DAT Review · Organic Chemistry
Radical Halogenation and Anti-Markovnikov Addition
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In 30 seconds
Scope: Radical chain mechanisms (initiation, propagation, termination), selectivity in chlorination vs. bromination, radical stability trends, allylic/benzylic bromination with NBS, and the anti-Markovnikov addition of HBr with peroxides. Expect 1–3 questions. The radical mechanism for HBr + peroxides is a classic DAT favorite — know it cold.
The college version
Core Review
Radical Chain Mechanism
All radical halogenation of alkanes follows three stages:
1. Initiation: Homolytic cleavage of halogen by heat (Δ) or light (hν).
X₂ → 2 X• (X = Cl or Br)
2. Propagation: Two repeating steps that form product and regenerate the chain carrier.
X• + R-H → H-X + R• R• + X₂ → R-X + X•
The net reaction: R-H + X₂ → R-X + H-X
3. Termination: Any two radicals combine (destroys radical intermediates, stops chain).
X• + X• → X₂ R• + X• → R-X R• + R• → R-R
Selectivity: Chlorination vs. Bromination
This is a kinetic selectivity argument explained by Hammond's postulate.
| Property | Chlorination (Cl₂, hν) | Bromination (Br₂, hν) |
|---|---|---|
| Reactivity | Very reactive, low selectivity | Less reactive, high selectivity |
| Selectivity ratio (3°:2°:1°) | ~5:4:1 | ~1600:80:1 |
| Transition state | Early (reactant-like) | Late (product-like) |
| Product distribution | Statistical, mixtures | Highly selective for most stable radical |
| Best for | Large excess alkane or when mixtures acceptable | Selective bromination at most substituted position |
Why bromine is more selective: The C-Br bond is weaker and the Br• radical is more stable than Cl•. The bromination transition state is later (more product-like), so it better "feels" the difference in radical stability. Chlorination's early transition state is less discriminating.
Radical Stability
3° (tertiary) > 2° (secondary) > 1° (primary) > Methyl
Radicals are electron-deficient (7 electrons on carbon) and are stabilized by hyperconjugation and inductive effects from adjacent alkyl groups — same trend as carbocation stability.
Allylic and benzylic radicals are exceptionally stable due to resonance delocalization. Allylic (~allyl) radicals have the unpaired electron spread over two carbons. Benzylic radicals are spread into the aromatic ring.
Allylic and Benzylic Bromination (NBS)
N-Bromosuccinimide (NBS) provides a low, steady concentration of Br₂, favoring radical substitution at the allylic or benzylic position over alkene addition.
Mechanism overview: NBS reacts with trace HBr to generate Br₂ in situ. The low Br₂ concentration favors radical substitution over ionic addition to the double bond. The net result is replacement of an allylic/benzylic H with Br.
Cyclohexene + NBS (hν, CCl₄) → 3-bromocyclohexene
Anti-Markovnikov Addition of HBr (Peroxide Effect)
This only works with HBr — NOT HCl or HI.
Mechanism (radical addition):
- Initiation: RO-OR → 2 RO• (peroxide homolysis)
- RO• + HBr → ROH + Br•
- Propagation 1: Br• adds to the LESS substituted carbon of the alkene (forms the MORE stable radical). This is the selectivity-determining step.
- Propagation 2: The carbon radical abstracts H from HBr, regenerating Br•.
The result: Br adds to the LESS substituted carbon, and H adds to the MORE substituted carbon — opposite of Markovnikov's rule.
Why Br• adds to the less substituted carbon: It forms the more stable (more substituted) carbon radical. A secondary radical is more stable than a primary radical.
Why only HBr?
- HCl: The H-Cl bond (431 kJ/mol) is too strong for easy homolysis by radicals. The Cl• addition to alkenes is endothermic.
- HI: The C-I bond is too weak; iodine radicals are too stable to propagate efficiently. Also, HI can reduce peroxides directly.
- HBr: Both propagation steps are exothermic — the "Goldilocks" hydrogen halide.
Common Traps
- Extending anti-Markovnikov to HCl or HI: Only HBr works. The DAT will offer HCl/peroxides as a distractor.
- Confusing NBS bromination with Br₂ addition: NBS gives substitution at the allylic position; Br₂ (in CCl₄, dark) gives anti addition across the double bond.
- Forgetting that homolytic cleavage requires initiation: Heat (Δ) or light (hν) — you must show this step. No initiation = no reaction.
- Writing ionic mechanisms for radical reactions: Fishhook arrows (single-barbed), not double-barbed curved arrows.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Radical reactions are like a chain letter — someone starts it (initiation), it gets passed along (propagation), and eventually two people with letters meet and stop (termination). Bromine is like a picky eater who only wants the "best" (most substituted) carbon. Chlorine is hungrier and less picky — it'll eat from any carbon. The peroxide trick with HBr is like flipping a switch: instead of following the "H goes to less substituted" normal rule, the radical path reverses it.
Key takeaways
- Bromination is highly selective for the most substituted carbon; chlorination gives statistical mixtures.
- NBS brominates at allylic/benzylic positions — the double bond/ring stays intact.
- Anti-Markovnikov HBr addition requires peroxides and works ONLY with HBr.
- Radical stability mirrors carbocation stability: 3° > 2° > 1° > methyl.
- Propagation steps must sum to the net reaction and regenerate the chain-carrying radical.
- Predict the major monobromination product of 2-methylbutane with Br₂/hν. Answer: 2-bromo-2-methylbutane. Bromination is highly selective for 3° C-H bonds. C-2 (tertiary) is brominated almost exclusively despite having only one hydrogen, compared to 2° and 1° positions that have many more hydrogens but far lower reactivity per hydrogen.
- Propene + HBr + peroxides → ? Answer: 1-bromopropane (anti-Markovnikov). The Br• adds to the less substituted C-1 (forming the more stable 2° radical), then H abstraction gives 1-bromopropane. Without peroxides, the product would be 2-bromopropane (Markovnikov).
- Cyclohexene + NBS (hν, CCl₄) — what is the product and why not 1,2-dibromocyclohexane? Answer: 3-bromocyclohexene (allylic bromination). NBS maintains a very low concentration of Br₂, which favors radical substitution (H abstraction at the allylic position) over ionic addition to the double bond. If Br₂ were used instead, the product would be trans-1,2-dibromocyclohexane.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Write complete radical chain mechanisms with initiation, propagation, and termination steps
- Predict product distributions from radical chlorination vs. bromination
- Explain selectivity differences using Hammond's postulate
- Apply allylic/benzylic bromination using NBS
- Explain why only HBr (not HCl or HI) undergoes anti-Markovnikov addition with peroxides
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