In 1929, Professor S. Kharasch of the University of Chicago observed the anti-Markovnikov addition of HBr to an alkene due to the presence of peroxides in the reaction medium.
![hbr addition with peroxides adición hbr con peróxidos](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos01.png)
Bromine is added to the less substituted carbon of the alkene, while hydrogen is attached to the carbon with more substituents (antiMarkovnikov)
![antimarkovnikov HBr addition adición de HBr antimarkovnikov](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos02.png)
In the absence of peroxides, the addition of HBr to the alkene is Markovnikov, that is, the bromine adds to the most substituted carbon.
![antimarkovnikov HBr addition adición de HBr antimarkovnikov](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos03.png)
The addition of hydrogen bromide to an alkene follows two different mechanisms depending on the presence or absence of peroxides in the reaction medium.
In the absence of peroxides, a carbocation forms at the most substituted position of the alkene.
![antimarkovnikov HBr addition adición de HBr antimarkovnikov](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos04.png)
In the presence of peroxides, a radical mechanism is followed, with formation of a radical-type intermediate on the most substituted carbon of the alkene.
Let's see the mechanism of the following global reaction:
![antimarkovnikov HBr addition adición de HBr antimarkovnikov](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos05.png)
a) initiation
The initiation consists of two stages:
Stage 1. The peroxide dissociates into two alkoxy radicals.
Stage 1. The peroxide dissociates into two alkoxy radicals.
![addition of HBr with peroxides adición de HBr con peróxidos](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos06.png)
Stage 2. Abstraction of hydrogen from hydrogen bromide by the alkoxy radical, forming bromine radicals that pass to the propagation stage.
![addition of HBr with peroxides adición de HBr con peróxidos](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos07.png)
b) Propagation
Propagation consists of two stages:
Stage 3. The bromo radical is added to the alkene.
![addition of HBr with peroxides adición de HBr con peróxidos](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos08.png)
Stage 4. The radical formed on carbon abstracts hydrogen from hydrogen bromide, forming new bromine radicals.
![addition of HBr with peroxides adición de HBr con peróxidos](/images/stories/organica-i/alquenos-reacciones/hbr-peroxidos/hbr-peroxidos09.png)
This addition is anti-Markovnikov since the radical formed in step 3 is tertiary (highly stabilized by hyperconjugation). A Markovnikov addition would generate a much more unstable secondary radical.