Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

A regioselective reaction yields multiple possible constitutional isomers, but one product predominates. In hydrohalogenation of unsymmetrical alkenes, regioselectivity occurs because hydrogen adds to the vinylic carbon with more hydrogens, while the halide adds to the more substituted carbon. This selectivity results from differences in carbocation intermediate stability.

Markovnikov's rule states that hydrogen preferentially adds to the vinylic carbon bearing the greater number of hydrogens, with the negative part of the reagent adding to the more substituted vinylic carbon. This occurs because the more substituted carbocation intermediate is more stable and forms via a lower activation energy pathway, making it the thermodynamically favored product.

Tertiary carbocations are more stable than primary or secondary carbocations because their formation requires lower activation energy. According to Hammond's postulate, the transition state of the endergonic protonation step structurally resembles the carbocation product. Since the reaction pathway favors lower energy barriers, the more stable tertiary carbocation intermediate is preferentially formed.

The protonation step, where the alkene donates electron pairs to the hydrogen bromide proton to form a carbocation, is the rate-determining step. This step is endergonic with high activation energy and occurs slowly. The subsequent halide ion combination is exergonic with low activation energy and proceeds rapidly.

When hydrohalogenation generates a new chiral center, the planar carbocation intermediate allows the halide nucleophile to approach from either above or below the plane with equal probability. This results in a racemic mixture of enantiomers rather than a single stereoisomer, making the reaction non-stereospecific despite being regioselective.

Anti-Markovnikov products form when the bromine adds to the less substituted vinylic carbon, opposite to Markovnikov's prediction. This occurs when addition reactions proceed via mechanisms other than electrophilic addition, allowing hydrogen to add to the more substituted carbon instead of the less substituted one.

Hammond's postulate states that the transition state in an endergonic process structurally resembles the products, which are closer in energy. In alkene protonation, the transition state resembles the carbocation intermediate. Since the more stable tertiary carbocation requires lower activation energy to form, the reaction preferentially follows this pathway, resulting in regioselectivity.