Markonikoff's and anti-Markonikoff's rule

Markonikoff’s Rule

During the addition of a reagent to an unsymmetrical alkene, positive part of the reagent adds to the double bonded carbon having more number of hydrogen and negative part of the reagent adds to the double bonded carbon atom having less number of hydrogen. This is known as Markonikoff’s or Markonikov’s rule

Anti – Markonikoff’s Rule

In the presence of peroxide the positive part of the reagent adds to the double bonded carbon atom having less number of hydrogen and negative part of the reagent adds to the double bonded carbon having more number of hydrogen. This reaction occurs between unsymmetrical alkene and HBr. This is called Anti – Markonikoff’s rule or peroxide effect or Kharasch effect.

Why HBr is used in Peroxide effect ?

Peroxide effect is seen in HBr only but not in HCl and HI. Peroxide effect is a free radical forming reaction i.e. the reaction takes place by forming the free radical of alkyl peroxide (R-O-O-R’). The free radical cannot break the H-Cl bond due to it’s high dissociation energy. The peroxide free radical can split the H – I molecule but as Iodine is a powerful reducing agent, the atom of iodine combines to give I2.