Radical reactions are extremely useful in organic synthesis because they allow bond formation or modification under conditions that are often milder or more selective than ionic reactions. Here’s a detailed explanation:
1. Selective Halogenation
- Radicals can replace hydrogen atoms with halogens (Cl, Br) selectively.
- Example: Bromination of alkanes using radicals can target the most stable radical position (usually tertiary > secondary > primary).
- Useful for preparing halogenated intermediates for further reactions.
2. Anti-Markovnikov Addition
- Radical reactions enable anti-Markovnikov addition of HBr to alkenes.
- This is important when the usual ionic addition (Markovnikov) gives the “wrong” product.
- Radical initiation is often done with peroxides.
3. Polymerization
- Radical reactions are the basis of free radical polymerization.
- Monomers like ethylene, styrene, and methyl methacrylate can be linked into polymers using radical initiators.
- Allows synthesis of plastics, synthetic rubbers, and resins.
4. Formation of Carbon–Carbon Bonds
- Radicals can couple to form new C–C bonds, which is often difficult under ionic conditions.
- Useful in building complex carbon skeletons in organic molecules.
5. Radical Rearrangements
- Radicals can rearrange to form more stable intermediates, allowing structural modifications.
- This can be exploited to shift functional groups or change the position of double bonds in synthesis.
6. Remote Functionalization
- Radicals can abstract hydrogens from distant positions in a molecule, allowing selective functionalization at positions hard to reach via normal ionic chemistry.
- Example: Functionalizing a tertiary carbon far from existing functional groups.
Advantages of Radical Reactions in Synthesis:
- Can occur under mild conditions (sometimes just light or heat).
- Can functionalize unactivated carbons.
- Often regioselective based on radical stability.
- Useful in making polymers, halides, and complex carbon frameworks.