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In the presence of peroxide, HCl and HI do not give anti-Markovnikov’s addition to alkenes because :

Option: 1

One of the steps is endothermic in HCl and HI      

Option: 2

Both HCl and HI are strong acids  

Option: 3

 HCl  is oxidizing and the HI is reducing 

Option: 4

 All the steps are exothermic in HCl and HI      

Answers (1)


As we have learnt,

Antimarkovnikov's Rule / Peroxide effect / Kharash effect -

In presence of peroxide addition of HBr to unsymetrical alkene takes place opposite to Markovnikov's Rule.

The mechanism of the reaction is given as 

(1) Chain Initiation 

\mathrm{(a)R - O - O - R \overset{\Delta}{\longrightarrow} 2 R - \dot O }

\mathrm { (b) R\dot O + H - X \longrightarrow R - OH + \dot X}

(2) Chain Propagation 

\mathrm { (a)R-CH=CH_2 + \dot X \longrightarrow \underset{More\ stable}{R- \dot CH-CH_2X} + \underset{Less\ stable}{R-CHX-\dot CH_2}}

\mathrm { (b){R- \dot CH-CH_2X}+ HX \longrightarrow \underset{major}{R-CH_2- CH_2X}}

In the case of HCl, the step numbered 1(b) is endothermic while in the case of HI, the step numbered 2(a) is endothermic.

In case of HBr, all the above steps are exothermic.

Hence, the correct answer is Option (1)

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