Overview
This lecture introduces hydrohalogenation, the first alkene addition reaction, covering its mechanisms, regiochemistry (Markovnikov vs anti-Markovnikov), stereochemistry, and special cases involving peroxides.
Hydrohalogenation Basics
- Hydrohalogenation adds a hydrogen (H) and a halogen (Cl, Br, I) across a carbon–carbon double bond (alkene).
- Typical reagents are HCl, HBr, or HI.
- The reaction is the reverse of elimination (dehydrohalogenation); instead of forming an alkene, you consume an alkene and make two new sigma bonds.
Markovnikov Addition & Mechanism
- Standard hydrohalogenation follows Markovnikov’s rule: H adds to the less substituted carbon, halogen adds to the more substituted carbon.
- The reaction proceeds through a carbocation intermediate, making it subject to carbocation rearrangements if a more stable carbocation is possible.
- No stereoselectivity is observed unless a chiral center is formed.
- Mechanism: alkene (nucleophile) attacks the acidic hydrogen (from HX), forming a carbocation; halide ion (nucleophile) then attacks the carbocation.
Special Case: HBr with Peroxide (Anti-Markovnikov Addition)
- When HBr is combined with a peroxide (ROOR), the addition follows anti-Markovnikov’s rule: Br attaches to the less substituted carbon.
- This mechanism involves radicals, not carbocations, so no carbocation rearrangement occurs.
- No stereoselectivity is present, and only one achiral product forms if no chiral centers are created.
Stereochemistry and Chiral Centers
- If hydrohalogenation forms a chiral center, both enantiomers (R and S) are produced in equal amounts (racemic mixture).
- It is best to draw both enantiomers explicitly in your answers as most professors prefer this format.
Key Terms & Definitions
- Hydrohalogenation — Addition of H and halogen (Cl, Br, I) across a double bond.
- Markovnikov Addition — H adds to less substituted carbon; halogen to the more substituted.
- Anti-Markovnikov Addition — Halogen adds to less substituted carbon; occurs only with HBr + peroxide.
- Carbocation — Positively charged carbon intermediate in standard hydrohalogenation.
- Radical Mechanism — Reaction pathway with unpaired electrons, relevant to HBr + peroxide reactions.
- Chiral Center — Carbon atom bonded to four different groups, leading to stereoisomers (enantiomers).
Action Items / Next Steps
- Practice drawing full mechanisms for hydrohalogenation reactions.
- Learn to identify Markovnikov versus anti-Markovnikov products.
- Draw both enantiomers when chiral centers are formed in products.
- Review radical mechanisms in preparation for the peroxide case.