Markovnikov's rule is a principle in organic chemistry that predicts the outcome of the addition reactions of protic acids (HX) or other polar reagents to asymmetric alkenes. The rule states:

• When HX (where H is hydrogen and X is a halogen or other electronegative group) adds to an asymmetric alkene, the hydrogen (H) attaches to the carbon atom with the greater number of hydrogen atoms already bonded to it.
• Meanwhile, the halide (X) or electronegative part attaches to the carbon atom with fewer hydrogen atoms but more alkyl substituents (more substituted carbon).

This happens because the addition involves formation of a carbocation intermediate, and the major product is formed via the more stable carbocation. More substituted carbocations (those attached to more alkyl groups) are more stable due to electron-donating effects and hyperconjugation. Thus, the reaction favors formation of the intermediate that leads to the more stable carbocation and hence the major product. In simpler terms: In addition reactions, "the rich get richer," meaning the carbon with more alkyl groups gets the electronegative group, and the carbon with more hydrogens gets the hydrogen atom. This rule is useful for predicting the major product in electrophilic addition reactions involving alkenes and helps understand regioselectivity in such reactions. Examples include hydrohalogenation, where HCl or HBr adds to an alkene, or hydration reactions forming alcohols, following the same pattern of attachment based on carbocation stability. In summary, Markovnikov's rule guides the regiochemistry of addition reactions on asymmetric alkenes by dictating that the hydrogen atom adds to the carbon with more hydrogens, while the other group adds to the more substituted carbon with fewer hydrogens, leading to the most stable carbocation intermediate and thus the major product.