Electrophilic addition to alkenes can produce stereoisomers

NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase.

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When cyclohexene is treated with bromine in carbon tetrachloride, the racemic anti-1,2-dibromocyclohexane is obtained exclusively. The bromonium ion is opened with inversion in an S_N2 reaction. As the bromine can attack from either side we get both stereoisomers and a racemic mixture.