Free Radical Mechanism – From Intuition to Precision
Imagine you have a long chain of paperclips linked together. Now imagine someone snips one link in the middle. That single cut doesn't just break the chain — it creates two new ends, each hungry to grab onto something. That's the core idea of a free radical mechanism: a reaction that proceeds through species with an unpaired electron — a "hungry" atom or molecule that desperately wants to pair up.
The Intuition: Why Radicals Are Special
Most chemical bonds involve paired electrons — two electrons spinning in opposite directions, like a stable couple. A free radical is the chemical equivalent of a lone wolf: it has one unpaired electron, making it highly reactive. It will do almost anything to find a partner — steal an electron from a neighbour, donate its own, or break another bond to create more radicals.
This creates a chain reaction. One radical reacts, produces another radical, which reacts again, and so on — like a row of dominoes falling one after another. That's why free radical mechanisms are often called chain reactions.
The Precise Statement
A free radical mechanism is a stepwise reaction pathway involving species with unpaired electrons (free radicals). It proceeds through three distinct phases:
Initiation – A stable molecule is broken to produce two free radicals. This usually requires energy — heat (thermolysis) or light (photolysis).
Propagation – Radicals react with stable molecules to produce new radicals. This step repeats many times, forming the chain.
Termination – Two radicals combine to form a stable product, ending the chain.
General pattern:
Initiation: A−Bhν or ΔA⋅+B⋅
Propagation: A⋅+C−D→A−C+D⋅
Termination: A⋅+D⋅→A−D
A Concrete Example: Chlorination of Methane
This is the classic textbook example, and it appears in almost every Indian board exam (Class 11/12, JEE, NEET).
Overall reaction:
CH4+Cl2hνCH3Cl+HCl
Step-by-step mechanism:
Initiation – Chlorine molecule absorbs UV light and splits:
Cl2hν2Cl⋅
Propagation – Two steps that repeat:
Chlorine radical attacks methane:
Cl⋅+CH4→HCl+CH3⋅
Methyl radical attacks another chlorine molecule:
CH3⋅+Cl2→CH3Cl+Cl⋅
Notice: the Cl⋅ consumed in step 1 is regenerated in step 2. This is the chain — one radical keeps producing another.
Termination – Any two radicals meet:
Cl⋅+Cl⋅→Cl2
CH3⋅+CH3⋅→C2H6
CH3⋅+Cl⋅→CH3Cl
Watch out
A common mistake: students think termination only happens when the same radicals combine. In reality, any two radicals can terminate — including cross-combination (like CH3⋅+Cl⋅). Also, termination steps are rare because radical concentrations are very low.
Key Characteristics to Remember
Free radicals are neutral — they have no charge, only an unpaired electron. Don't confuse them with ions.
They are highly reactive — lifetimes are typically microseconds or less. …
A common initiator used in free radical addition polymerisation is benzoyl peroxide.
Structure: C6H5−C(=O)−O−O−C(=O)−C6H5 (two benzoyl groups linked through a peroxide, −O−O−, bond).
On heating, the weak O−O bond homolytically cleaves to generate two benzoyloxy free radicals, which readily lose CO2 to give phenyl radicals — these radicals initiate the polymerisation by adding to the monomer's double bond: