Addition of HBr to Alkenes
Addition of HBr turns an alkene into an alkyl bromide. Step-by-step mechanism, reagents and practice.
What happens
Addition of HBr turns an alkene into an alkyl bromide. The C=C double bond breaks and the two parts of HBr add across it: H goes to one carbon, Br to the other. The reaction follows Markovnikov's rule, H adds to the carbon that already has more hydrogens, Br to the more substituted carbon, because that pathway forms the more stable carbocation intermediate.
Reaction summary
Propene + HBr → 2-Bromopropane (Markovnikov addition)
Reagents and conditions
- Starting material: Propene (alkene)
- Reagents: HBr
- Products: 2-Bromopropane (Markovnikov product)
Mechanism
- Protonation: The alkene's π bond attacks H⁺ from HBr; the H–Br bond breaks toward Br, forming a carbocation.
- Nucleophilic Attack: Bromide's lone pair attacks the carbocation, forming the new C–Br bond.
- Product: The neutral alkyl bromide is formed.
Selectivity
- Regioselectivity: Markovnikov: H adds to the carbon that already has more hydrogens; Br ends up on the more substituted carbon.
Common mistakes
- Carbocation rearrangements can occur, a hydride or alkyl shift may happen if it gives an even more stable cation. Don't assume the product is always unrearranged.
Memory trick
"Markovnikov = the rich get richer": the carbon that already has more H gets the new H; Br goes to the more substituted carbon.