Alcohol to haloalkane (HBr): practice and mechanism
The protonated OH group leaves as water and bromide takes its place: alcohol → bromoalkane.
Example
The acid protonates the OH group, turning it into a good leaving group (H₂O). Bromide replaces it: via SN2 for primary alcohols, via the carbocation (SN1) for tertiary ones.
Mechanism step by step
- 1
HBr protonates the OH group to –OH₂⁺.
- 2
Primary: Br⁻ attacks from the back while water leaves (SN2).
- 3
Tertiary: water leaves first (carbocation), then Br⁻ attacks (SN1).
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: Acid-catalysed dehydration (E1) · Alcohol to chloroalkane (thionyl chloride)
Frequently asked questions
Why doesn't OH⁻ leave directly?
OH⁻ is a strong base and therefore a poor leaving group. Once protonated, neutral water leaves, which is much easier.
Does elimination compete?
With tertiary alcohols and heat, yes; that is why you work cool with concentrated HBr.