Free Radical Halogenation Mechanism - NEET Notes, Formula & Common Mistakes
Hydrocarbons·2 min read·NEET 2026
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).
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.
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.
Frequently Asked Questions
What are the three main steps in free radical halogenation?▾
The three main steps are Initiation, Propagation, and Termination. Initiation involves the homolytic fission of the halogen molecule to form radicals. Propagation is the chain-carrying step where radicals react with stable molecules. Termination occurs when two radicals combine to form a stable, non-radical product.
Why is bromination more selective than chlorination?▾
Bromination is a slower and more endothermic reaction than chlorination. Due to its higher activation energy, the transition state resembles the product (alkyl radical), allowing for greater differentiation between the stabilities of primary, secondary, and tertiary radicals, leading to higher selectivity.
What conditions are required for free radical halogenation?▾
Free radical halogenation of alkanes typically requires either ultraviolet (UV) light (hν) or high temperatures. These conditions provide the necessary energy to break the halogen-halogen bond homolytically, initiating the radical chain reaction.
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