Question

why does hydrogen bromide is used in antimarkonovis reaction ?

Answer 1

A process is spontaneous if its ∆G is negative or if it is exothermic. Anti Markownikoff rule is a free radical mechanism and the mechanism can be divided into three steps.

If all the steps are exothermic, the process is driven towards stability and thus, it is feasible.

In case of HBr, all the three steps are exothermic. Thus HBr shows anti markownikoff.

In case of HCl, the first and the last steps are endothermic whereas in case of HI, the second step is endothermic. Therefore HI and HCl do not show anti MR.