Electrophilic addition — anti-selective
Halohydrin formation from an alkene + Br₂/H₂O
How the Halohydrin formation from an alkene + Br₂/H₂O works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Br₂ in H₂O (or NBS, H₂O/DMSO). Conditions: Water as solvent and nucleophile, 0–25 °C. Bonds formed: C–Br and C–O σ bonds; bonds broken: C=C π bond; Br–Br. Stereochemistry: Anti addition (trans Br/OH via bromonium). Rate and selectivity: Faster for electron-rich alkenes; OH at the more substituted carbon.
At a glance
Step 1: π bond attacks Br₂, bromonium forms
The alkene's π electrons attack Br₂, but instead of an open carbocation, they form a bridged, three-membered bromonium ion — bromine partially bonded to both carbons at once.
Step 2: Water opens the ring from the back
Water attacks the bromonium ion from the face opposite bromine (anti), preferentially at the more substituted carbon where positive character is greatest, opening the ring.
Step 3: Deprotonation gives the halohydrin
Loss of a proton gives the halohydrin. Because the nucleophile is forced to attack from the opposite face, the Br and OH end up anti to each other on the final product.
Abbreviations and terms
- DMSO
- Dimethyl sulfoxide. Polar aprotic solvent; also the oxidant in Swern/Kornblum-type reactions.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- NBS
- N-Bromosuccinimide. Source of a low, steady concentration of Br₂/Br• for allylic/benzylic bromination.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Halohydrin formation from an alkene + Br₂/H₂O | Br₂ addition to an alkene (vicinal dihalide) | Acid-catalyzed hydration of an alkene | |
|---|---|---|---|
| Reagents | Br₂ in H₂O (or NBS, H₂O/DMSO) | Br₂ | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O |
| Conditions | Water as solvent and nucleophile, 0–25 °C | CH₂Cl₂ or CCl₄, 25 °C, dark | 25–100 °C; heat and excess water favour the alcohol |
| Stereochemistry | Anti addition (trans Br/OH via bromonium) | Anti addition via bromonium; trans product (racemic) | Planar carbocation: racemic; Markovnikov regiochemistry |
| Rate / selectivity | Faster for electron-rich alkenes; OH at the more substituted carbon | Bromonium formation rate-limiting; fast | Protonation of the alkene is rate-determining |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Halohydrin formation from an alkene + Br₂/H₂O?
Br₂ in H₂O (or NBS, H₂O/DMSO). Conditions: Water as solvent and nucleophile, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–Br and C–O σ bonds. Broken: C=C π bond; Br–Br.
What is the stereochemical outcome?
Anti addition (trans Br/OH via bromonium)
What controls the rate or the selectivity?
Faster for electron-rich alkenes; OH at the more substituted carbon
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go