Markovnikova

Markovnikova refers to a set of rules in organic chemistry that predict the regiochemistry of electrophilic addition reactions to unsymmetrical alkenes and alkynes. Specifically, it describes how a hydrogen atom and a halogen atom (or another group) add across a double or triple bond. The original formulation, developed by Vladimir Markovnikov in 1870, states that in the addition of a protic acid HX to an alkene, the hydrogen atom adds to the carbon atom that already has the greater number of hydrogen atoms attached. Consequently, the halide (X) adds to the more substituted carbon atom.

This rule is a consequence of the stability of the carbocation intermediate formed during the reaction. In the addition of HX, the proton (H+) first adds to one of the carbon atoms of the double bond, forming a carbocation. The more substituted carbocation is generally more stable due to hyperconjugation and inductive effects. Therefore, the addition of the proton occurs in a way that favors the formation of the more stable carbocation. The nucleophilic part of the acid (X-) then attacks this more stable carbocation, leading to the observed product distribution.

While Markovnikov's rule is a powerful predictive tool, it has limitations and exceptions. For instance, in the presence of peroxides, the addition of HBr to alkenes proceeds via a free radical mechanism, which leads to anti-Markovnikov addition, where the hydrogen atom adds to the more substituted carbon. This inverse relationship is known as the Kharasch effect. Modern understanding of electrophilic additions often involves considering frontier molecular orbitals and reaction mechanisms rather than relying solely on empirical rules.