Markovnikov's rule states that in the addition of an unsymmetrical reagent to an unsymmetrical alkene, the major product is the one in which the more highly substituted carbon atom of the double bond becomes bonded to the negative part of the adding reagent, whereas the positive part of a polar reagent adds to the less substituted carbon atom.
*For example*, in the addition of HBr to propene, the major product is 2-bromopropane, in which the bromine atom is bonded to the more highly substituted carbon atom.
This rule can be explained by considering the stability of the intermediate carbocations formed in the addition reaction. The more highly substituted carbocation is more stable because it has more alkyl groups bonded to the positive carbon atom, which helps to disperse the positive charge. The less substituted carbocation is less stable because it has fewer alkyl groups bonded to the positive carbon atom, which makes the positive charge more concentrated.
The following table summarizes the predictions of Markovnikov's rule for the addition of various reagents to alkenes:
Reagent | Major Product
--- | ---
HX | Alkyl halide
H2O | Alcohol
ROH | Ether
NH3 | Amine
RMgX | Grignard reagent
LiAlH4 | Aluminum hydride
Exceptions to Markovnikov's Rule
There are some exceptions to Markovnikov's rule. One exception is the addition of HBr to alkenes in the presence of peroxides. In this case, the major product is the anti-Markovnikov product, in which the bromine atom is bonded to the less substituted carbon atom.
The peroxy radical can abstract a hydrogen atom from the less substituted carbon atom of the alkene, forming a more stable allylic radical. The allylic radical then reacts with HBr to form the anti-Markovnikov product.
Another exception to Markovnikov's rule is the addition of water to alkenes in the presence of an acid catalyst. In this case, the major product is the Markovnikov product, but the reaction proceeds through a different mechanism.
The acid catalyst protonates the alkene, forming a carbocation. The carbocation then reacts with water to form the Markovnikov product.
The following table summarizes the exceptions to Markovnikov's rule:
Reagent | Major Product
--- | ---
HBr (peroxides) | Anti-Markovnikov product
H2O (acid catalyst) | Markovnikov product
Regioselectivity vs. Stereoselectivity
Regioselectivity refers to the preference for one product over another based on the position of the atoms that are bonded together. Stereoselectivity refers to the preference for one product over another based on the spatial arrangement of the atoms in the product.
In the addition of an unsymmetrical reagent to an unsymmetrical alkene, both regioselectivity and stereoselectivity can be observed. The regioselectivity of the reaction is determined by Markovnikov's rule, while the stereoselectivity of the reaction is determined by the geometry of the alkene.
The following table summarizes the regio- and stereoselectivity of the addition of various reagents to alkenes:
Reagent | Regioselectivity | Stereoselectivity
--- | --- | ---
HX | Markovnikov | Anti addition
H2O | Markovnikov | Syn addition
ROH | Markovnikov | Syn addition
NH3 | Markovnikov | Syn addition
RMgX | Markovnikov | Syn addition
LiAlH4 | Markovnikov | Syn addition
