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Anti Markonikov's rule - HBr

Why anti Markonikov's rule is applicable only in case of HBr during its addition to unsymmetrical alkenes in presence of peroxides?


The anti markonikov's addition of HBr to unsymmetrical alkenes in presence of peroxides and light is a free radical addition reaction. The crucial step is abstraction of H* free radical from the hydrogen bromide by the organic free radical (alkoxy or alkyl free radicals).

abstraction of H free radical from HBr

This step is exothermic (why?) and hence formation of Br* free radical is feasible. Hence the reaction can proceed further. 

However, HCl and HI do not undergo any free radical additions to alkenes in presence of peroxides.


In case of HCl, the abstraction of H* from HCl is endothermic (why?) and hence further reaction is not possible.

And in case of HI , even though the abstraction of H* from HI is exothermic, the addition of I* free radical to alkene is endothermic. Hence addition with HI is less likely to occur.

Why the abstraction of H* free radical from HBr is exothermic, whereas it is endothermic with HCl?

Since H-Cl bond is stronger than H-Br, breaking H-Cl bond requires more energy. The energy released in the formation new O-H bond cannot compensate for this as with HBr. Hence the abstraction from H-Cl is endothermic.

What about HF?

H-F bond is even more stronger than H-X bonds in other halides. Hence we can safely assume that in this case too the abstraction of H* free radical is not feasible due to endothermic nature.

Remember that addition of H-Br through electrophilic addition mechanism is also not possible under normal conditions due to same reason.

Related questions 

1) What are endothermic and exothermic reactions?

2) What is anti-markonikov's rule?

3) What is Kharash effect?

4) What is the mechanism of electrophilic addition reaction of alkenes?

Author: Aditya vardhan Vutturi