Electrophilic addition to alkenes: Bromine and pent-2-ene

Unsymmetrical bromonium ions open to give racemates

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Conjugate addition mechanism

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Alkenes react on both faces to produce a 1:1 mixture of enantiomeric bromonium ions. Each of these bromonium ions can be opened at both ends by the liberated bromide with inversion. The racemic mixtures of bromonium ions produces a racemic mixture of dibromides. Significantly unsymmetrical bromonium ions may display a preference for opening at one end rather than the other but this example will be close to 1:1.