Addition of bromine: practice and mechanism
Br₂ adds to both carbons of the double bond (via the bromonium ion): a vicinal dibromide forms.
Example
Bromine adds via a cyclic bromonium ion; bromide attacks from the opposite side (anti addition). A vicinal dibromide forms, with bromine on both former double-bond carbons. Decolourising bromine water is the classic test for alkenes.
Mechanism step by step
- 1
Br₂ is polarised at the double bond; the near bromine is attacked electrophilically.
- 2
A cyclic bromonium ion (three-membered ring with Br⁺) forms and Br⁻ leaves.
- 3
Br⁻ attacks from the opposite side and opens the ring: anti addition, trans dibromide.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
The cyclic bromonium ion
The π electrons of the double bond attack a bromine atom and the Br–Br bond breaks. Instead of an open carbocation a three-membered ring of two carbons and a positively charged bromine forms. Because the bromine shields one face of the molecule, the bromide ion can only attack from the opposite side, which is why the addition is anti.
Stereochemistry by example
Cyclohexene gives only trans-1,2-dibromocyclohexane. (E)-But-2-ene gives the meso compound, (Z)-but-2-ene the racemate of the two enantiomers. Exams like to ask exactly this link.
In water: bromohydrin
With an excess of water, water instead of bromide opens the bromonium ion. A bromohydrin forms, again anti, with the OH on the more substituted carbon (Markovnikov).
Typical mistakes
Often confused with: Addition of HBr (Markovnikov) · Catalytic hydrogenation
Frequently asked questions
Why is bromine water decolourised?
The brown Br₂ is consumed and the dibromide is colourless. That is the classic test for C=C.
Why doesn't benzene react this way?
Addition would destroy the aromatic system. Benzene only reacts with a Lewis acid and then substitutes.
Why is bromine addition anti?
A cyclic bromonium ion forms first; bromide attacks from the back side, so the two bromines end up trans.
How does bromine show a C=C double bond?
Brown bromine water is decolourised because bromine adds to the double bond.
What is a cyclic bromonium ion?
A three-membered ring of two carbons and a positively charged bromine. It forms as an intermediate when bromine adds to a C=C double bond and explains why the two bromines end up trans in the product.