Anti Markovnikov Rule

(Khrash Effect)

Addition of hydrogen bromide to unsymmetrical alkenes in the presence of organic peroxide takes a cause opposite to the suggested by markovnikov rule.

Anti markovnikov rule addition of hydrogen bromide caused by the presence of peroxide is known as peroxide effect.

Propylene react with hydrogen bromide in the presence of peroxide the major product is n-propyl bromide where as the absence of a peroxide the major product is propyl bromide.

Mechanism:-

Propylene reacts with the hydrogen bromide in the presence of peroxide by a free radical mechanism.

Step1 :

Peroxide dissociates with alkoxy free radical.

Step2 :

Alkoxy free radical attacks hydrogen bromide to form a bromide atom (free radical)

Step3 :

Bromide atom attack propylene to give a primary free radical and secondary free radical.

Order of stability free radical is greater than secondary, secondary is greater than primary. the more stable secondary free radical is stable immediately.

Step4 :

More stable free radical the hydrogen to form anti markovnikov product and bromide free radical.