Electrophilic addition — anti-selective
Br₂ addition to an alkene (vicinal dihalide)
How the Br₂ addition to an alkene (vicinal dihalide) 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. Conditions: CH₂Cl₂ or CCl₄, 25 °C, dark. Bonds formed: two C–Br (vicinal, anti); bonds broken: C=C π; Br–Br. Stereochemistry: Anti addition via bromonium; trans product (racemic). Rate and selectivity: Bromonium formation rate-limiting; fast.
At a glance
Step 1: π bond attacks Br₂
The alkene's π electrons attack Br₂ directly, exactly as in halohydrin formation covered earlier, forming a bridged, three-membered bromonium ion rather than an open carbocation.
Step 2: Backside attack by bromide
With no water or other external nucleophile present, the only nucleophile available is the bromide ion just released from the same Br₂ molecule — it attacks the bromonium ion from the face opposite the existing bromine.
Step 3: Anti addition: vicinal dibromide
This anti addition gives a vicinal dihalide with defined relative stereochemistry (anti, not syn) — the classic decolorization of bromine's orange-red color on contact with an alkene is a widely used qualitative test for carbon-carbon unsaturation.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Br₂ addition to an alkene (vicinal dihalide) | Halohydrin formation from an alkene + Br₂/H₂O | Acid-catalyzed hydration of an alkene | |
|---|---|---|---|
| Reagents | Br₂ | Br₂ in H₂O (or NBS, H₂O/DMSO) | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O |
| Conditions | CH₂Cl₂ or CCl₄, 25 °C, dark | Water as solvent and nucleophile, 0–25 °C | 25–100 °C; heat and excess water favour the alcohol |
| Stereochemistry | Anti addition via bromonium; trans product (racemic) | Anti addition (trans Br/OH via bromonium) | Planar carbocation: racemic; Markovnikov regiochemistry |
| Rate / selectivity | Bromonium formation rate-limiting; fast | Faster for electron-rich alkenes; OH at the more substituted carbon | 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 Br₂ addition to an alkene (vicinal dihalide)?
Br₂. Conditions: CH₂Cl₂ or CCl₄, 25 °C, dark.
Which bonds are formed and which are broken?
Formed: two C–Br (vicinal, anti). Broken: C=C π; Br–Br.
What is the stereochemical outcome?
Anti addition via bromonium; trans product (racemic)
What controls the rate or the selectivity?
Bromonium formation rate-limiting; fast
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