Final answer:
The regioselectivity of Markovnikov addition is explained by the formation of the most stable carbocation intermediate in electrophilic addition reactions to unsymmetrical alkenes.
Step-by-step explanation:
The theoretical explanation for the regioselectivity of Markovnikov addition lies in the stability of carbocation intermediates formed during electrophilic addition reactions. In such reactions involving unsymmetrical alkenes, two different carbocations can form, but the more stable carbocation leads to the major product. This stability often arises from the carbocation being more substituted, having more alkyl groups attached to it, which provide electron-donating inductive effects and stabilize the positive charge.
For example, when hydrohalic acids like HBr are added to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogens (and thus fewer alkyl substituents), and the bromine atom attaches to the more substituted carbon. This outcome is induced because forming a secondary or tertiary carbocation is energetically more favorable than forming a primary carbocation. Therefore, the regioselectivity observed in these additions follows Markovnikov's rule, which states that the electrophile adds to the carbon atom that has more hydrogen atoms attached.