Final answer:
The presence or absence of peroxides determines the regiochemical outcome of HBr addition to alkenes. Without peroxides, Markovnikov addition occurs, while with peroxides, anti-Markovnikov addition takes place, altering the position where bromine attaches to the alkene.
Step-by-step explanation:
The regiochemical outcome of HBr addition can be controlled by choosing whether or not to use peroxides. In the absence of peroxides, HBr adds to alkenes in a Markovnikov addition, where the hydrogen atom of HBr attaches to the carbon with the most hydrogens and the bromine attaches to the carbon with fewer hydrogens. This addition follows the rule that the major product is the one where the more substituted carbon (the one with more carbon atoms attached) gets the bromine. However, when peroxides are present, the reaction goes through a radical mechanism, resulting in anti-Markovnikov addition, where the bromine attaches to the carbon with more hydrogens, making it the minor product in normal conditions. This shift in the reaction outcome is due to the different reaction pathways available in the presence of peroxides, which facilitate the formation of a more stable radical intermediate leading to the anti-Markovnikov product.