Why only HBr and not HCL and HI give anti-Markonikov product ?

HBr gives anti-Markovnikov product because of the presence of a peroxide initiator, such as benzoyl peroxide, which initiates a free radical chain reaction. This leads to the formation of a more stable radical intermediate, which then reacts with the alkene to give the anti-Markovnikov product.

On the other hand, HCl and HI do not give anti-Markovnikov products because they do not have the ability to form the stable radical intermediate required for the anti-Markovnikov addition. Instead, they follow the traditional Markovnikov addition mechanism, where the hydrogen adds to the carbon with more hydrogen atoms already attached, leading to the formation of the Markovnikov product.

HBr and HI are able to give anti-Markovnikov products because they are stronger acids than HCl. This means that they are more likely to protonate the alkene, which leads to the formation of a more stable carbocation. The more stable carbocation is then more likely to react with the nucleophile to form the anti-Markovnikov product.

In the case of HCl, it is not as strong an acid as HBr or HI, so it is less likely to protonate the alkene. This means that the carbocation that is formed is less stable, and is therefore more likely to react with the nucleophile to form the Markovnikov product.