Addition of HBr to alkynes: practice and mechanism
Two equivalents of HBr both add Markovnikov: both Br end up on the same, more substituted carbon (geminal dibromide).
Example
HBr adds twice following Markovnikov. After the first addition the bromine stabilises the cation on the same carbon (resonance with its lone pairs), so both bromines end up on the same carbon: a geminal dibromide.
Mechanism step by step
- 1
The first HBr protonates the terminal carbon; the more stable (inner) vinyl cation forms and Br⁻ adds there.
- 2
The second HBr protonates the vinyl bromide again at the terminal carbon. The cation on the carbon bearing Br is stabilised by its lone pair.
- 3
Br⁻ adds to this carbon: both Br are on the same carbon (geminal dibromide).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be the vicinal dibromide (Br on two adjacent carbons) from Br₂. With HBr the Markovnikov rule decides twice: both Br go to the same carbon.
Often confused with: Addition of bromine to alkynes · Hydration of alkynes
Frequently asked questions
What do geminal and vicinal mean?
Geminal: both groups on the same carbon. Vicinal: on two adjacent carbons (as in Br₂ addition).
What happens with peroxides?
As with alkenes the addition then runs radically and anti-Markovnikov: the Br goes to the terminal carbon.