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Also known as: Bromination of Alkenes, Anti Addition
Electrophilic addition of Br₂ or Cl₂ to alkenes proceeds through a cyclic halonium ion intermediate, giving exclusively anti addition (trans product). This is one of the classic tests for unsaturation — Br₂ decolorization.
The π electrons of the alkene attack Br₂, displacing Br⁻ and forming a cyclic bromonium ion intermediate. The three-membered ring bridges both carbons.
Bromonium ion prevents rotation — locks geometry
Br⁻ attacks the bromonium ion from the back side (anti to the existing Br), opening the ring. This gives exclusively anti addition product.
Anti addition — both Br's end up on opposite faces
Cyclohexene
Br₂ in CCl₄
trans-1,2-Dibromocyclohexane
cis-2-Butene
Br₂ in CH₂Cl₂
(2R,3R) + (2S,3S)-2,3-Dibromobutane
Classic method for introducing two halogen atoms across a double bond. Bromonium ion intermediate is key to stereocontrol.