Final answer:
Regioselectivity in the radical addition of HBr to 2-methylpropene is determined by the formation of the more stable radical intermediate, which leads to the major product due to the stability offered by hyperconjugation and inductive effects from methyl groups.
Step-by-step explanation:
The regioselectivity observed in the radical addition of HBr to 2-methylpropene reflects the preference for the formation of the more stable free radical intermediate. When HBr is added to 2-methylpropene, a 2-carbon radical is generated preferentially over a 1-carbon radical due to the former's greater stability. This preference is due to the hyperconjugation and the inductive effects provided by the adjacent methyl groups, which stabilize the radical.
The addition of HBr to 2-methylpropene could yield two different radicals, however, the major product is typically the one that comes from the more stable radical intermediate. The stability of carbocations and radicals play a critical role in determining the major product in such chemical reactions.
In summary, when HBr is added to the unsymmetrical alkene 2-methylpropene, the regioselectivity we observe is due to the formation of the more stable radical intermediate based on the relative stability of the possible intermediates, resulting in the formation of the corresponding bromide as the major product.