AntiMarkownikowRegel

The Anti-Markovnikov rule, also known as the Kharasch effect, describes the regioselectivity of electrophilic addition reactions of hydrogen halides to alkenes and alkynes under specific conditions. Typically, Markovnikov's rule predicts that the hydrogen atom of the electrophile will add to the carbon atom of the double or triple bond that already has the greater number of hydrogen atoms. This results in the halide atom attaching to the more substituted carbon.

However, the Anti-Markovnikov rule applies when the addition of hydrogen halides to alkenes is initiated by free radicals. This usually occurs in the presence of peroxides or other radical initiators, and in the absence of polar solvents. In such conditions, the addition proceeds in the opposite manner to that predicted by Markovnikov's rule. The hydrogen atom adds to the carbon atom with fewer hydrogen atoms, and the halide atom attaches to the less substituted carbon.

This regiochemical outcome is explained by a free radical chain mechanism. The peroxide initiates the reaction by forming a halogen radical. This radical then adds to the alkene, forming a carbon radical. The carbon radical then abstracts a hydrogen atom from a hydrogen halide molecule, propagating the chain and forming the anti-Markovnikov addition product. The regioselectivity is determined by the relative stability of the intermediate carbon radicals. The less substituted carbon radical is more stable, leading to the observed anti-Markovnikov addition. This effect is particularly significant in the addition of HBr.