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Please tell me the concept behind this question of organic chemistry. A reaction in the presence of HBr

Please tell me the concept behind this question of organic chemistry.  A reaction in the presence of HBr

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3 Answers

Shah Rutvi
10 Points
5 years ago
HBr in presence of peroxide is used to generate free radical. HF,  HCL are not used in this process because second step involved in the reaction is endothermic.. While this is not seen in HBR. And with HI reversible reaction takes place. 
Ashutosh Pokhriyal
8 Points
5 years ago
I know this. It is there in theory . Please tell me its answer and mechanism involved.
I am confused about cleavage of double bond present there. 
Samyak Jain
333 Points
5 years ago
Addition of HBr in presence of peroxide follows free radical mechanism and 
takes place according to anti-Markownikoff’s rule. 
In this reaction stability of free radical decides the major product.
Initially homolytic bond cleavage of H2O2 takes place producing 2 OHo radicals.
Then homolytic bond cleavage of HBr takes place, producing Bro and Ho ;
this Ho combines with one of OHo radicals to give H2O.
After homolytic break of pi bond of the double bond, two carbon free radicals are formed.
Of which carbon free radical attached to phenyl is resonance stabilised and thus more stable than the carbon free radical attached to two methyl groups whose +H = 6.
Therefore, Bro generated above forms a bond with less stable carbon radical and again Ho produced by homolytic break of HBr is linked with carbon radical having phenyl ring.
So, I think 2nd option is correct.

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