Bromination of ethene: electrophilic addition of Br₂

Bromine water is decolourised by alkenes. The alkene's π electrons polarise the Br–Br bond and attack it, forming a three-membered bromonium ion, which is then opened by bromide. Some school courses draw a simple carbocation instead; the bromonium ion explains why the two bromines add to opposite faces.

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Electrophilic addition (via bromonium ion)

The C=C π bond attacks one bromine atom of Br₂, pushing out bromide and forming a cyclic bromonium ion. Bromide then attacks a carbon from the back, opening the ring to give 1,2-dibromoethane.

Overall reaction

1. 1Formation of the bromonium ionelectrophilic attack

   ethene

   +

   bromine

   bromonium ion

   +

   bromide ion

   Electron movement

   1. The π electrons attack the nearer bromine atom (made δ+ by the electron-rich alkene).
   2. The Br–Br bond breaks and the far bromine leaves as Br⁻.
   3. A lone pair on the attacked bromine bonds to the second carbon, closing a three-membered ring.

   As Br₂ approaches the alkene its electrons are repelled, making the nearer bromine δ+. The bromonium ion spreads the positive charge over a ring, so no free carbocation forms.

2. 2Bromide opens the ringnucleophilic attack (ring opening)

   bromonium ion

   +

   bromide ion

   1,2-dibromoethane

   Electron movement

   1. A lone pair on the bromide ion attacks one ring carbon, from the side opposite the ring bromine.
   2. The C–Br⁺ bond breaks, opening the ring and making that bromine neutral.

   Bromide can only attack from the face opposite the bulky bromonium bridge, so the two bromines end up on opposite faces (anti addition).