Electrophilic Addition of Br2 to an Alkene

•	A possible mechanism for the electrophilic addition of Br2 to an alkene is outlined below.

 

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As a bromine molecule approaches the nucleophilic alkene, the Br-Br bond becomes polarized. The electron pair from the double bond then attacks the polarized bromine forming a C-Br bond and displacing a bromide ion. The intermediate electrophilic carbocation then immediately reacts with the nucleophilic bromide ion to give the dibromo addition product.

•	Although this mechanism looks reasonable it is not consistent with the know facts. In particular, it does not explain the stereochemistry of halogen addition.