Question

Define Kharasch effect?

Answer 1

Kharasch effect also known as Anti-Markovnikov reaction is an organic reaction. It was first discovered by Morris. S Kharasch in 1940. It is an additional reaction. The Kharasch effect is also sometimes called the peroxide effect.

Complete answer:
Kharasch effect is the addition of $HBr$ to unsymmetrical alkenes in the presence of Peroxide. It gives a product contrary to what Markovnikov addition would give. The reaction proceeds via a free radical mechanism.
$RCH = C{H_2} + HBr\xrightarrow[{hv}]{{R - 0 - 0 - R}}RC{H_2}C{H_2}Br$
The bond dissociation energy of peroxide is less; it breaks easily in the presence of sunlight to form radicals which initiates the reaction.
Only addition of $HBr$ to alkene shows the kharasch effect. $HI$ and $HCl$ can’t show this reaction because they have one endothermic step. They can show Markovnikov addition where the reaction proceeds via carbocation formation.
Markovnikov rule states that when polar reagents undergo an additional reaction with unsymmetrical alkenes, the negative part of the addendum adds to the carbon atom having less number of hydrogen atoms. For example:
$RCH = C{H_2} + HCl \to RCHCl - C{H_2}$
Here $Cl$ adds to the carbon having less number of hydrogen. This reaction takes place via carbocation intermediate.
In the presence of peroxide, the addition of $HBr$ takes place contrary to Markovnikov rule thus giving different products. This is known as the Kharasch effect or peroxide effect.

Additional Information:
Peroxide is a necessary reagent in this reaction as it will initiate the reaction. Peroxide is an unstable molecule which will form radicals very easily.

Note:
The Anti-Markovnikov addition proceeds via free radical contrary to the Markovnikov addition, thus giving us different products. Also, it is important to note that only addition with $HBr$ shows Kharasch effect.