7.7 : アルケンへの求電子付加の位置選択性: マルコフニコフの法則

If a set of reactants can yield multiple constitutional isomers, but the formation of one product is favored, the reaction is regioselective.       

For instance, when an unsymmetrical alkene, such as 2-methylpropene, undergoes hydrobromination, two bromoalkane products seem to be possible. 

Here, the difference in relative stabilities of the two carbocation intermediates determines the final product. 

According to Markovnikov's rule, hydrogen preferentially adds to the vinylic carbon bearing the greater number of hydrogens. Consequently, the negative part of the reagent adds to the more substituted vinylic carbon.

First, the alkene donates a pair of electrons to the proton of the hydrogen bromide to form a carbocation. The protonation step is endergonic and has a high energy of activation. Since it occurs slowly, it is also the rate-determining step. 

The next step is exergonic and has a low activation energy. It involves a rapid combination of the carbocation with the halide ion.

Hammond's postulate indicates that the transition state in the endergonic protonation step will structurally resemble the product, which is the carbocation intermediate.

Protonation can yield either 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 carbocation intermediate results in a pair of enantiomers, and the reaction is not stereospecific.

Exceptions to Markovnikov's addition occur when the bromine adds to the less substituted vinylic carbon, yielding the anti-Markovnikov product.