Markovnikov's rule

Markovnikov's rule is an scientific law in organic chemistry. It predicts the isomer made by addition reactions to an alkene. The rule was discovered by Vladimir Markovnikov in 1870, and is now a common example of regioselectivity in chemistry education and basic research.

The simple version of Markovnikov's rule says that

When a reaction adds hydrogen and a nucleophile X to an alkene with alkyl substituents, H adds to the side with less alkyl groups and X adds to the side with more.

When the product of a reaction matches Markovnikov's rule, it is called Markovnikov addition; when a reaction makes the other product it is anti-Markovnikov addition.

Markovnikov and anti-Markovnikov addition are related to but different from syn and anti addition, which focuses on the stereochemistry of the addition rather than the structural isomers.

Mechanism

In Markovnikov addition, the reaction starts with protonation of the alkene to make a carbocation. Because alkyl carbocations with a greater degree of substitution are more stable, the more stable intermediate has the positive charge on the more substituted carbon. The nucleophile then makes a bond with the carbocation to make the final product.

Exceptions and generalisation

Markovnikov's rule as stated only applies to alkenes with alkyl substituents. Other functional groups can change the stability of the carbocations in ways different from alkyl groups. When other groups are present, Markovnikov's rule can be rewritten as

When a reaction adds hydrogen and a nucleophile X to an alkene with a carbocation reaction intermediate, adds to the atom that has the positive charge in the most stable intermediate.

For example, isobutenylbenzene has one phenyl group on one side of the alkene and two methyl groups on the other. Addition to this compound will add the nucleophile on the carbon with the phenyl group, not the methyl groups, because one phenyl substituent stabilises a carbocation more than two methyl groups.

The rule of "reactions happen in places that make the most stable intermediate" is seen in other regioselective reactions like electrophilic aromatic substitution.

References