Alcohol to bromoalkane (PBr₃): practice and mechanism
PBr₃ turns the OH group into a good leaving group and bromide replaces it via SN2 (with inversion). No carbocation, no rearrangement; suited to primary and secondary alcohols.
Example
The alcohol attacks PBr₃ and becomes a good leaving group, which bromide replaces via SN2. Because no carbocation forms there are no rearrangements; tertiary alcohols hardly react this way (use HBr for those).
Mechanism step by step
- 1
The alcohol oxygen attacks the phosphorus of PBr₃ and Br⁻ leaves. A protonated alkyl dibromophosphite forms.
- 2
The O–PBr₂ group is a good leaving group. Bromide attacks the carbon from the back (SN2).
- 3
The bromoalkane (with inversion) and HOPBr₂ form; one PBr₃ can convert three alcohols this way.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Alcohol to haloalkane (HBr) · Alcohol to chloroalkane (thionyl chloride)
Frequently asked questions
PBr₃ or HBr?
For primary and secondary alcohols PBr₃ (SN2, no rearrangement). For tertiary alcohols HBr (SN1 via the stable cation).
What is the chlorine counterpart?
SOCl₂ (thionyl chloride) or PCl₃ convert alcohols into chloroalkanes.