Why is anti-Markovnikov HBr addition (with peroxides) opposite to Markovnikov ionic HBr addition? Explain in terms of the radical intermediate stability.
23. Sketch the mechanism of NBS allylic bromination of cyclohexene to 3-bromocyclohexene.
24. Compare ionic and radical mechanisms for HBr + propene. What's different about the regiochemistry, and why?
25. Why are polyunsaturated fatty acids (PUFAs) vulnerable to lipid peroxidation? Sketch how vitamin E quenches the resulting peroxyl radical.
Answer key
- b — Radical = unpaired electron.
- b — Homolysis = 1 electron each.
- b — Chain reaction has 3 stages.
- d — 3° most stable (alkyl).
- b — Br more selective.
- b — Peroxide for anti-Markovnikov.
- a — NBS at allylic.
- b — Fish-hook = 1 electron.
- b — LDPE = radical.
- b — Antioxidants quench radicals.
- a — Allyl resonance.
- a — Benzyl resonance into ring.
- a — ROS initiates peroxidation.
- a — Vit E donates H.
- a — Endothermic abstraction = selective.
- a — Hock process.
- a — Resonance lowers BDE.
- a — Anti-Markovnikov mechanism.
- a — Cl exothermic = less selective.
- a — Photoredox description.
21. Methane + Cl₂ + light → CH₃Cl + HCl. - Initiation: $Cl_2 + h\nu \to 2 Cl^{\bullet}$ (UV homolyzes Cl-Cl bond). - Propagation: 1. $CH_4 + Cl^{\bullet} \to CH_3^{\bullet} + HCl$ (H abstraction; rate-limiting; Cl• consumed, methyl radical formed). 2. $CH_3^{\bullet} + Cl_2 \to CH_3Cl + Cl^{\bullet}$ (Cl₂ consumed; CH₃Cl formed; Cl• regenerated). - The chain repeats: net effect is CH₄ + Cl₂ → CH₃Cl + HCl. - Termination: any combination of two radicals, e.g., $2 Cl^{\bullet} \to Cl_2$, or $CH_3^{\bullet} + Cl^{\bullet} \to CH_3Cl$, or $2 CH_3^{\bullet} \to C_2H_6$.
22. HBr + propene: - Ionic (no peroxides; polar solvent): H⁺ adds to terminal C (less-substituted); cation forms at central C (more-substituted = more-stable; 2°). Br⁻ attacks the cation. Markovnikov: 2-bromopropane. - Radical (peroxides): Br• adds to terminal C (less-substituted; the resulting alkyl radical at central C is more-stable, 2°). H abstraction from HBr gives 1-bromopropane (Br on terminal C) — anti-Markovnikov. The difference: in ionic mechanism, H+ adds first → cation forms at the OTHER C → product has X at the C where the cation was. In radical mechanism, Br• adds first → radical forms at the OTHER C → product has Br at the C where Br first added. The intermediate at the "more-substituted" C is in both cases more stable, but the geometric outcome is reversed.
23. NBS + cyclohexene → 3-bromocyclohexene: - Initiation: peroxide or light generates Br• and HBr in trace amounts. - NBS slowly releases Br₂: NBS + HBr → succinimide + Br₂ (low [Br₂]). - Propagation: 1. $Br^{\bullet} + \text{cyclohexene}-CH_2 \to \text{cyclohexene}-CH^{\bullet} + HBr$ (allylic H abstraction; the resulting allylic radical is resonance-stabilized). 2. $\text{cyclohexene}-CH^{\bullet} + Br_2 \to \text{cyclohexene}-CHBr + Br^{\bullet}$ (allylic radical + Br₂; new C-Br bond forms; chain continues). - The HBr formed in step 1 reacts with NBS to generate more Br₂ in low concentration; the cycle continues. - Net: the allylic H is replaced by Br; cyclohexene → 3-bromocyclohexene.
24. HBr + propene: - Ionic: H+ to terminal CH₂; cation at central C (2°, stable). Br⁻ attacks. Markovnikov product: 2-bromopropane. - Radical (peroxide): Br• adds to terminal CH₂ (the radical forms at the central C, 2°, stable). H abstraction completes. Anti-Markovnikov product: 1-bromopropane. The selectivity is determined by which intermediate is more stable: in ionic, the cation; in radical, the radical. Both intermediates are at the same position (central C, more-substituted), but the geometric outcome differs because the order of attack reverses. Key insight: in both mechanisms, the more-stable intermediate forms; but the products differ because of the different mechanisms (electrophile-first vs. radical-first).
25. PUFAs (polyunsaturated fatty acids): - Have bis-allylic C-H bonds (a C-H between two C=C bonds). The H abstraction here is particularly easy because the resulting radical is doubly resonance-stabilized. - BDE of bis-allylic C-H ≈ 75 kcal/mol (much lower than typical alkyl 100 kcal/mol). - Easy initiation; rapid chain propagation.
Lipid peroxidation chain: 1. ROS abstracts bis-allylic H from PUFA → lipid radical (L•). 2. L• + O₂ → LOO• (peroxyl radical). 3. LOO• + LH (another PUFA) → LOOH + L• (chain continues).
Vitamin E (α-tocopherol) quenches: - Vitamin E has a phenolic O-H bond that is weak (BDE ~78 kcal/mol; the resulting phenoxyl radical is resonance-stabilized into the chromanol ring). - LOO• + Vit E-OH → LOOH + Vit E-O• (the phenoxyl radical). - Vit E-O• is resonance-stabilized; unreactive on biological timescale; eventually reduced back to Vit E-OH by ascorbic acid (vitamin C). - Net effect: the peroxidation chain is broken; LOOH is formed (later detoxified by glutathione peroxidase).
This is one of the most important antioxidant chemistries in biology. Vitamin E saves cell membranes from radical damage.