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THEORY REACTIONS OF ALKENES
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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.

adición hbr con peróxidos
Bromine is added to the less substituted carbon of the alkene, while hydrogen is attached to the carbon with more substituents (antiMarkovnikov)
adición de HBr antimarkovnikov
In the absence of peroxides, the addition of HBr to the alkene is Markovnikov, that is, the bromine adds to the most substituted carbon.
adición de HBr antimarkovnikov

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.
adición de HBr antimarkovnikov
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:
adición de HBr antimarkovnikov
a) initiation
The initiation consists of two stages:
Stage 1. The peroxide dissociates into two alkoxy radicals.
adición de HBr con peróxidos
Stage 2. Abstraction of hydrogen from hydrogen bromide by the alkoxy radical, forming bromine radicals that pass to the propagation stage.
adición de HBr con peróxidos
b) Propagation
Propagation consists of two stages:
Stage 3. The bromo radical is added to the alkene.
adición de HBr con peróxidos
Stage 4. The radical formed on carbon abstracts hydrogen from hydrogen bromide, forming new bromine radicals.
adición de HBr con peróxidos
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.