91Ó°ÊÓ

When the unsymmetrical reagent (HBr, HCl, etc) is added to an unsymmetrical alkene, the positive part of the unsymmetrical reagent goes to that unsaturated bond (double or triple) having a greater number of hydrogen atoms.

The product formed from such a reaction is governed by the anti-Markovnikov rule.

Anti- Markovnikov rule governs the product for many chemical reactions.

In the initiation step, radicals are formed.

Initiation step

In the propagation step, the bromide radical adds to triple bond of pent-1-yne to generate alkyl radical on the secondary carbon atom, and then the secondary alkyl radical abstract hydrogen atom from HBr to generate product and bromide radical.

Mechanism showing the reaction of pent-1-yne with HBr in the presence of peroxides

The anti- Markovnikov orientation result when pent-1-yne react with HBr in the presence of peroxide to form a more stable free radical. The more substituted radical is more stable; thus, $2^0$radical is more stable than $1^0$radical.

Hence, the bromine radical first attack forms a more stable $2^0$radical forming the anti- Markovnikov oriented product.