The addition of unsymmetrical reagents (like HX, X=Cl, Br, I) to unsymmetrical alkenes in presence of a peroxide occurs in such a way that the negative part of the addendum goes to the carbon atom of the double bond which carries higher number of hydrogen atoms, is given by which rule?

Anti-Markovnikov's rule ( Kharasch effect)

The correct answer is option 4. Anti-Markovnikov's rule ( Kharasch effect).

The rule that describes the addition of unsymmetrical reagents (like \(HX\), where \(X = Cl, Br, I\)) to unsymmetrical alkenes in the presence of a peroxide, such that the negative part of the addendum goes to the carbon atom of the double bond which carries a higher number of hydrogen atoms, is known as Anti-Markovnikov's rule (Kharasch effect).

Anti-Markovnikov's Rule (Kharasch effect):

In the presence of peroxides, the addition of hydrogen halides \((HX)\) to alkenes occurs in such a way that the halogen (negative part of the addendum) attaches to the carbon atom with more hydrogen atoms. This is the opposite of what happens in the absence of peroxides (where Markovnikov's rule applies).

Mechanism: This occurs via a free radical mechanism initiated by the peroxide. The peroxide initiates the formation of free radicals, leading to the anti-Markovnikov addition.

Example: For the addition of HBr to propene \((CH_3-CH=CH_2)\) in the presence of a peroxide:

According to Anti-Markovnikov's rule:

\(CH_3-CH=CH_2 + HBr \overset{\text{peroxide}}{\longrightarrow} CH_3-CH_2-CH_2Br \)

Here, the bromine (Br) attaches to the carbon with more hydrogen atoms (the terminal carbon).

Why Not the Other Rules

Markovnikov's Rule: States that in the absence of peroxides, the halogen attaches to the carbon with fewer hydrogen atoms (the more substituted carbon).

Saytzeff Rule: Primarily applies to elimination reactions, predicting the formation of the most substituted (more stable) alkene.

Anti-Saytzeff Rule: Is not typically used in this context and generally refers to cases where the less substituted alkene is formed in elimination reactions, which is not relevant to the addition of \(HX\) to alkenes.

Summary

In the presence of peroxides, the correct rule governing the addition of unsymmetrical reagents to unsymmetrical alkenes, where the negative part of the addendum attaches to the carbon with more hydrogen atoms, is the anti-Markovnikov's rule (Kharasch effect).