Q.Discuss Markownikov's rule and Kharash effect.

Markownikov's rule predicts where $H$ and $X$ add on an unsymmetrical alkene (via the more stable carbocation); the Kharash/peroxide effect reverses this outcome specifically for $HBr$ by switching the mechanism to a free-radical chain reaction.

Markownikov's Rule:

When a protic acid $HX$ (such as $HBr$, $HCl$, $HI$, or $H_2SO_4$) adds to an unsymmetrical alkene, the rule states: the hydrogen atom of $HX$ adds to the carbon of the double bond that already carries the greater number of hydrogen atoms, while $X$ attaches to the carbon bearing fewer hydrogens (the more substituted carbon). This is often paraphrased as "the rich get richer."

Mechanistic reason: the reaction proceeds by an ionic (electrophilic addition) mechanism. $H^+$ first adds to one of the double-bond carbons, generating a carbocation at the other carbon. Since more substituted carbocations (tertiary > secondary > primary) are more stable due to greater electron-donation (hyperconjugation/inductive effect) from adjacent alkyl groups, the pathway that forms the more stable, more substituted carbocation is strongly favoured. The nucleophile $X^-$ then attacks this more stable carbocation, so $X$ ends up on the more substituted carbon — exactly as the empirical rule predicts.

Example: propene + $HBr$ → 2-bromopropane (major product), since the secondary carbocation $CH_3-\overset{+}{C}H-CH_3$ is more stable than the primary carbocation, so $Br^-$ attacks that carbon.

Kharash Effect (Peroxide Effect / Anti-Markownikov Addition):

When the addition of $HBr$ only (not $HCl$ or $HI$) to an unsymmetrical alkene is carried out in the presence of peroxides ($R-O-O-R$), the product obtained is the opposite (anti-Markownikov) of what Markownikov's rule predicts — $Br$ ends up on the carbon with more hydrogens.

Mechanistic reason: peroxides decompose to generate free radicals, which switch the mechanism from ionic to free-radical chain addition:

1. Peroxide homolysis generates $RO\cdot$ radicals, which abstract an H from $HBr$ to generate $Br\cdot$ radicals. …