Addition of bromine to alkynes: practice and mechanism
With two equivalents of Br₂ there are two additions: the triple bond becomes a 1,1,2,2-tetrabromoalkane (two Br on each carbon).
Example
With two equivalents of bromine the triple bond is fully saturated: each of the two carbons ends up with two bromine atoms (tetrabromoalkane).
Mechanism step by step
- 1
Br₂ adds to the triple bond via a (bridged) bromonium ion.
- 2
Bromide opens it from the opposite side: a trans dibromoalkene forms.
- 3
The second equivalent of Br₂ adds to the remaining double bond: a 1,1,2,2-tetrabromoalkane.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Addition of HBr to alkynes · Addition of bromine
Frequently asked questions
Can you stop after one equivalent?
Yes, with exactly 1 equiv. of Br₂ you get the trans-1,2-dibromoalkene, because the brominated double bond is more electron-poor and slower.
Is this a test for multiple bonds?
Yes: the brown bromine solution decolourises with both alkenes and alkynes.