Bondline / Reactions

Bromination of an alkene

Br₂ adds across the C=C through a cyclic bromonium ion, so the two bromines end up anti (on opposite faces).

Addition to alkenes · 2 steps

Conditions

Br₂ in CH₂Cl₂ (in water, a bromohydrin forms instead). Decolourises bromine water: a test for C=C.

Mechanism in 2 steps

Red curly arrows show where each pair of electrons moves.

BrBrBr+Br−++
Step 1The π bond attacks the nearer bromine, which uses a lone pair to bond to the other carbon; bromide leaves. A three-membered bromonium ion forms.
Br+Br−BrBr+
Step 2Bromide opens the ring from the back face, giving anti addition.

About addition to alkenes reactions

The π electrons of a C=C are exposed and electron-rich, so alkenes react with electrophiles. The two new groups add across the double bond.

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