First of all, we should know about the reaction which occurs here. Later on we will discuss the peroxide effect in each case of HX.
The reaction occurs is an addition reaction of an alkene. The hydrogen halide (HX, where X is F, Cl, Br, I) molecule adds to the double bonded carbon atoms to form an alkyl halide.
A good way to think of the addition reaction is that the pi bond acts as a nucleophile which attacks on the proton (from HX) which acts as an electrophile. As an alternative method, you can view the first step of the reaction as protonation of pi bond.
Peroxide effect is only applicable to HBr due to symmetrical breaking of H-Br bond.
What is Markovnikov addition rule?
When a protic acid (HX) is added to an asymmetric alkene, the acidic hydrogen attaches itself to the carbon having a greater number of hydrogen substituents, whereas the halide group attaches itself to the carbon atom which has a greater number of alkyl substituents.
To simplify the rule, it can also be stated as – “Hydrogen is added to the carbon with the most hydrogen and the halide is added to the carbon with least hydrogen”.
Remember it like: “H अपने भाइयों के पास जाएगा”
As a result, it means that the positive part of HX is added to carbon having more hydrogen and the negative part is added to carbon with least hydrogen, at the double bond.
What is Anti-Markovnikov’s addition rule?
In an addition reaction of HX to an alkene, the hydrogen atom of HX becomes bonded to the carbon atom that has the least number of hydrogen atoms, while halide group becomes bonded to carbon atom that has the most number of hydrogen atoms.
To simplify the rule, it can also be stated as – “Hydrogen is added to the carbon with the least hydrogen and the halide is added to the carbon with most hydrogen”.
As a result, it means that the positive part of HX is added to carbon having least hydrogen and the negative part is added to carbon with most hydrogen, at the double bond.
How does HBr follow peroxide effect?
When HBr with Peroxide is given in an addition reaction of alkene, then apply Anti- Markovnikov addition rule (peroxide effect), while with other alkyl halides, i.e., HCl and HI, apply Markovnikov addition rule only (if they are mentioned with peroxide also).
The mechanism of addition of HBr to unsymmetrical alkene (say propene) in the presence of peroxide is free radical mechanism, i.e., H-Br undergo homolytic fission to form free radicals, H˙ and Br˙.
Why does HCl not follow peroxide effect?
HCl is a very stable acid due to which the H-Cl bond (BE 430 kJ/mol) is stronger than H-Br bond (BE 378 kJ/mol) and is not broken symmetrically by the free radicals generated by peroxide. It breaks via heterolytic fission into H+ and Cl- ions.
Hence, the free radical addition of HCl to alkene is not possible.
Why does HI not follow peroxide effect?
In case of HI, the H-I bond (BE 297 kJ/mol) is weaker than H-Br bond and undergo homolytic fission readily to form iodine free radical. But the iodine free radical has a greater tendency to combine amongst themselves to form iodine molecule rather than add to the ethylenic bond.
Hence, HI does not respond to the peroxide effect.
It can be concluded that, only HBr show peroxide effect. If HCl or HCl/Peroxide or HI or HI/Peroxide is given in reaction, then only follow Markovnikov’s addition rule.
If HBr is given then follow Markovnikov’s addition rule and for HBr/Peroxide follow Anti-Markovnikov’s addition rule.
The reactions are given below:
Read more: Mechanism of addition of HOCl to an alkene
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