Free Radical Halogenation Mechanism
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Direct answer
Halogenation of an alkane under ultraviolet light or heat is a radical chain in three acts: initiation splits X2 into 2X•; propagation alternates X• + R–H → HX + R• with R• + X2 → RX + X•; termination pairs any two radicals. Selectivity between C–H positions is the scoring idea — chlorination is near-unselective (3°:2°:1° ≈ 5:3.8:1 per hydrogen) while bromination is fiercely selective (1600:82:1) — and because the intermediate radical is trigonal planar, any stereocentre created at that carbon emerges racemic. Fluorination runs explosive, iodination stalls endothermic; the practicable pair is chlorine and bromine.
What you must remember
- Chain equations: Cl2 →(hν) 2Cl•; Cl• + CH4 → HCl + CH3•; CH3• + Cl2 → CH3Cl + Cl•; termination gives CH3Cl, C2H6 (a trace of ethane is the mechanism's fingerprint) or Cl2.
- Radical stability order: tertiary > secondary > primary > methyl, driven by hyperconjugation and, for benzylic and allylic positions, resonance; C–H bond enthalpies fall in the same order, tertiary near 400 against methyl's 435 kJ/mol.
- Selectivity numbers: per-hydrogen reactivity 5 : 3.8 : 1 for chlorination and 1600 : 82 : 1 for bromination — the most quoted pair in the chapter.
- Product-ratio arithmetic: multiply each position's hydrogen count by its reactivity; propane chlorination gives 6 × 1 against 2 × 3.8, or about 44 : 56 for the 1-chloro to 2-chloro split.
- Stereochemistry: the planar sp2 radical accepts attack on either face, so a new stereocentre at the radical carbon forms as a racemic pair.
- Halogen extremes: fluorination releases too much heat to control; iodination fails because the H-abstraction step is endothermic and the chain dies.
- Allylic chemistry: N-bromosuccinimide keeps bromine at low steady concentration, favouring allylic substitution over addition to the double bond.
- Photochemical yield: one photon can power thousands of propagation cycles before termination — the amplification oxygen or radical scavengers quench.
Predicting the chlorination mixture
Run propane through the arithmetic. Six equivalent primary hydrogens each react with relative rate 1; two secondary hydrogens each react at 3.8. Expected 1-chloropropane against 2-chloropropane = 6 : 7.6, roughly 44 : 56 — the secondary product dominates despite fewer sites, because per-hydrogen reactivity outweighs statistics. Switch to bromination and the same alkane collapses to a single product: 6 × 1 against 2 × 82 gives 6 : 164, about 3 : 97 in favour of 2-bromopropane. The conceptual engine is the Hammond postulate: chlorine's hydrogen abstraction is exothermic, so its transition state is early and develops little radical character — almost no discrimination; bromine's abstraction is endothermic, the transition state late and product-like, so the stability of the radical being formed weighs fully — hence the 1600-fold preference for tertiary positions. Same mechanism, one thermodynamic sign flipped, a completely different product sheet.
Selectivity and stereochemistry in questions
JEE Main asks mechanism labels — identify the initiation, propagation and termination steps — plus reagent outcomes and the counting arithmetic above. Advanced goes stereochemical: monochlorination of an alkane where the radical carbon becomes stereogenic yields a racemate, and counting questions that include stereoisomers (the two enantiomers of 2-chlorobutane count separately) punish candidates who count constitutions only. Assertion-reason regulars: "alkane halogenation needs light" (initiation energy); "iodination of alkanes is impractical" (endothermic abstraction, equilibrium reversed); "benzylic C–H reacts fastest" (resonance-stabilised radical). The recurring errors are statistical-only counting that forgets the 3.8 and 82 multipliers, and drawing inversion or retention at the radical carbon — it racemises, with no backside-attack logic anywhere in sight.
Frequently asked questions
Why is bromination far more selective than chlorination?
Bromine's hydrogen-abstraction step is endothermic with a late, radical-like transition state, so tertiary radical stability weighs heavily; chlorine's exothermic early transition state barely discriminates.
Write the propagation steps of methane chlorination.
Cl• + CH4 → HCl + CH3•, then CH3• + Cl2 → CH3Cl + Cl• — the pair consumes no net radicals and turns over until quenched.
Predict the monochlorination ratio for propane.
Six primary hydrogens at rate 1 against two secondary at 3.8 gives 6 : 7.6 — roughly 44 per cent 1-chloropropane to 56 per cent 2-chloropropane.
What stereochemistry results at a newly chiral radical centre?
The planar sp2 radical is trapped from both faces, giving a one-to-one racemic mixture — neither inversion nor retention applies.
Why is direct iodination of alkanes impractical?
The iodine radical abstracts hydrogen endothermically, the chain dies, and accumulating HI pushes the equilibrium backwards — practical iodination needs an oxidant to remove HI.