Alkene Bromination

Animated mechanism for the anti addition of Br₂ to ethylene. The direction of attack can be explained by the molecular orbitals of the reactants.

1. The π electrons of ethylene act as a nucleophile, attacking one of the bromine atoms in Br₂. This forms a cyclic bromonium ion intermediate and releases Br^-.
2. The cyclic bromonium ion is stabilized by sharing the positive charge between both carbons through a three-membered ring structure.
3. A Br^- ion performs an SN2 attack, opening the bromonium ring to yield the anti addition product.

This stereospecific anti addition mechanism explains why bromine addition to alkenes always gives trans dibromide products.