Anti-Markovnikov addition is a type of chemical reaction where a substituent (like a halogen or hydroxyl group) adds to the less substituted carbon of a double bond, which is the opposite of what happens in a normal (Markovnikov) addition. In the context of radical reactions, this happens because the reaction proceeds through free radicals rather than carbocations. Let me explain clearly:
1. Normal (Markovnikov) vs. Anti-Markovnikov
- Markovnikov addition: In ionic reactions, the more electronegative group (like –OH or –Cl) attaches to the more substituted carbon of a double bond because the reaction goes through a carbocation intermediate, which is more stable on the more substituted carbon.
- Anti-Markovnikov addition: In radical reactions, the group attaches to the less substituted carbon because the reaction goes through a radical intermediate, and the more stable radical forms on the more substituted carbon.
2. Why Radical Reactions Give Anti-Markovnikov Products
- The reaction starts when a radical initiator (like peroxides) produces free radicals.
- The radical reacts with a molecule like H–Br, forming a bromine radical.
- The bromine radical adds to the double bond, generating a more stable carbon radical (usually on the more substituted carbon).
- This carbon radical then reacts with another H–Br molecule, forming the product.
- Because the radical forms on the more substituted carbon, the hydrogen ends up on the more substituted carbon and the halogen (or group) ends up on the less substituted carbon—hence anti-Markovnikov.
3. Example
- Hydrobromination in the presence of peroxides:
- HBr adds to an alkene, but the bromine attaches to the less substituted carbon.
- This is opposite to the normal HBr addition without radicals, which follows Markovnikov’s rule.
4. Key Points
- Anti-Markovnikov addition occurs only under radical conditions, often initiated by peroxides or light.
- It is common for reactions like HBr addition to alkenes, hydroboration-oxidation, and some other radical additions.
- The reaction is guided by radical stability, not carbocation stability.