What NEET Asks
- Questions on identifying initiation, propagation, and termination steps.
- Predicting major products based on radical stability and selectivity.
- Understanding the relative reactivity of halogens and different types of H atoms.
- Typically 1-2 questions directly or indirectly related to this mechanism.
Key Points
- Initiation: Homolytic fission of the halogen molecule (X₂) by UV light or heat generates free radicals (X•).
- Propagation: A chain reaction where a radical reacts with a stable molecule to form a new radical and a new molecule. Includes H-abstraction (R-H + X• → R• + H-X) and halogen-abstraction (R• + X₂ → R-X + X•).
- Termination: Two free radicals combine to form a stable molecule, ending the chain. (e.g., X• + X• → X₂, R• + R• → R-R, R• + X• → R-X).
- Radical Stability: 3° alkyl radical > 2° alkyl radical > 1° alkyl radical > methyl radical. This dictates regioselectivity.
- Reactivity Order of Halogens: F₂ > Cl₂ > Br₂ > I₂. Fluorination is explosive, iodination is reversible.
- Selectivity Order: Br₂ is much more selective than Cl₂, favoring substitution at the most stable radical position (3° > 2° > 1°). Chlorination is less selective.
Must-Know Formula / Reaction
Overall Reaction: R-H + X₂ --(hν/heat)--> R-X + H-X
- R-H: Alkane (reactant)
- X₂: Halogen (Cl₂ or Br₂)
- hν/heat: Energy for homolytic fission
- R-X: Haloalkane (product)
- H-X: Hydrogen halide (byproduct)
Common Mistakes
- Don't confuse homolytic fission (forms radicals) with heterolytic fission (forms ions).
- Students often overlook or misidentify the various possible termination steps.
- Failing to consider the relative selectivity of Cl₂ vs. Br₂ when predicting major products.
Rapid Revision
Free radical halogenation proceeds via initiation (radical formation), propagation (chain reaction), and termination (radical combination). Remember radical stability (3°>2°>1°) guides product formation. Bromination is highly selective for the most stable radical, while chlorination is less selective.