Radical reactions are a major class of reactions in organic chemistry, and they generally proceed through species with unpaired electrons. These reactions are usually classified based on how radicals are involved and what happens to them. Here’s a clear breakdown:
1. Radical Substitution
- In radical substitution, a radical replaces an atom (usually hydrogen) in a molecule.
- Common in halogenation of alkanes.
- Mechanism involves three steps:
- Initiation – formation of radicals.
- Propagation – radicals react with stable molecules to form new radicals.
- Termination – two radicals combine to end the chain.
- Example: Chlorination of methane to form chloromethane.
2. Radical Addition
- Radicals add across double or triple bonds of alkenes or alkynes.
- Often used in anti-Markovnikov addition of HBr to alkenes (in the presence of peroxides).
- The radical attacks the multiple bond, forming a new radical, which then reacts further to complete the addition.
3. Radical Polymerization
- Radicals initiate the linking of monomers to form polymers.
- Example: Polymerization of ethylene to form polyethylene.
- Mechanism is also initiation → propagation → termination.
4. Radical Rearrangement
- Radicals can rearrange within a molecule to form more stable radicals.
- Often occurs if forming a more substituted or resonance-stabilized radical is possible.
- Example: Shifting of a radical from a primary to a secondary carbon.
5. Radical Coupling
- Two radicals combine to form a stable covalent bond.
- This is the termination step in many radical reactions, but can also be used synthetically.
Key Points to Remember:
- Radical reactions usually involve initiation, propagation, and termination.
- They are common in halogenation, addition, polymerization, and rearrangements.
- Radicals are highly reactive and seek to pair their unpaired electron, which drives the reaction.