Convert alcohol (R-OH) to alkyl bromide (R-Br) using PBr₃, PBr₅, or HBr with H₂SO₄—these reagents replace the hydroxyl group with bromine.
Common Methods:
1. Phosphorus Tribromide (PBr₃) - Most Common
- 3R-OH + PBr₃ → 3R-Br + H₃PO₃
- Mild conditions, good yields
- Works for primary and secondary alcohols
2. Phosphorus Pentabromide (PBr₅)
- R-OH + PBr₅ → R-Br + POBr₃ + HBr
- More reactive than PBr₃
3. HBr with H₂SO₄
- R-OH + HBr → R-Br + H₂O
- Requires heating
- H₂SO₄ removes water (shifts equilibrium)
Reaction Mechanism:
- Nucleophilic substitution (SN1 or SN2)
- OH group is replaced by Br atom
Importants Points:
- Best reagent: PBr₃ for most alcohols
- Reaction type: Nucleophilic substitution
- Alternatives: PBr₅ or HBr/H₂SO₄