Initiation: X-X → 2X· (UV light or heat provides energy for homolytic bond cleavage)
Propagation (chain-carrying steps):
- X· + R-H → R· + H-X (H-abstraction — rate-determining step)
- R· + X-X → R-X + X· (halogen transfer)
Termination (any radical + radical combination):
- R· + R· → R-R
- R· + X· → R-X
- X· + X· → X-X
Selectivity: Br2 is far more selective than Cl2 because the H-abstraction step for Br· is endothermic (late, product-like transition state that distinguishes between 1°/2°/3° radicals). For Cl·, it's exothermic (early TS, less discriminating).
Relative reactivity per H: For Br2: 3° : 2° : 1° = 1600 : 82 : 1. For Cl2: 5.0 : 3.8 : 1.0.