Radicals react with alkenes through addition reactions, usually following a chain reaction mechanism. Here’s a clear explanation step by step:
1. Nature of the Reaction
- Alkenes have a carbon-carbon double bond, which is rich in electrons.
- Radicals are highly reactive species with an unpaired electron.
- The radical attacks the double bond, forming a new radical intermediate.
- This intermediate can then react further to propagate the reaction.
2. Mechanism of Radical Addition to Alkenes
Radical reactions with alkenes generally follow a three-step chain mechanism:
a) Initiation
- A radical is generated from an initiator (like peroxides or light).
- Example: A halogen radical forms from a molecule like HBr under light or peroxide.
b) Propagation
- The radical adds to the alkene:
- One carbon of the double bond forms a bond with the radical species.
- The other carbon becomes a carbon-centered radical, which is more stable if it’s on a more substituted carbon (due to radical stability).
- This new radical then reacts with another molecule (like HBr), forming the final product and regenerating a radical that continues the chain.
c) Termination
- Two radicals combine to form a stable product, stopping the chain reaction.
- This can happen by:
- Combination of two radicals, or
- Disproportionation, where one radical abstracts a hydrogen from another.
3. Outcome
- The radical adds across the double bond of the alkene.
- The position where the radical adds depends on radical stability, often leading to anti-Markovnikov products in reactions like HBr addition under radical conditions.
- The reaction can produce polymers if the process repeats many times (radical polymerization).
4. Key Points
- Radical addition to alkenes is regioselective, guided by radical stability.
- The reaction often requires initiators or energy sources like heat or light.
- It is widely used in industrial polymerization and in forming functionalized organic molecules.