The Anti-Markovnikov rule is a regiochemistry rule that describes the addition of a nucleophile to an unsymmetrical alkene, where the substituent is bonded to a less substituted carbon instead of the more substituted carbon/Alkenes/Reactivity_of_Alkenes/Free_Radical_Reactions_of_Alkenes/Radical_Additions%3A_Anti-Markovnikov_Product_Formation). This is in contrast to the Markovnikov rule, which states that in an unsymmetrical alkene, the addition of a nucleophile will attach to the more substituted carbon.

The Anti-Markovnikov rule is also known as the peroxide effect or the Kharasch effect. It is observed in reactions such as the addition of hydrogen bromide to propene in the presence of peroxide. The peroxide compound involved helps in the generation of free radicals, which attack the hydrogen halide to form halide radicals through hemolysis. The generated halide radical then attacks the alkene molecule to form an alkyl radical through hemolysis. The alkyl radical then attacks the hydrogen halide to form an alkyl halide through homolytic cleavage of the hydrogen halide bond.

The Anti-Markovnikov rule is important in organic chemistry because it can lead to the formation of different products in reactions. For example, the Anti-Markovnikov addition of hydrogen bromide to propene results in the formation of 2-bromopropane, while the Markovnikov addition of hydrogen bromide to propene results in the formation of 1-bromopropane.