Electrophilic Addition of HX to Alkenes
Because tertiary and secondary carbocations are more stable than primary carbocations, Markovnikov addition is observed in the electrophilic addition of HX to alkenes, so the product formed is the one with the halogen substituent upon the more highly substituted carbon. Also, rearrangement (hydride or methyl shift to form a more stable carbocation) might occur, typical of reactions that have a carbocation intermediate. Remember that electrophilic addition will not be observed in the presence of peroxides. Peroxides initiate anti-Markovnikov addition via free-radical addition. An interesting fact about electrophilic addition of HX to alkenes, is that the more acidic the hydrogen halide, the more electrophilic it will be. HF, for example, only a weak acid, does not react.