If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the greater number of hydrogens, and the negative part of the reagent adds to the more substituted vinylic carbon. Thus, one constitutional isomer is preferred, and the hydrohalogenation of alkenes is highly regioselective. Here, the regioselectivity is a consequence of the relative stabilities of the carbocation intermediates for each product.
The reaction begins with the transfer of a pair of electrons from the alkene to the proton of the hydrogen bromide, resulting in a carbocation. The protonation step is endergonic with a high energy of activation. It is also the slow rate-determining step. The rapid combination of the carbocation with the halide ion in the next step is exergonic with low activation energy.
Hammond's postulate indicates that the structure of the transition state in an endergonic process resembles that of the products as they are closer in energy. Thus, in the protonation step, the transition state structurally resembles the carbocation intermediate. Protonation can either yield the less substituted primary carbocation or, the more substituted tertiary carbocation. Tertiary carbocations are more stable than their secondary or primary counterparts, as their formation requires lower activation energy. Thus, the reaction prefers the pathway with the lower energy barrier via the more stable intermediate, resulting in regioselectivity.
When hydrohalogenation generates a new chiral center, the planar nature of the carbocation intermediate indicates that the nucleophile can approach from above or below the plane with equal probability, resulting in a racemic mixture of products.
When addition reactions proceed via mechanisms other than electrophilic addition, the hydrogen can add to the more substituted carbon, yielding the anti-Markovnikov product.
