Free-Radical Halogenation

Why free-radical halogenation matters

• Industrial methane chlorination ~10⁵ ton/yr. Methane + Cl2 with UV light at ~400 °C produces a statistical mixture of CH3Cl, CH2Cl2, CHCl3, and CCl4, each separated by distillation. CH3Cl is a methylating agent and silicone monomer (~1M ton/yr); CH2Cl2 is a paint-stripper solvent; CHCl3 is the feedstock for HCFC-22 refrigerant.
• Vinyl chloride monomer / PVC at ~30M ton/yr. Ethylene + Cl2 → 1,2-dichloroethane (free-radical chlorination of the saturated chain), then thermal cracking → vinyl chloride monomer. PVC is the world's third-most-used polymer ($65B/yr global market) — and the upstream radical chlorination is what feeds it.
• Selectivity textbook for Hammond's postulate. The bromination 3°:1° rate ratio of ~1600:1 (vs chlorination 5:1) is the canonical illustration that an endothermic step has a late, product-like transition state and therefore reflects bond-energy differences between substrates. Every undergraduate organic class teaches this comparison.
• NBS allylic bromination is undergraduate-standard. Cyclohexene + NBS in CCl4 with peroxide initiator → 3-bromocyclohexene. The product is the launch point for SN2/SN1 substitution, E2 elimination, and Suzuki cross-coupling — three subsequent classes of reactions in introductory synthesis. NBS is in 95% of organic chemistry teaching labs.
• Pharmaceutical pheromone syntheses. Insect pheromones often have brominated allylic precursors made by Wohl-Ziegler. Companies like Bedoukian Research and Suterra produce ~100 ton/yr of pheromones for pest control via free-radical bromination as a key step.
• Mechanistic foundation of polymer chemistry. Radical chain polymerization (PE, PS, PMMA — ~200M ton/yr combined) uses the same initiation-propagation-termination logic. Understanding alkane halogenation prepares chemists to read polymer kinetics — the chain-length expression k_p[M]/√(2k_t·k_d[I]) is the same algebra.
• Hydrocarbon C-H functionalization frontier. Modern C-H activation chemistry (Pd, Rh, photoredox) was inspired by the simplicity of radical halogenation and aims to extend the same selectivity principles to non-hazardous functionalizing reagents. Radical relay catalysts (decatungstate, iron Quinoline complexes) are essentially designer chain carriers.

Common misconceptions

• Chlorination always gives one product. Almost never — for a substrate with multiple C-H types, chlorination gives a statistical mixture barely weighted toward 3° C-H. Propane + Cl2 gives ~57% 1-chloropropane and ~43% 2-chloropropane (corrected for H count, the per-H rate ratio is ~3.6:1 for 2° vs 1°). Synthetic chlorinations are most useful when the substrate has only one type of C-H.
• Bromination of methane is industrially relevant. No — bromine is too expensive (~$5/kg vs Cl2 ~$0.50/kg) and methane bromination's product CH3Br is a banned ozone depletor. Industry brominates only when selectivity makes high-cost feasible (allylic, benzylic, or 3° positions on more complex substrates).
• Radical halogenation is always destructive. Selectivity is the issue, not yield — well-controlled bromination at allylic positions is >90% yield. The reaction is "destructive" only when chain length is short or when multiple halogens add (poly-chlorination of methane).
• Light is the only initiator. Heat (above ~150 °C for Cl2), peroxides (BPO, AIBN, dilauroyl peroxide), and photoinitiators (benzophenone) all work. Industrial methane chlorination uses thermal initiation at 350-450 °C with no light. Pharmaceutical Wohl-Ziegler uses peroxide initiation in refluxing CCl4 (76 °C).
• The propagation chain runs forever. Chain lengths are 10³ to 10⁶ events typically. Termination — two radicals combining — is rare per cycle (because [R•] is ~10⁻⁹ M) but eventually wins when X2 is depleted or when an inhibitor (O2, hydroquinone) traps radicals. Industrial reactors continuously feed Cl2 to maintain steady-state propagation.
• Stereochemistry is preserved. Free-radical halogenation racemizes a stereogenic center because the planar sp²-hybridized R• intermediate is attacked from both faces equally. So bromination of (R)-2-methylbutane at C3 gives the same racemate ratio whichever enantiomer you start with.

Chain mechanism

The free-radical halogenation chain has three distinct stages with very different kinetics. Initiation is a slow, energy-input step: light or heat homolytically cleaves the X-X bond into two X• radicals. The X-X bond dissociation energies are 58 kcal/mol for Cl2, 46 kcal/mol for Br2, and 36 kcal/mol for I2 — all easily broken by ~300 nm UV light or thermal energy at >200 °C. Initiation rates are low because X-X bonds are stable; only ~10⁻⁶ to 10⁻⁹ of the X2 molecules are dissociated at any moment.

Propagation is the heart of the reaction. Step 1: an X• radical abstracts a hydrogen atom from R-H: X• + R-H → HX + R•. The thermodynamics depend on relative bond strengths (C-H ~98 kcal/mol vs H-Cl 103, H-Br 87, H-I 71 kcal/mol), giving ΔH of -5 (Cl) to +27 (I) kcal/mol. Step 2: the carbon radical R• reacts with X2 (X-X bond ~46-58 kcal/mol breaks, R-X bond ~70-85 kcal/mol forms): R• + X2 → R-X + X•. This step is always exothermic and faster than step 1. The new X• carries the chain by abstracting another H, and propagation cycles through 10³ to 10⁶ events per initiation. The chain length is determined by the ratio k_propagation/k_termination.

Selectivity is governed by Hammond's postulate applied to step 1. In bromination (ΔH ≈ +11 kcal/mol, endothermic), the transition state is late and product-like — the C-H bond is mostly broken when the new H-Br bond forms — so the activation energy reflects almost the full C-H bond dissociation energy. The 3° C-H (BDE ~96 kcal/mol) abstracts ~5 kcal/mol more easily than 1° C-H (~101 kcal/mol), giving exp(5000/RT) ≈ 5000-fold rate difference at 25 °C; statistical correction for H-count gives the observed ~1600:1. In chlorination (ΔH ≈ -5 kcal/mol, exothermic), the transition state is early and reactant-like — the C-H bond is barely broken — so activation energy depends only weakly on C-H bond strength. The 3°:1° rate ratio collapses to ~5:1. Termination — R• + R•, R• + X•, X• + X• — runs at the diffusion limit (10⁹ M⁻¹s⁻¹) but has tiny rate because [R•] and [X•] are very low.

Free-radical vs ionic halogenation, and selectivity by halogen

Reaction	Mechanism	Substrate	Conditions	3°:2°:1° selectivity	Industrial use
Free-radical bromination	Homolytic chain (Br•)	Saturated R-H	Br2 + hν or heat	~1600 : 80 : 1	Pharma (kg scale)
Free-radical chlorination	Homolytic chain (Cl•)	Saturated R-H	Cl2 + hν or heat	~5 : 4 : 1	Methane → CHnCl4-n ~10⁵ t/yr
Free-radical iodination	Endothermic chain — does not propagate	—	I2 + hν	—	Not practical
Free-radical fluorination	Wildly exothermic; uncontrolled	R-H	F2 (highly diluted)	~1:1:1 (statistical)	Industrial PFC manufacture
Wohl-Ziegler (allylic NBS)	Homolytic chain at low [Br2]	Allylic / benzylic	NBS in CCl4 + peroxide	Allylic » non-allylic (~50:1)	Steroids, pheromones, pharma
Ionic Br2 addition (alkene)	Heterolytic — Markovnikov + anti	Alkene C=C	Br2 in CCl4, dark	Anti-stereospecific, no Markovnikov override	Bromine-water test; vicinal dibromides
Friedel-Crafts halogenation	Heterolytic EAS (Lewis acid)	Aromatic C-H	Br2 + FeBr3 or Cl2 + AlCl3	Activator/deactivator-directed	Aryl halides for pharma
HX addition (alkene)	Heterolytic — Markovnikov	Alkene C=C	HBr, HCl in polar solvent	Markovnikov regiochemistry	Alkyl halide synthesis

Famous and industrial applications

• Methane chlorination at ~10⁵ ton/yr globally. The Hoechst, Solvay, and Olin processes use UV-photoinitiated CH4 + Cl2 in tubular reactors at 400 °C. Distillation separates CH3Cl (methyl chloride, silicone-precursor monomer ~1M ton/yr; methylation reagent), CH2Cl2 (paint stripper, pharma solvent ~600k ton/yr), CHCl3 (HCFC-22 refrigerant precursor ~500k ton/yr), and CCl4 (legacy refrigerant precursor; production now restricted by Montreal Protocol).
• Vinyl chloride / PVC at ~30M ton/yr. Ethylene + Cl2 → 1,2-dichloroethane (radical chlorination of the saturated dichloride, then thermal cracking → vinyl chloride monomer + HCl). PVC, the world's third-most-used polymer, is built from this chain. The radical chlorination contributes ~$2-3 of the ~$10/kg PVC cost.
• Wohl-Ziegler allylic bromination in steroids. The classical synthesis of cortisone (Reichstein 1936) uses allylic bromination to functionalize the steroid C-17 position. Modern progesterone, testosterone, and synthetic estrogen routes still use NBS at kilogram scale to install bromides for subsequent SN2 substitution or elimination.
• Pheromone manufacturing. Bedoukian Research (Connecticut) and Suterra (Oregon) produce 50-100 ton/yr of insect pheromones for agricultural pest control. Most are unsaturated long-chain acetates; the syntheses use Wohl-Ziegler bromination to install allylic bromides as cross-coupling partners, then Suzuki/Negishi to install the fatty chain.
• Benzylic chlorination for water treatment. Benzylic chlorides like benzyl chloride (PhCH2Cl) are produced at ~150,000 ton/yr globally by photochemical chlorination of toluene; downstream products include benzyl alcohol (Friedel-Crafts), quaternary ammonium salts (cationic surfactants for water treatment), and pharmaceutical intermediates. The Lonza-Visp facility produces ~30,000 ton/yr by free-radical chlorination of toluene.