Free Radical Halogenation Alkanes — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Free-Radical Halogenation of Alkanes Alkanes (R–H) undergo substitution with halogens (X2) via a radical chain mechanism under UV light or heat to form alkyl halides (R–X) and hydrogen halides (HX). The mechanism involves initiation (homolysis of X2), propagation (H-abstraction and halogen transfer), and termination (radical recombination). Reactivity and selectivity follow F > Cl > Br > I (reactivity), with bromination being slower but more selective than chlorination. Under UV light, greenish-yellow Cl2 or reddish-brown Br2 is decolorized; misty white fumes of HCl/HBr appear and may be detected by moist litmus (turns red due to acid). Reaction mixtures can warm due to exothermicity; over-chlorination leads to a mixture of products unless excess alkane is used. Overall enthalpy for methane monochlorination is exothermic (ΔH ≈ −90 kJ·mol⁻¹); bromination is slightly exothermic (≈ −10 kJ·mol⁻¹) but with an endothermic H-abstraction step; iodination is endothermic overall (≈ +50 kJ·mol⁻¹) and reversible, hence not feasible without an oxidant to remove HI. Fluorination is highly exothermic and often uncontrollable. 1. Initiation: Homolysis of halogen under light or heat: X2 h ;or ; 2 ,X 2. Propagation (H-abstraction): X + R - H R + H - X. For bromination this step is endothermic and rate-determining (high selectivity); for chlorination it is exothermic (lower selectivity). 3. Propagation (halogen transfer): R + X2 R - X + X , regenerating the halogen radical and sustaining the chain. 4. Chain length and inhibitors: Chains propagate for many cycles; radical inhibitors (e.g., O2) quench by forming peroxy radicals: X + O2 XO2 (inhibits reaction). 5. Termination: Radical recombination removes radicals from the system: X + X X2; R + X R - X; R + R R - R. 6. Site selectivity: More stable radicals form preferentially (3° > 2° > 1°). Product distribution depends on both number of equivalent H atoms and reactivity factors (chlorination ~ 3°:2°:1° = 5:3.8:1; bromination ~ 1600:80:1). Assuming iodination proceeds like chlorination/bromination — it is endothermic and reversible (does not proceed appreciably without an oxidant). Confusing the peroxide effect (anti-Markovnikov addition to alkenes) with radical substitution of alkanes — peroxides are not required here. Ignoring selectivity: bromination is more selective than chlorination (3° > 2° > 1°); F is too reactive (poor selectivity), I is essentially unreactive. Forgetting to multiply by the number of equivalent hydrogens when predicting major product ratios. Not controlling multiple substitution — excess alkane is needed to favor monosubstitution in chlorination. Assuming stereochemistry is retained at a chiral center — radical substitution proceeds via a planar radical leading to racemization.